TY - JOUR
AU - Mesbahi, Asghar
AB - Abstract Despite all advantages for using high-energy photons for radiotherapy, high-energy photon beams (≥10 MV) induce photonuclear and neutron capture interactions, which result in producing radionuclide byproducts inside the Linac head and bunker, exposing radiation therapy technologists (RTTs) and patients to excessive dose. By the use of higher photon energy, greater number of monitor unit, greater field size and adding treatment accessories, induced dose rate become greater in the isocenter mainly due to activation of high-Z materials inside the Linac head. Activated radionuclides disintegrate with γ, β+ and β− rays with half-lives between 2 min up to more than 5 years. Several researches estimated additional exposure to an RTT depend on treatment strategies, beam energy, and delay time before entrance to the treatment room between 0.1 and 4.9 mSv/y and proposed at least 2 min delay before entrance to the treatment room after treatments with high-energy photon beams. INTRODUCTION External-beam radiotherapy by means of high-energy photons generated by medical linear accelerators (Linacs) is the most common modality for treating cancers. High-energy photons provide numerous advantages in comparison with lower-energy photons, such as better dose delivery for deep-rooted malignant tumors, minor skin dose and lower sensitivity to tissue homogeneities(1, 2), but on the other hand, for photon energies more than 6 MV and mostly above 10 MV, the components constituting the Linac head, the treatment room, and in the patient’s body become radioactive because of photonuclear, neutron capture(3–5) and electronuclear(4, 6–8) interactions. The photoneutron byproducts mostly exist inside the initial beam and remain so even after the beam has been switched off and consequently exposing radiation therapy technologists (RTTs) to unwanted and excessive radiation dose(9–12). High-energy photons cause radioactive yields initially by (γ,n) interactions by emitting a neutron from a target nuclide. Other fewer common procedures are photodisintegration—(γ,2n), (γ,p), etc.(9). Inside the Linac head, the patient’s body, and the surrounding materials around the room(11, 13–15). A representation of radiotherapy room and the location of RTT is illustrated in Figure 1. These processes induce radioactivity in the irradiated materials, a process known as ‘photon activation’(9, 16). The cross-sections of photonuclear reactions are greater than those of electronuclear reactions (e,e′n) triply in the energy range produced by therapeutic Linacs(17). Thus, the neutron contamination remains a particularly important issue for therapeutic X-ray beams used in external radiotherapy. Whereas recommended by International Commission of Radiological Protection (ICRP) neutrons weighting factor is equal to 2(18), so neutrons are more biologically damaging compared to photon beams(19). Furthermore, thermal and resonance neutrons undergo (n,γ) interactions and induce radioactivity in the Linac head and other components in the vicinity of treatment room(9, 12). Figure 1. View largeDownload slide The schematic representation of treatment room from top view. Figure 1. View largeDownload slide The schematic representation of treatment room from top view. In spite of all high-energy photon advantages, the photoneutrons increase the unwanted dose to patients as well as RTTs(6, 10, 14, 20). Several studies have estimated the time of disintegration from 1 min up to more than 5 years after the last exposure(3, 21, 22). Consequently, it can be considered as the highest source of occupational radiation exposure for RTTs, who enter the treatment room immediately after the end of the irradiation to operate the patient or to change of the treatment elements, are exposed to increased radiation levels(6, 10, 12, 23). Konefal et al.(7,) reported that the fluence of thermal neutrons after the emission of a 20 MV photon beam is ~106 neutrons/cm−2 Gy−1. Israngkul-Na-Ayuthaya et al.(24,) calculated the average neutron energy of 0.25 MV, and they have detected a gamma dose rate of 4.14 μSv/h as a consequence of neutron induced activation inside the Linac head. Saeed et al.(25,) identified air photoactivation after operation of a 15 MV Linac and reported a production rate of a 836.8 Bq/Gy for 13N. Additionally, Tana et al.(15,) showed that the dose to RTT’s lung can be negligible for photon beam energies below 18 MV. Ho et al.(12) measured the highest gamma dose rate of 7 μSv/h nearly inside the treatment hall and the highest dose received by RTTs was estimated at 5 mSv/y. However, it should be mentioned here that the occupational radiation exposure of RTTs due to the induced radioactivity depends upon several parameters, including the type of Linac and its component, dose rate, the energy of photon beam, entry time after beam-off(7, 9, 22, 26). Several studies reported the presence of various radionuclides in the treatment room(3, 4, 21, 26–31), dose rate due to induced radioactivity in the different point at treatment room(14, 22), and factors that affect induced radioactivity, annual occupational exposure. The occupational received dose rate was estimated to be 0.5–5 mSv/y(10, 12, 23). Also, dose rate after operation an 18 MV accelerator at the isocenter was estimated ~7–17 μSv/h depends upon several factors like dose rate, field size, etc.(11, 21, 22). Donadile et al.(10) indicated that ~54% of the annual effective dose to RTTs is owing to the induced radioactivity and 7% owing to neutrons. The purpose of this article was to review and summarize the sources of RTT’s received dose from induced radioactivity from Linac head. Moreover, the physics of induced radioactivity after high-energy Linac operation, induced radioisotopes from the Linac head and other components of treatment room were reviewed. PHYSICS OF INDUCED RADIOACTIVITY IN HIGH-ENERGY RADIATION THERAPY After generation of photons from the Bremsstrahlung phenomenon in the high-Z target inside a high-energy Linac, resulting photons will interact with the electron shell of equipment in their pathway, and with the nuclei. In the energy range of medical Linac, i.e. up to 25 MV, the most significant reaction is the photonuclear effect. This reaction of photons with materials found in the head of Linac results in generation of fast neutrons and neutron-deficient radionuclides(3, 26). The reaction can be shown as follows(5, 22, 26, 32):   ANZA(γ,n)AN−1ZA−1 (1)where, A = number of nucleons, Z = atomic number and N = number of neutrons. The neutrons have a wide spectral distribution with a maximum energy, more than 10 MV(7, 33–35) and the subsequent nucleus will emit a positron (β+) or decay by electron capture, mainly, due to the neutron deficiency. Then, they lose energy and slow down via multiple interactions with surrounding materials and undergo nuclear reactions. In the energy range of medical Linac the most significant reaction is the ‘neutron capture’ that can be defined as follows(5, 22, 26, 32, 36):   ANZA(n,γ)AN+1ZA+1 (2) In most cases, the subsequent nucleus is radioactive again and because of additional neutron it will decay by β− production. Consequently, it can be predictable that the activation products made by neutron capture are spread over the entire treatment hall(30), and not just in the pathway of the primary beam(35), because the neutron flux reduces slightly with the increasing distance from the Linac head. Neutron capture happens at thermal and resonance energies, and the radiation level of thermal and resonance neutron becomes more important in the distant locations from the head of the Linac(4). It should be noted here that the energy range of neutrons depends directly on the nominal energy of the therapeutic X-ray beam(4, 7). When a medical Linac operates in photon energy, <10 MV neutron production is negligible(7, 10, 35, 37). For nucleus with atomic number more than carbon, the threshold energy for removing a neutron from a nucleus is ~6 up to 16 MV; because threshold energy is reduced with increasing atomic number(12, 19). Additionally, the neutron binding energy for heavy nuclei is smaller compared to others and so, the photonuclear cross-section becomes higher for them. As it was noted, the cross-section of nuclear interaction of photons for elements with high atomic number is high and for light atomic number is low, therefore, light elements activation is not common as heavy elements (Table 1)(12, 21). In the Linac head and bunker, threshold energy for most of the isotopes is ~8 MV(4). Neutron production occurs inside Linac head components, patient’s body and as well in treatment room building(1, 43) (Figure 1). Several researches determined that target, collimation system and particularly multi-leaf collimators (MLCs)(7, 19) are the most important components which contribute in neutron production due to higher atomic number and intense photon fluence in these regions(12, 30, 44, 45). A number of articles have reported the threshold energy for several metals constituting the Linac head. For instance, for lead and tungsten, it is between 6 and 8 MV, for aluminum and iron is 13 MV and finally for copper is 10 MV(12). For example, a study showed that for a treatment with a 15 MV photon beam, the average energy of produced neutrons is between 1 and 3 MV, although the maximum of 7.6 MV was reported(7, 12, 19, 46). As a matter of fact, the produced neutron energy strongly depends on interacting photon beam energy. Several studies indicated that there are multiple factors, which are influencing the neutron production process in radiation therapy; such as different treatment techniques, the accelerator type due to the various target material and other machinery components, using of beam modifying accessories (such as wedges and MLCs) that made of high atomic number metals(7, 31, 37, 47–49). Table 1. Radionuclides byproducts inside the Linac head and bunker. Activated radioisotope  Activation process  Decay mechanism  Half-life  Main gamma rays (keV)  Threshold energy (MV)  Cross-section (barns)  Main location  22Na  23Na(γ, n)22Na  EC, β+  2.6 y  1275  12.42d  0.1  Concrete, outer casing material  24Na  23Na(n,γ)24Na  IT, β−  15 h  1369, 2754  12.1d  0.53b  Concrete, outer casing material  28Al  27Al(n, γ)28Al 29Si(γ, p)28Al  β−, γ β−  2.3 min  1779  13.03d 12.3d  0.23b  Treatment coach, electron absorber, wedge tray, PI  34mCl  35Cl(γ, n)34mCl  β+, EC, IT  32 min  147, 1178, 2129  12.65d  0.09b    38Cl  37Cl(n, γ)38Cl  β−  37.24 min  1642, 2168  n.d.  100b  Outer caning material  51Cr  50Cr(n, γ)51Cr 52Cr(γ, n) 51Cr  EC  27.7 d  320  n.d.  15.9b 4.09c  Wall concrete, stainless steel, flattening filter  52Mn  54Fe(γ,np)52Mn  β+, EC  5.59  744, 936, 1434  20.91d  0.48c  Wedge, stainless steel  54Mn  55Mn(γ,n)54Mn 56Fe(γ,np)54Mn  β−, EC  312.3 d  835  10.23d 20.41d  0.56c n.d.  Wedge, stainless steel, wall, flattening filter  56Mn  55Mn(n,γ)56Mn  β−, γ  2.58 h  847, 1811, 2113  10.6d  13.4b  Wedge, stainless steel, flattening filter  57Mn  58Fe(γ,p)57Mn  β−  1.45 min  122, 692  10.0d  n.d.  Linac head, wedge, wall  53Fe  54Fe(γ, n)53Fe  β+, EC  8.51 min  377, 378, 511a(14)  13.4d  0.21c  Head, wedge, bunker  55 Fe  54Fe(n,γ)55Fe  β−  2.68 y  n.d.  n.d.  2.25b  Steel component of Ledite material  59Fe  59Co(n, p)59Fe  β−  44.5 d  1099, 1292  n.d.  44c  Stainless steel, flattening filter  57Co  59Co(γ,2n)57Co 58Ni(γ, p)57Co  β+  271.8 d  122, 136  19.3d  0.15c 0.58c  Stainless steel flattening filter Nickel parts  58Co  59Co(γ, n)58Co 60Ni(γ,np)58Co  β+, EC  70.78 d  811, 864, 1 675 511a(14)  10.45d 19.99d  0.44c 0.94c  Stainless steel flattening filter  60Co  61Ni(γ, p)60Co 59Co(n, γ)60Co  β−  5.3 y  1173, 1333  9.86d  n.d. 37.1b  Stainless steel flattening filter  57Ni  58Ni(γ, n)57Ni  β+, EC  35.6 h  127, 1378, 1920  12.22d  0.27c  Flattening filter, MLC Stainless steel  62Cu  63Cu(γ, n)62Cu 64Zn(γ,np)62Cu  β+, EC  9.74 min  876, 1173  10.85d 7.71  0.61c n.d.  Target, flattening filter, PI  64Cu  65Cu(γ, n)64Cu  β+, β−, EC  12.7 h  1346  9.9d  0.06c  Electronic modules, collimation system, target  66Cu  65Cu(n, γ)66Cu  β−  5.1 min  1039.23  n.d.  2.14b  Electronic modules, accelerating structure, target, copper admixture  63Zn  64Zn(γ, n)63Zn  β+, EC  38.4 min  670, 962, 1412 511a(14)  11.9d  0.09c  Electron applicator  65Zn  66Zn(γ, n)65Zn  β−, β+  244.3 d  1116  11.1d  0.03c  Copper admixture  82Br  81Br(n, γ)82Br  β−, γ  35.34 h  554, 619, 777  n.d.  2.7b  Electronic modules, admixture to outer casing materials  99Mo  100Mo(γ,n)99Mo  β+  66.0 h  181, 739, 778  8.29d  0.81c  Stainless steel, collimation system  120Sb  121Sb(γ,n)120Sb  β+, EC  15.9 min  703, 989, 1172  9.24d  0.3 atc 15 MV    122Sb  121Sb(n,γ)122Sb  β−, β+, EC  2.7 d  564, 693, 1257  9.0d  6.01b  Electronic modules (contacts), admixture of Pb head cover  124Sb  123Sb(n,γ)124Sb  β−  60.3 d  603, 723, 1691  n.d.  4.1b  Electronic modules (contacts), admixture of Pb head cover  184Re  185Re(γ,n)184Re  β+  38.0 d  111, 792, 903  7.67d  n.d.    187W  186 W(n,γ)187W  β−, γ  23.72 h  134, 479, 686 551, 618, 744, 772(29)  n.d.  37.9c  Collimation system, target  185 mW  184W(n,γ)185 mW 186W(γ,n)185 mW  β−  1.67 min  132, 188  7.5d  1.69b 1.12 at 12 MV  Collimation system, target  181W  182W(γ,n) 181W  EC  122.2 d  58  8.07d  n.d.  Collimation system, target  196Au  197Au(γ,n)196Au 197Au(γ,n)196Au  β+, β−, EC  6.2 d  333, 356, 426  8.07d  2.37 at 16 MV  Wave-guide, Target of Primus, electronic contacts  198Au  197Au (n,γ) 198Au  β−  2.7 d  412  8.07d  99.2b  Electronic contacts, elements of Clinac 2100 target  27Mg  27Al(n, p)27Mg  β−  9.46 min  844, 1015  n.d.  0.075c  PI  41Ar  40Ar(n,γ) 41Ar  β−  1.827 h  n.d.  n.d.  0.6b  Air  13N  14N (γ,n) 13N  β+  10 min  511a  10.6d  0.09 at 23 MVc  Air, human dudy  11C  12C(γ,n)11C  β+  20.3 min  n.d.  18.72d  n.d.  air  15O  16O(γ, n)15O  EC, β+  2.04 min  511a  15.7d  0.06 at 22 MVc  Air, wall concrete  116 mIn  115In(n,γ) 116 mIn  β−  54.3 min  1097, 1293, 1508  n.d.  205.02b    139Ba  138Ba(n,γ) 139Ba  β−  83 min  165  n.d.  0.35b  Admixture of Pb head cover  131Ba  130Ba(n,γ)131Ba  β−  11.5 d  123.8, 216.1, 373.2, 496.3  n.d.  11.2b    182Ta  183W(γ,p)182Ta  β−  115 d  1121,1189,1221  7.2d  0.05c  Target, collimation system, flattening filter, electronic modules  183Ta  184W(γ,p)183Ta  β−  5.1 d  246, 354  7.7d  0.05 at 23 MVc  Target, collimation system, flattening filter  185Ta  186W(γ,p)183Ta  β−  49 min  174, 178  8.4d  0.05 at 23 MV  Target, collimation system, flattening filter  49Ca  48Ca(n,γ) 49Ca  β−  8.8 min  3084  n.d.  1.09b  Flattening filter, Wall-mounted laser, concrete  47Ca  48Ca(γ, n) 47Ca  β−  4.5 d  1297  9.9d  n.d.  Concrete  52V  53Cr(γ,p)52V  β−, γ  3.75 min  1434  11.1d  n.d.  Linac head shielding  53V  54Cr(γ,p)52V  β−  1.6 min  1006, 1289  12.4d  n.d.  Linac head shielding  206*Tl  206Pb(γ,p) 206*Tl  IT  3.8 min  687, 1022  7.5d  n.d.  Linac head shielding  38K  39K(γ, n)38K  β+  7.6 min  511a, 2168  13.1d  0.1 at 20 MVc  Wall-mounted laser  43K  44Ca(γ,p) 43K  β−  22.3 h  373, 617  12.2d  n.d.  Bunker  207 mPb  207Pb(n,n’)207 mPb  IT  0.8 s  596.702  6.7  12.1b  Collimation system, Linac head shielding  203Pb  EC, β+  β−  51.9 h  279, 401, 681  8.39d  2.9 at 13 MVc    137Cs  136Cs(n,γ)137Cs fission  β−  30.07 y  661.65  n.d.  1.3b    37S  36S(n,γ)37S  β−  5.07 min  3103  n.d.  0.15b    Activated radioisotope  Activation process  Decay mechanism  Half-life  Main gamma rays (keV)  Threshold energy (MV)  Cross-section (barns)  Main location  22Na  23Na(γ, n)22Na  EC, β+  2.6 y  1275  12.42d  0.1  Concrete, outer casing material  24Na  23Na(n,γ)24Na  IT, β−  15 h  1369, 2754  12.1d  0.53b  Concrete, outer casing material  28Al  27Al(n, γ)28Al 29Si(γ, p)28Al  β−, γ β−  2.3 min  1779  13.03d 12.3d  0.23b  Treatment coach, electron absorber, wedge tray, PI  34mCl  35Cl(γ, n)34mCl  β+, EC, IT  32 min  147, 1178, 2129  12.65d  0.09b    38Cl  37Cl(n, γ)38Cl  β−  37.24 min  1642, 2168  n.d.  100b  Outer caning material  51Cr  50Cr(n, γ)51Cr 52Cr(γ, n) 51Cr  EC  27.7 d  320  n.d.  15.9b 4.09c  Wall concrete, stainless steel, flattening filter  52Mn  54Fe(γ,np)52Mn  β+, EC  5.59  744, 936, 1434  20.91d  0.48c  Wedge, stainless steel  54Mn  55Mn(γ,n)54Mn 56Fe(γ,np)54Mn  β−, EC  312.3 d  835  10.23d 20.41d  0.56c n.d.  Wedge, stainless steel, wall, flattening filter  56Mn  55Mn(n,γ)56Mn  β−, γ  2.58 h  847, 1811, 2113  10.6d  13.4b  Wedge, stainless steel, flattening filter  57Mn  58Fe(γ,p)57Mn  β−  1.45 min  122, 692  10.0d  n.d.  Linac head, wedge, wall  53Fe  54Fe(γ, n)53Fe  β+, EC  8.51 min  377, 378, 511a(14)  13.4d  0.21c  Head, wedge, bunker  55 Fe  54Fe(n,γ)55Fe  β−  2.68 y  n.d.  n.d.  2.25b  Steel component of Ledite material  59Fe  59Co(n, p)59Fe  β−  44.5 d  1099, 1292  n.d.  44c  Stainless steel, flattening filter  57Co  59Co(γ,2n)57Co 58Ni(γ, p)57Co  β+  271.8 d  122, 136  19.3d  0.15c 0.58c  Stainless steel flattening filter Nickel parts  58Co  59Co(γ, n)58Co 60Ni(γ,np)58Co  β+, EC  70.78 d  811, 864, 1 675 511a(14)  10.45d 19.99d  0.44c 0.94c  Stainless steel flattening filter  60Co  61Ni(γ, p)60Co 59Co(n, γ)60Co  β−  5.3 y  1173, 1333  9.86d  n.d. 37.1b  Stainless steel flattening filter  57Ni  58Ni(γ, n)57Ni  β+, EC  35.6 h  127, 1378, 1920  12.22d  0.27c  Flattening filter, MLC Stainless steel  62Cu  63Cu(γ, n)62Cu 64Zn(γ,np)62Cu  β+, EC  9.74 min  876, 1173  10.85d 7.71  0.61c n.d.  Target, flattening filter, PI  64Cu  65Cu(γ, n)64Cu  β+, β−, EC  12.7 h  1346  9.9d  0.06c  Electronic modules, collimation system, target  66Cu  65Cu(n, γ)66Cu  β−  5.1 min  1039.23  n.d.  2.14b  Electronic modules, accelerating structure, target, copper admixture  63Zn  64Zn(γ, n)63Zn  β+, EC  38.4 min  670, 962, 1412 511a(14)  11.9d  0.09c  Electron applicator  65Zn  66Zn(γ, n)65Zn  β−, β+  244.3 d  1116  11.1d  0.03c  Copper admixture  82Br  81Br(n, γ)82Br  β−, γ  35.34 h  554, 619, 777  n.d.  2.7b  Electronic modules, admixture to outer casing materials  99Mo  100Mo(γ,n)99Mo  β+  66.0 h  181, 739, 778  8.29d  0.81c  Stainless steel, collimation system  120Sb  121Sb(γ,n)120Sb  β+, EC  15.9 min  703, 989, 1172  9.24d  0.3 atc 15 MV    122Sb  121Sb(n,γ)122Sb  β−, β+, EC  2.7 d  564, 693, 1257  9.0d  6.01b  Electronic modules (contacts), admixture of Pb head cover  124Sb  123Sb(n,γ)124Sb  β−  60.3 d  603, 723, 1691  n.d.  4.1b  Electronic modules (contacts), admixture of Pb head cover  184Re  185Re(γ,n)184Re  β+  38.0 d  111, 792, 903  7.67d  n.d.    187W  186 W(n,γ)187W  β−, γ  23.72 h  134, 479, 686 551, 618, 744, 772(29)  n.d.  37.9c  Collimation system, target  185 mW  184W(n,γ)185 mW 186W(γ,n)185 mW  β−  1.67 min  132, 188  7.5d  1.69b 1.12 at 12 MV  Collimation system, target  181W  182W(γ,n) 181W  EC  122.2 d  58  8.07d  n.d.  Collimation system, target  196Au  197Au(γ,n)196Au 197Au(γ,n)196Au  β+, β−, EC  6.2 d  333, 356, 426  8.07d  2.37 at 16 MV  Wave-guide, Target of Primus, electronic contacts  198Au  197Au (n,γ) 198Au  β−  2.7 d  412  8.07d  99.2b  Electronic contacts, elements of Clinac 2100 target  27Mg  27Al(n, p)27Mg  β−  9.46 min  844, 1015  n.d.  0.075c  PI  41Ar  40Ar(n,γ) 41Ar  β−  1.827 h  n.d.  n.d.  0.6b  Air  13N  14N (γ,n) 13N  β+  10 min  511a  10.6d  0.09 at 23 MVc  Air, human dudy  11C  12C(γ,n)11C  β+  20.3 min  n.d.  18.72d  n.d.  air  15O  16O(γ, n)15O  EC, β+  2.04 min  511a  15.7d  0.06 at 22 MVc  Air, wall concrete  116 mIn  115In(n,γ) 116 mIn  β−  54.3 min  1097, 1293, 1508  n.d.  205.02b    139Ba  138Ba(n,γ) 139Ba  β−  83 min  165  n.d.  0.35b  Admixture of Pb head cover  131Ba  130Ba(n,γ)131Ba  β−  11.5 d  123.8, 216.1, 373.2, 496.3  n.d.  11.2b    182Ta  183W(γ,p)182Ta  β−  115 d  1121,1189,1221  7.2d  0.05c  Target, collimation system, flattening filter, electronic modules  183Ta  184W(γ,p)183Ta  β−  5.1 d  246, 354  7.7d  0.05 at 23 MVc  Target, collimation system, flattening filter  185Ta  186W(γ,p)183Ta  β−  49 min  174, 178  8.4d  0.05 at 23 MV  Target, collimation system, flattening filter  49Ca  48Ca(n,γ) 49Ca  β−  8.8 min  3084  n.d.  1.09b  Flattening filter, Wall-mounted laser, concrete  47Ca  48Ca(γ, n) 47Ca  β−  4.5 d  1297  9.9d  n.d.  Concrete  52V  53Cr(γ,p)52V  β−, γ  3.75 min  1434  11.1d  n.d.  Linac head shielding  53V  54Cr(γ,p)52V  β−  1.6 min  1006, 1289  12.4d  n.d.  Linac head shielding  206*Tl  206Pb(γ,p) 206*Tl  IT  3.8 min  687, 1022  7.5d  n.d.  Linac head shielding  38K  39K(γ, n)38K  β+  7.6 min  511a, 2168  13.1d  0.1 at 20 MVc  Wall-mounted laser  43K  44Ca(γ,p) 43K  β−  22.3 h  373, 617  12.2d  n.d.  Bunker  207 mPb  207Pb(n,n’)207 mPb  IT  0.8 s  596.702  6.7  12.1b  Collimation system, Linac head shielding  203Pb  EC, β+  β−  51.9 h  279, 401, 681  8.39d  2.9 at 13 MVc    137Cs  136Cs(n,γ)137Cs fission  β−  30.07 y  661.65  n.d.  1.3b    37S  36S(n,γ)37S  β−  5.07 min  3103  n.d.  0.15b    n.d.: Not determined. PI: Portal imager. aAnnihilation gamma rays. bCross-sections derived from ‘The Jeff nuclear data library, report 21’(38), ‘a nuclear cross-section data, IAEA 1989’(39), and ‘Handbook on photonuclear data for applications cross-section and spectra, IAEA 2000’(40). cDerived from ‘Handbook on nuclear activation data, IAEA’(41). dDerived from’ ‘handbook on photonuclear data for applications cross-section and spectra’(42). Table 1. Radionuclides byproducts inside the Linac head and bunker. Activated radioisotope  Activation process  Decay mechanism  Half-life  Main gamma rays (keV)  Threshold energy (MV)  Cross-section (barns)  Main location  22Na  23Na(γ, n)22Na  EC, β+  2.6 y  1275  12.42d  0.1  Concrete, outer casing material  24Na  23Na(n,γ)24Na  IT, β−  15 h  1369, 2754  12.1d  0.53b  Concrete, outer casing material  28Al  27Al(n, γ)28Al 29Si(γ, p)28Al  β−, γ β−  2.3 min  1779  13.03d 12.3d  0.23b  Treatment coach, electron absorber, wedge tray, PI  34mCl  35Cl(γ, n)34mCl  β+, EC, IT  32 min  147, 1178, 2129  12.65d  0.09b    38Cl  37Cl(n, γ)38Cl  β−  37.24 min  1642, 2168  n.d.  100b  Outer caning material  51Cr  50Cr(n, γ)51Cr 52Cr(γ, n) 51Cr  EC  27.7 d  320  n.d.  15.9b 4.09c  Wall concrete, stainless steel, flattening filter  52Mn  54Fe(γ,np)52Mn  β+, EC  5.59  744, 936, 1434  20.91d  0.48c  Wedge, stainless steel  54Mn  55Mn(γ,n)54Mn 56Fe(γ,np)54Mn  β−, EC  312.3 d  835  10.23d 20.41d  0.56c n.d.  Wedge, stainless steel, wall, flattening filter  56Mn  55Mn(n,γ)56Mn  β−, γ  2.58 h  847, 1811, 2113  10.6d  13.4b  Wedge, stainless steel, flattening filter  57Mn  58Fe(γ,p)57Mn  β−  1.45 min  122, 692  10.0d  n.d.  Linac head, wedge, wall  53Fe  54Fe(γ, n)53Fe  β+, EC  8.51 min  377, 378, 511a(14)  13.4d  0.21c  Head, wedge, bunker  55 Fe  54Fe(n,γ)55Fe  β−  2.68 y  n.d.  n.d.  2.25b  Steel component of Ledite material  59Fe  59Co(n, p)59Fe  β−  44.5 d  1099, 1292  n.d.  44c  Stainless steel, flattening filter  57Co  59Co(γ,2n)57Co 58Ni(γ, p)57Co  β+  271.8 d  122, 136  19.3d  0.15c 0.58c  Stainless steel flattening filter Nickel parts  58Co  59Co(γ, n)58Co 60Ni(γ,np)58Co  β+, EC  70.78 d  811, 864, 1 675 511a(14)  10.45d 19.99d  0.44c 0.94c  Stainless steel flattening filter  60Co  61Ni(γ, p)60Co 59Co(n, γ)60Co  β−  5.3 y  1173, 1333  9.86d  n.d. 37.1b  Stainless steel flattening filter  57Ni  58Ni(γ, n)57Ni  β+, EC  35.6 h  127, 1378, 1920  12.22d  0.27c  Flattening filter, MLC Stainless steel  62Cu  63Cu(γ, n)62Cu 64Zn(γ,np)62Cu  β+, EC  9.74 min  876, 1173  10.85d 7.71  0.61c n.d.  Target, flattening filter, PI  64Cu  65Cu(γ, n)64Cu  β+, β−, EC  12.7 h  1346  9.9d  0.06c  Electronic modules, collimation system, target  66Cu  65Cu(n, γ)66Cu  β−  5.1 min  1039.23  n.d.  2.14b  Electronic modules, accelerating structure, target, copper admixture  63Zn  64Zn(γ, n)63Zn  β+, EC  38.4 min  670, 962, 1412 511a(14)  11.9d  0.09c  Electron applicator  65Zn  66Zn(γ, n)65Zn  β−, β+  244.3 d  1116  11.1d  0.03c  Copper admixture  82Br  81Br(n, γ)82Br  β−, γ  35.34 h  554, 619, 777  n.d.  2.7b  Electronic modules, admixture to outer casing materials  99Mo  100Mo(γ,n)99Mo  β+  66.0 h  181, 739, 778  8.29d  0.81c  Stainless steel, collimation system  120Sb  121Sb(γ,n)120Sb  β+, EC  15.9 min  703, 989, 1172  9.24d  0.3 atc 15 MV    122Sb  121Sb(n,γ)122Sb  β−, β+, EC  2.7 d  564, 693, 1257  9.0d  6.01b  Electronic modules (contacts), admixture of Pb head cover  124Sb  123Sb(n,γ)124Sb  β−  60.3 d  603, 723, 1691  n.d.  4.1b  Electronic modules (contacts), admixture of Pb head cover  184Re  185Re(γ,n)184Re  β+  38.0 d  111, 792, 903  7.67d  n.d.    187W  186 W(n,γ)187W  β−, γ  23.72 h  134, 479, 686 551, 618, 744, 772(29)  n.d.  37.9c  Collimation system, target  185 mW  184W(n,γ)185 mW 186W(γ,n)185 mW  β−  1.67 min  132, 188  7.5d  1.69b 1.12 at 12 MV  Collimation system, target  181W  182W(γ,n) 181W  EC  122.2 d  58  8.07d  n.d.  Collimation system, target  196Au  197Au(γ,n)196Au 197Au(γ,n)196Au  β+, β−, EC  6.2 d  333, 356, 426  8.07d  2.37 at 16 MV  Wave-guide, Target of Primus, electronic contacts  198Au  197Au (n,γ) 198Au  β−  2.7 d  412  8.07d  99.2b  Electronic contacts, elements of Clinac 2100 target  27Mg  27Al(n, p)27Mg  β−  9.46 min  844, 1015  n.d.  0.075c  PI  41Ar  40Ar(n,γ) 41Ar  β−  1.827 h  n.d.  n.d.  0.6b  Air  13N  14N (γ,n) 13N  β+  10 min  511a  10.6d  0.09 at 23 MVc  Air, human dudy  11C  12C(γ,n)11C  β+  20.3 min  n.d.  18.72d  n.d.  air  15O  16O(γ, n)15O  EC, β+  2.04 min  511a  15.7d  0.06 at 22 MVc  Air, wall concrete  116 mIn  115In(n,γ) 116 mIn  β−  54.3 min  1097, 1293, 1508  n.d.  205.02b    139Ba  138Ba(n,γ) 139Ba  β−  83 min  165  n.d.  0.35b  Admixture of Pb head cover  131Ba  130Ba(n,γ)131Ba  β−  11.5 d  123.8, 216.1, 373.2, 496.3  n.d.  11.2b    182Ta  183W(γ,p)182Ta  β−  115 d  1121,1189,1221  7.2d  0.05c  Target, collimation system, flattening filter, electronic modules  183Ta  184W(γ,p)183Ta  β−  5.1 d  246, 354  7.7d  0.05 at 23 MVc  Target, collimation system, flattening filter  185Ta  186W(γ,p)183Ta  β−  49 min  174, 178  8.4d  0.05 at 23 MV  Target, collimation system, flattening filter  49Ca  48Ca(n,γ) 49Ca  β−  8.8 min  3084  n.d.  1.09b  Flattening filter, Wall-mounted laser, concrete  47Ca  48Ca(γ, n) 47Ca  β−  4.5 d  1297  9.9d  n.d.  Concrete  52V  53Cr(γ,p)52V  β−, γ  3.75 min  1434  11.1d  n.d.  Linac head shielding  53V  54Cr(γ,p)52V  β−  1.6 min  1006, 1289  12.4d  n.d.  Linac head shielding  206*Tl  206Pb(γ,p) 206*Tl  IT  3.8 min  687, 1022  7.5d  n.d.  Linac head shielding  38K  39K(γ, n)38K  β+  7.6 min  511a, 2168  13.1d  0.1 at 20 MVc  Wall-mounted laser  43K  44Ca(γ,p) 43K  β−  22.3 h  373, 617  12.2d  n.d.  Bunker  207 mPb  207Pb(n,n’)207 mPb  IT  0.8 s  596.702  6.7  12.1b  Collimation system, Linac head shielding  203Pb  EC, β+  β−  51.9 h  279, 401, 681  8.39d  2.9 at 13 MVc    137Cs  136Cs(n,γ)137Cs fission  β−  30.07 y  661.65  n.d.  1.3b    37S  36S(n,γ)37S  β−  5.07 min  3103  n.d.  0.15b    Activated radioisotope  Activation process  Decay mechanism  Half-life  Main gamma rays (keV)  Threshold energy (MV)  Cross-section (barns)  Main location  22Na  23Na(γ, n)22Na  EC, β+  2.6 y  1275  12.42d  0.1  Concrete, outer casing material  24Na  23Na(n,γ)24Na  IT, β−  15 h  1369, 2754  12.1d  0.53b  Concrete, outer casing material  28Al  27Al(n, γ)28Al 29Si(γ, p)28Al  β−, γ β−  2.3 min  1779  13.03d 12.3d  0.23b  Treatment coach, electron absorber, wedge tray, PI  34mCl  35Cl(γ, n)34mCl  β+, EC, IT  32 min  147, 1178, 2129  12.65d  0.09b    38Cl  37Cl(n, γ)38Cl  β−  37.24 min  1642, 2168  n.d.  100b  Outer caning material  51Cr  50Cr(n, γ)51Cr 52Cr(γ, n) 51Cr  EC  27.7 d  320  n.d.  15.9b 4.09c  Wall concrete, stainless steel, flattening filter  52Mn  54Fe(γ,np)52Mn  β+, EC  5.59  744, 936, 1434  20.91d  0.48c  Wedge, stainless steel  54Mn  55Mn(γ,n)54Mn 56Fe(γ,np)54Mn  β−, EC  312.3 d  835  10.23d 20.41d  0.56c n.d.  Wedge, stainless steel, wall, flattening filter  56Mn  55Mn(n,γ)56Mn  β−, γ  2.58 h  847, 1811, 2113  10.6d  13.4b  Wedge, stainless steel, flattening filter  57Mn  58Fe(γ,p)57Mn  β−  1.45 min  122, 692  10.0d  n.d.  Linac head, wedge, wall  53Fe  54Fe(γ, n)53Fe  β+, EC  8.51 min  377, 378, 511a(14)  13.4d  0.21c  Head, wedge, bunker  55 Fe  54Fe(n,γ)55Fe  β−  2.68 y  n.d.  n.d.  2.25b  Steel component of Ledite material  59Fe  59Co(n, p)59Fe  β−  44.5 d  1099, 1292  n.d.  44c  Stainless steel, flattening filter  57Co  59Co(γ,2n)57Co 58Ni(γ, p)57Co  β+  271.8 d  122, 136  19.3d  0.15c 0.58c  Stainless steel flattening filter Nickel parts  58Co  59Co(γ, n)58Co 60Ni(γ,np)58Co  β+, EC  70.78 d  811, 864, 1 675 511a(14)  10.45d 19.99d  0.44c 0.94c  Stainless steel flattening filter  60Co  61Ni(γ, p)60Co 59Co(n, γ)60Co  β−  5.3 y  1173, 1333  9.86d  n.d. 37.1b  Stainless steel flattening filter  57Ni  58Ni(γ, n)57Ni  β+, EC  35.6 h  127, 1378, 1920  12.22d  0.27c  Flattening filter, MLC Stainless steel  62Cu  63Cu(γ, n)62Cu 64Zn(γ,np)62Cu  β+, EC  9.74 min  876, 1173  10.85d 7.71  0.61c n.d.  Target, flattening filter, PI  64Cu  65Cu(γ, n)64Cu  β+, β−, EC  12.7 h  1346  9.9d  0.06c  Electronic modules, collimation system, target  66Cu  65Cu(n, γ)66Cu  β−  5.1 min  1039.23  n.d.  2.14b  Electronic modules, accelerating structure, target, copper admixture  63Zn  64Zn(γ, n)63Zn  β+, EC  38.4 min  670, 962, 1412 511a(14)  11.9d  0.09c  Electron applicator  65Zn  66Zn(γ, n)65Zn  β−, β+  244.3 d  1116  11.1d  0.03c  Copper admixture  82Br  81Br(n, γ)82Br  β−, γ  35.34 h  554, 619, 777  n.d.  2.7b  Electronic modules, admixture to outer casing materials  99Mo  100Mo(γ,n)99Mo  β+  66.0 h  181, 739, 778  8.29d  0.81c  Stainless steel, collimation system  120Sb  121Sb(γ,n)120Sb  β+, EC  15.9 min  703, 989, 1172  9.24d  0.3 atc 15 MV    122Sb  121Sb(n,γ)122Sb  β−, β+, EC  2.7 d  564, 693, 1257  9.0d  6.01b  Electronic modules (contacts), admixture of Pb head cover  124Sb  123Sb(n,γ)124Sb  β−  60.3 d  603, 723, 1691  n.d.  4.1b  Electronic modules (contacts), admixture of Pb head cover  184Re  185Re(γ,n)184Re  β+  38.0 d  111, 792, 903  7.67d  n.d.    187W  186 W(n,γ)187W  β−, γ  23.72 h  134, 479, 686 551, 618, 744, 772(29)  n.d.  37.9c  Collimation system, target  185 mW  184W(n,γ)185 mW 186W(γ,n)185 mW  β−  1.67 min  132, 188  7.5d  1.69b 1.12 at 12 MV  Collimation system, target  181W  182W(γ,n) 181W  EC  122.2 d  58  8.07d  n.d.  Collimation system, target  196Au  197Au(γ,n)196Au 197Au(γ,n)196Au  β+, β−, EC  6.2 d  333, 356, 426  8.07d  2.37 at 16 MV  Wave-guide, Target of Primus, electronic contacts  198Au  197Au (n,γ) 198Au  β−  2.7 d  412  8.07d  99.2b  Electronic contacts, elements of Clinac 2100 target  27Mg  27Al(n, p)27Mg  β−  9.46 min  844, 1015  n.d.  0.075c  PI  41Ar  40Ar(n,γ) 41Ar  β−  1.827 h  n.d.  n.d.  0.6b  Air  13N  14N (γ,n) 13N  β+  10 min  511a  10.6d  0.09 at 23 MVc  Air, human dudy  11C  12C(γ,n)11C  β+  20.3 min  n.d.  18.72d  n.d.  air  15O  16O(γ, n)15O  EC, β+  2.04 min  511a  15.7d  0.06 at 22 MVc  Air, wall concrete  116 mIn  115In(n,γ) 116 mIn  β−  54.3 min  1097, 1293, 1508  n.d.  205.02b    139Ba  138Ba(n,γ) 139Ba  β−  83 min  165  n.d.  0.35b  Admixture of Pb head cover  131Ba  130Ba(n,γ)131Ba  β−  11.5 d  123.8, 216.1, 373.2, 496.3  n.d.  11.2b    182Ta  183W(γ,p)182Ta  β−  115 d  1121,1189,1221  7.2d  0.05c  Target, collimation system, flattening filter, electronic modules  183Ta  184W(γ,p)183Ta  β−  5.1 d  246, 354  7.7d  0.05 at 23 MVc  Target, collimation system, flattening filter  185Ta  186W(γ,p)183Ta  β−  49 min  174, 178  8.4d  0.05 at 23 MV  Target, collimation system, flattening filter  49Ca  48Ca(n,γ) 49Ca  β−  8.8 min  3084  n.d.  1.09b  Flattening filter, Wall-mounted laser, concrete  47Ca  48Ca(γ, n) 47Ca  β−  4.5 d  1297  9.9d  n.d.  Concrete  52V  53Cr(γ,p)52V  β−, γ  3.75 min  1434  11.1d  n.d.  Linac head shielding  53V  54Cr(γ,p)52V  β−  1.6 min  1006, 1289  12.4d  n.d.  Linac head shielding  206*Tl  206Pb(γ,p) 206*Tl  IT  3.8 min  687, 1022  7.5d  n.d.  Linac head shielding  38K  39K(γ, n)38K  β+  7.6 min  511a, 2168  13.1d  0.1 at 20 MVc  Wall-mounted laser  43K  44Ca(γ,p) 43K  β−  22.3 h  373, 617  12.2d  n.d.  Bunker  207 mPb  207Pb(n,n’)207 mPb  IT  0.8 s  596.702  6.7  12.1b  Collimation system, Linac head shielding  203Pb  EC, β+  β−  51.9 h  279, 401, 681  8.39d  2.9 at 13 MVc    137Cs  136Cs(n,γ)137Cs fission  β−  30.07 y  661.65  n.d.  1.3b    37S  36S(n,γ)37S  β−  5.07 min  3103  n.d.  0.15b    n.d.: Not determined. PI: Portal imager. aAnnihilation gamma rays. bCross-sections derived from ‘The Jeff nuclear data library, report 21’(38), ‘a nuclear cross-section data, IAEA 1989’(39), and ‘Handbook on photonuclear data for applications cross-section and spectra, IAEA 2000’(40). cDerived from ‘Handbook on nuclear activation data, IAEA’(41). dDerived from’ ‘handbook on photonuclear data for applications cross-section and spectra’(42). Several formulas have proposed and recommended in the published articles to determine the neutron induced radioactivity. For estimating radioisotopes produced by neutron capture, Cho et al. used the following formula(36):   dN(r,E,t)dt=Ф(r,E,t)σ(E)NT(r,E,t)–λN(r,E,t) (3)where N(r, E, t) = number of radionuclides generated by neutron capture [nuclide/cm3], Ф(r,E,t) = neutron flux [neutrons/cm3 s], σ(E) = (n,γ) reaction cross-section [cm3], NT(r, E, t) = number of target nuclei irradiated by neutrons [nuclide/cm3], λ = transformation constant [s−1]. The answer to Eq. (3) is as follows:   Nact(t)=1−e−λtλ∬ФσNTdVdE (4)where Nact(t) is the number of radionuclides produced by neutron capture(36). As mentioned before, induced activity after electron beam operation is negligible. For instance, Keehan et al. (2015) showed that the activation for a 500 MU photon beam delivering is 250 times higher than for count rate resulted from a 5000 MU electron beam in 18 MV mode. It is worth to note that the induced activity in electron mode increased 3.8 times by using of an applicator(6). In addition, several investigations reported the possibility of induced radioactivity for therapeutic electron beams. In a study by Wang et al., they concluded that induced radioactivity for a 22 MV electron beam was considerably lower than an 18 MV photon beam in so far as activation level after an electron beam was close to background level(4, 6, 22). INDUCED ACTIVITY INSIDE THE LINAC HEAD AND TREATMENT ROOM The most significant sources of activation in treatment room are (1) the Linac head, (2) patient support arrangements, (3) building supplies and (4) air(9). Several studies have reported the features and the levels of radionuclide energies that their type differ depending upon the Linac, the situation in the room in connect with the angle of the Linac head, the collimator opening, the irradiation history and room building(1, 11, 13, 14, 22, 50). The most important source of activation is Linac the head, especially the target, flattening filter and collimators because these components contain high atomic number elements(10, 21, 22, 30, 35). Fischer et al. noted that the photonuclear products was originated mostly from the inside of the Linac head, and their radiation is effectively shielded by the collimator jaws(22, 26), while neutron capture leads to induced activity outside it; therefore, these radioisotopes might be circulated all over the treatment room(12, 26, 35). Obviously, this increases the potential radiation protection problems for RTTs working in radiation therapy centers(6, 10, 12, 23). Israngkul-Na-Ayuthaya et al. determined activity of 28Al, 24Na, 54Mn and 60Co in the vicinity of Varian 23EX for 15 MV photon beam. The equivalent gamma dose rate immediately after irradiation was 4.14 μSv/h and reduced to 0.65 μSv/h after 5 min. They observed that the target was the main source of neutron production(24). Wang et al. applied the following formula to calculate the decay conduct of 28Al, 62Cu and 187W in the isocenter of a Varian Clinac 2300 C/D operating in 18 MV mode(22, 51):   Ẋ(t)=ẊB+(Ẋmax)Al(1−exp(λAlta))exp(λAlta)+(Ẋmax)Cu(1−exp(λCuta))exp(λCuta)+(Ẋmax)W(1−exp(λWta))exp(λWta) (5)where ẊB defines the background dose rate, Ẋmax defines the saturation activity level and lambda is decay constant for each radionuclides. The time of post-irradiation determined with ‘t’ and the ‘ta’ is irradiation time. Their measurements were in good agreement with the mathematical model. After delivering the 1000 MU dose, they observed that induced dose rate for Varian Clinac 2300 and 21EX operating in 18 MV were 13.3 and 12.2 μSv/h, respectively. Furthermore, they found the maximum equivalent dose rate in the isocenter of Varian Clinac 2300 C/D, which was ~13 μSv/h. According to Wang et al.(22,) results, after delivering the 12 000 MU dose at a dose rate of 400 MU/min, the induced activity reached at a saturation level and there was no increase with increasing in photon energy, dose, field size, etc. In this regard, Polaczek-Grelik et al.(31) determined that the activation of 28Al reached at a saturation level even through short beam emission and after 15 h the activation reached to its background level. It is in agreement with other author’s result(10, 14, 52). In 2008, Fischer et al. measured gamma spectra at the isocenter of four different types of high-energy medical Linacs. They used Siemens Primus, Varian Clinac 2100 C/D, and Elekta SL-18 in 15 MV mode and also General Electric Saturne 42 F in 18 MV mode. They detected activation of 24Na, 28Al, 51Cr, 54Mn, 56Mn, 57Co, 58Co, 60Co, 57Ni, 59Fe, 62Cu, 64Cu, 65Zn, 82Br, 99Mo, 122Sb, 124Sb, 184Re, 187W, 196Au and 203Pb(7). More information about each radioisotopes is summarized in Table 1. In addition, they indicated that as the time passed after operation of irradiation, the types of dominant radionuclides were changed(3). 28Al and 62Cu are short-lived nuclides (T1/2 < 10 min) that have the highest activation in the first minutes after irradiation in medical accelerators according to several studies(3, 21, 35, 53, 54), except General Electric Linac(3). Moreover, 56Mn, 64Cu, 57Ni, 24Na and 187W with a medium half-life increased during a working day(10). It can be predictable that at the end of a working day, the activity would be or close to its maximum(3). However, the radionuclides of 51Cr, 54Mn, 57Co, 58Co, 60Co, 59Fe, 124Sb, 82Br, 99Mo, 122Sb, 196Au and 203Pb with half-lifes more than a day are accumulated weekly or even annually(3, 6, 7, 10, 14, 22, 31, 51). Generally, radionuclides that activated through (γ,n) or directly with fast neutrons through (n,2n) have a long half-life(31). Wang et al. determined 30% additional activation after 10 fractions of treatment with photon beam. It is worth noting that in every accelerator, the induced dose due to short-lived radionuclides is significantly more than other radionuclides(22). Although, as 60Co found in all machines with 10μBq/g activity(29) would be a problem during operation of accelerators. Similarly, Polaczek-Grelik et al. analyzed three different kinds of Linacs for gamma emissions from activated radionuclides. They examined Siemens Primus in 15 MV mode, Varian Clinac 2100 in 18 MV mode and Varian Clinac 2300 in 20 MV mode. They cannot detect 62Cu and 99Mo like Fischer et al., nevertheless, they reported some other radionuclides that include 38Cl, 66Cu, 80Br, 139Ba, 182Ta, 185 mW and 198Au(31). Gamma-ray energies which emitted by radionuclides were between 166 and 2754 keV as seen in Table 1(3, 14, 31). As mentioned earlier, studies showed that neutron capture does not happen in the Linac head. In this study, it was determined that 185mW (Clinac) and 196Au (Primus) were activated due to the photonuclear reaction and although 185mW is a short-lived radioisotope, 196Au contribute in dose build-up weekly. They estimated effective half-life of accelerator heads ~10 min(31). Tungsten as the largest part of a Linac head components is the main activated radioisotope which is produced when a Linac operates with energies between 10 and 20 MV(3, 4, 21, 22, 29, 31, 35). Other atoms found in head components, such as 58Fe became less activated in comparison with other components made of stainless steel, due to its small neutron capture cross-section(31). Several studies determined 28Al, 24Na and 38Cl as important radionuclides, due to their short half-life, in radiological protection point of view(3, 14, 31). Although the majority of radionuclides were the same among different accelerators, but the amount of induced activity is increased with nominal potential of beam energies(4, 22, 31). In a simulation study by Juste et al. using MCNP6 Monte-Carlo code, they evaluated the neutron activation procedure in an Elekta Linac head. They showed that 187W is the most abundant radionuclide which produced in the Linac head, also the most activation of tungsten occurred in the target(32). Similarly, in a simulation study by Isolan et al. on Varian Clinac 2100 in addition to similar observed radionuclides, the activity of 207mPb, 137Cs and 40K was reported. Additionally, they calculated the activity of 187W ~1.57 MBq which in the experimental measurement, it was determined ~0.612 MBq(55). In 2011, Petrovic et al. using in situ gamma spectroscopy measured induced activity around an Elekta Precise in 18 MV mode. They approved the presence of 28Al, 24Na, 56Mn, 54Mn, 187W, 64Cu and 62Cu radionuclides and the highest induced dose rate in the first 2 min post-irradiation was 4 μSv/h at the isocenter while, the dose rate after 10 min was negligible. The induced dose rate after ~1.6 h slightly became constant and tends to be at the background level(53). In a similar study using in situ gamma spectrometry, Ateia et al. measured activation byproducts around a Siemens Primus operating at 18 MV energy. They determined activation of radionuclides same as the last study with a difference in the activity of 52V. Nevertheless, they did not find any presence of Co, Fe, Sb and Cr radioisotopes,(21) which were previously reported in other articles(3, 29, 31). A study of Janiszewska et al. on total body irradiation (TBI) detected similar radionuclides as mentioned before and only added 179W to the activated radioisotopes. They calculated effective half-life of Varian Clinac 2100 activity ~6.76 h without considering the short-lived radionuclides. On the other hand, apparent activity of the accelerator heads ~2 h after Linac operation, estimated to be 530.8 kBq, and leads to total effective dose rate of 40.4 μSv/h. Their spectrometric measurement indicated 187W, 56Mn and 57Ni as the main source of induced activity; as a consequence, effective dose rates immediate after beam operation were 51.5 μSv/h and after 6 h fell to 7 μSv/h(20). As the same, Keehan et al. (2015) expressed similar significant radionuclides for both Varian Clinac 21EX and Elekta Synergy in 18 MV irradiation mode for TBI. They concluded that long irradiation time, as the irradiation time increases, more nuclei decay occurs during the irradiation it leads to a lower net induced activity after irradiation was terminated(6). Kalef-erza et al. performed a spectroscopy following 18 MV X-ray irradiation of Varian Clinac 2100 DHX and measured induced radioactivity in several points around Linac head and treatment room after 100 MU irradiation. At the central beam exit window, an initial dose rate of 9 μSv/h was measured which had 30% reduction in 1.8 min and most of the remaining activity reduced with an effective half-life that was ~9.5 min(14). Nevertheless, for TBI, this magnitude increased to over 13 μSv/h and then reduced by a factor of 9.7 in 10 min after beam operation(6). In the same study, at the point 15 cm above the isocenter, a dose rate of 5.2 μSv/h with effective half-life between 1.6 and 9 min was found. Overall, other authors have reported a dose rate of 10 μSv/h at the isocenter in similar situation. Furthermore, 52V, 206*Tl, 15O, 124mSb and 56Mn have been detected as a consequence of induced activation in the Linac head shielding with a dose rate of 3 μSv/h(14). As many papers reported that collimation system is the most significant source of neutron production(7, 19), thus, induced activity differs significantly due to collimation system configuration. For collimators, dose rate was measured between 2.3 and 10 μSv/h in various distances(21). It has been shown that considering the collimator jaws as a secondary neutron source, the induced dose rate increased from 2.2 to 10 μSv/h and became more non-uniform as the distance enhanced between the Linac head and the detector(21). There are several studies of the effect of the physical wedge on the produced neutrons around a medical Linac(56). For example, Mesbahi et al. indicated that neutron dose equivalent in wedged beams was 6.5 times higher than beams without wedge filters(37, 52). As it mentioned before by increasing neutron dose, neutron capture reaction and as a consequence, induced dose will increase. Thus, in a study by Kalef-erza et al., a 45° lead filter resulted in 3.4 μSv/h dose rate, and the addition of a 30° steel filter to the Linac head increased dose rate from 9 to 12 μSv/h. Activated radionuclides were 124mSb and 206*Tl with half-life ranged between 1.7 and 8.5 min(14). On the other hand, Guo et al.(50,) after irradiation of 999 MU photon beam with CGR Saturne III in 25 MV found different results, which summarized in Table 2. As it can be seen, as the wedge angle increased from 15° to 30° and from 45° to 60°, induced dose rate increased. Furthermore, the activities of different wedge filters were more and faster than a block. de León-Martínez et al. just determined activation of 56Mn in the wedge filter after operation of Varian Clinac IX in 15 MV mode with a NaI (Tl) detector(13, 56). Vassiliev et al. measured induced radioactivity as a result of Varian Clinac 21EX, with and without flattening filter. They concluded that induced activity is reduced 20–30% for unflattened beam compared to flattened beams(57, 58). Their results are in good agreement of the results of Mesbahi et al. as they reported 54% reduction for neutron dose in unflattened beams(59). Furthermore, Vassiliev et al.(57,) found activation of 28Al, 62Cu and 49Ca elements at the head of the Linac. In another study, Ateia et al.(21) expressed that they did not observe the difference between gamma spectra with and without wedge filter. Table 2. Different wedges and radionuclides byproducts after delivering 999MU(50). Wedge degree  Produced radionuclides  Dose rate 1 min post-irradiation (μSv/h)  Dose rate several minutes post-irradiation (μSv/h)  15  57Ni, 61Co, 63Fe, 56Mn,49Cr,51Cr and 52V  82  After 82 min: 0.92  30  57Ni, 61Co, 63Fe, 56Mn,49Cr,51Cr and 52V  100  After 70 min: 2.2  45  187W, 62Cu, 57Ni and 61Co  70  After 75 min: 0.8  60  187W, 62Cu, 57Ni and 61Co  82  After 82 min: 0.92  Block  203Pb, 111Sn and 111mIn  3  After 66 min: 0.45  Wedge degree  Produced radionuclides  Dose rate 1 min post-irradiation (μSv/h)  Dose rate several minutes post-irradiation (μSv/h)  15  57Ni, 61Co, 63Fe, 56Mn,49Cr,51Cr and 52V  82  After 82 min: 0.92  30  57Ni, 61Co, 63Fe, 56Mn,49Cr,51Cr and 52V  100  After 70 min: 2.2  45  187W, 62Cu, 57Ni and 61Co  70  After 75 min: 0.8  60  187W, 62Cu, 57Ni and 61Co  82  After 82 min: 0.92  Block  203Pb, 111Sn and 111mIn  3  After 66 min: 0.45  Table 2. Different wedges and radionuclides byproducts after delivering 999MU(50). Wedge degree  Produced radionuclides  Dose rate 1 min post-irradiation (μSv/h)  Dose rate several minutes post-irradiation (μSv/h)  15  57Ni, 61Co, 63Fe, 56Mn,49Cr,51Cr and 52V  82  After 82 min: 0.92  30  57Ni, 61Co, 63Fe, 56Mn,49Cr,51Cr and 52V  100  After 70 min: 2.2  45  187W, 62Cu, 57Ni and 61Co  70  After 75 min: 0.8  60  187W, 62Cu, 57Ni and 61Co  82  After 82 min: 0.92  Block  203Pb, 111Sn and 111mIn  3  After 66 min: 0.45  Wedge degree  Produced radionuclides  Dose rate 1 min post-irradiation (μSv/h)  Dose rate several minutes post-irradiation (μSv/h)  15  57Ni, 61Co, 63Fe, 56Mn,49Cr,51Cr and 52V  82  After 82 min: 0.92  30  57Ni, 61Co, 63Fe, 56Mn,49Cr,51Cr and 52V  100  After 70 min: 2.2  45  187W, 62Cu, 57Ni and 61Co  70  After 75 min: 0.8  60  187W, 62Cu, 57Ni and 61Co  82  After 82 min: 0.92  Block  203Pb, 111Sn and 111mIn  3  After 66 min: 0.45  Gurjar et al. measured radiation level due to induced activity after operation of Varian Clinac DMX in 15 MV mode at the isocenter and close to Linac head ~10 μSv/h. They reported radiation level due to induced radioactivity following various dose rate and field size. For 10 × 10 cm2 field size following 100 MU irradiation, they detected 2.4 μSv/h dose rate at the treatment table (0.5 m lateral to isocenter)(11). Similarly, Kalef-erza et al. reported 2.5 μSv/h dose rate with 40% reduction in 1 min close to treatment table(12). As well at the location of the portal imager (PI) has found 10 μSv/h dose rate, which reduced to 6.5 μSv/h without phantom and also reduced to 1.5 μSv/h without both phantom and PI. Activated radionuclides were 62Cu and 27Mg with half-life ranged between 2 and 9.5 min(14). Furthermore, for TBI, induced dose rate was 31.3 μSv/h at the treatment table(6). Moreover, Wang et al.(22) reported that dose rate at treatment coach 2 min post-irradiation was more than 4 μSv/h and 60% higher than 1 m lateral isocenter. THE EFFECT OF FIELD SIZE AND DOSE RATE ON THE INDUCED ACTIVITY Several studies have shown that the increase in photon beam energy(3, 22, 31), field size, dose rate(6, 10, 11, 14, 22, 50) and also using MLCs causes to more induced activity. Ghavami et al.(52,) observed that when the field size increased from 10 × 10 cm2 to 30 × 30 cm2 for wedged beam, the neutron fluence became 3.84–7.2 times higher. In addition, Israngkul-Na-Ayuthaya et al.(24) observed that the neutron equivalent dose increased from 3.79 mSv/Gy to 6.75 mSv/Gy by increasing the field size of the photon beam from 2 × 2 cm2 to 30 × 30 cm2; furthermore, their results were in agreement with other similar studies(22, 50, 60). As a consequence of increasing field size, a greater amount of induced radionuclides with longer half-lives contributes in induced dose rate, which measured at the isocenter(22). For the photonuclear activation yields, the activation products are increased with the field size up to a saturation level, whereas activation products from neutron capture was independent of the photon field size(30). In a study by Donadile et al.(10), it was observed an enhancement of almost a factor of 5 in induced dose rate by increasing the field size, at the same number of MU. As a matter of fact, it can be predictable that induced activity will increase with dose rate. For instance, Gurjar et al.(11) approved induced activity enhancement with field size and determined a saturation level of the field size of 30 × 30 cm2. As shown in Figure 2, when delivered MU increased from 50 to 1000 MU, and field size increased from 5 × 5 to 40 × 40 cm2, induced dose rate increased from 0.4 μSv/h to 17.1 μSv/h(11). Otherwise, Keehan et al.(6) expressed that the induced radioactivity was independent of field size. Figure 2. View largeDownload slide The induced dose rate enhancement with field size and number of MU. Every group of columns demonstrates the induced dose rate for a specific field size. For each field size, the number of MUs was depicted increasingly and 50, 100, 200, 300, 400, 500 and 1000 MU were shown (Derived from Gurjar et al.(11).) Figure 2. View largeDownload slide The induced dose rate enhancement with field size and number of MU. Every group of columns demonstrates the induced dose rate for a specific field size. For each field size, the number of MUs was depicted increasingly and 50, 100, 200, 300, 400, 500 and 1000 MU were shown (Derived from Gurjar et al.(11).) Keehan et al.(6,) observed 42% increase in count rates and as a consequence in induced activity, as the dose rate increase from 400 to 600 MU/min. Otherwise, Donadile et al.(10) determined increasing in the induced dose by a factor 6 as the number of delivered MUs increased. Another point is that the dose rate because of neutrons and induced activity are decreased with the distance from Linac head(21, 22, 24, 31). For instance, Zabihzadeh et al.(60) expressed that by increasing distance from the Linac head, neutron dose is decreased significantly, especially along the maze. Also, it was observed that induced dose rate under the Linac head was two to three times greater than values measured inside the maze(14, 21, 29, 35). INDUCED ACTIVITY AFTER 10 MV BEAM OPERATION A number of studies indicated that neutron production for a 10 MV photon beam could be negligible and as a consequence, induced dose rate for Linacs operating in 10 MV is insignificant, and also they do not consider neutron production for a 6 MV beam(4, 22, 29, 45, 61). For instance, Wang et al.(22,) for Varian Clinac 18 operating in 10 MV mode reported that induced activity was 0.32 μSv/h at the isocenter. Likewise, Fujibuchi et al. by using of a germanium semiconductor survey meter around EXL-15SP and Varian Clinac iX in 10 MV mode detected similar radionuclides as other articles and appended 181W and 116mIn. Annihilation photons were released from 57Ni, 64Cu, 65Zn, 122Sb and 196Au with the half-life ranged between 10 and 15 h, and dose rate reduced up to 10% of its initial magnitude during a week(29). Ho et al.(12) measured induced dose rate after operation of an Elekta Precise in 10 MV mode posterior–anterior, ~3 μSv/h which after 15 min post-irradiation induced dose rate was nearby the background level. INDUCED ACTIVITY FROM THE WALL OF TREATMENT ROOM After operating high-energy Linacs, whereas photoneutrons scatter throughout the treatment room simply, induced activity occurs in the concrete wall of treatment room as well(56, 62). A number of articles approved activation of the treatment bunker after 18 and 15 MV beam operation(13, 28, 56, 62) although for 10 MV beam, no induced radioactivity was found on the wall of treatment room(29). Bieniasiewiez et al. measured the thermal and resonance neutron fluence, and neutron induced radioactivity inside two types of bunkers around an Elekta accelerator in 18 MV mode. The highest fluence was detected at the isocenter, and it was measured 1.4 × 104 cm−2 MU−1 for thermal neutron and for resonance neutron it was 0.7 × 104 cm−2 MU−1 inside bunkers. Their measurements have shown that changes in the construction of bunkers do not affect considerably on neutron fluence, similarly they found no significant change in the induced radioactivity inside both bunkers(28). However, their results are opposed to some articles that reported neutron fluence was increased in bunkers made of high density concretes(63–67). de León-Martínez et al. measured induced radioactivity in concrete following irradiation of Varian Clinac IX with a NaI(Tl) detector in 15 MV mode. They observed 54Mn, 24Na and 28Al radionuclides with 0.15, 0.50 and 1.99 Bq activity, respectively(13). Their results are in agreement with other articles(3, 14, 21). Similarly, Vega-Carillo et al. measured dose rate in three types of concretes and found similar radionuclides with different specific activity in each sample. They noted that the induced activity was constant at the first 10 cm depth in the wall. In the same study, for a Fe/concrete with 17 × 17 cm2 field size and 270° gantry angle dose rate was measured 1.3 μSv/h as a consequence of 15O, 57Mn, 124mSb, 53Fe and 38K activation. Additionally, for wall-mounted laser, it was observed a dose rate of 1.6 μSv/h coming from the activity of 28Al, 27Mg, 49Ca, 53Fe, 62Cu, 38K and 44Ar radionuclides(56). Juste et al. conducted a study on the induced radioactivity in medical Linac facilities using MCNP6 Monte-Carlo code following 15 MV irradiation with Elekta Precise medical Linac. They found an activation of 51Cr and 53Fe in the wall of treatment room(62). INDUCED ACTIVITY FROM THE AIR OF TREATMENT ROOM When Linac operating in 18 MV photon mode, the air in the bunker becomes activated due to the photoactivation reactions, mainly. The air is straightly activated just in the collimated photon field(30). Depend on photonuclear threshold energies of 12C, 16O, 14N and 41Ar can be activated in energies between 15 and 18 MV(15, 25, 30, 68). Tana et al. measured air photoactivation after operating Siemens Primus in 15 MV mode and Varian Clinac 2100 in 18 MV mode with different dose rates. Activation of 15O and 11C was negligible due to their high (γ,n) threshold energy(15, 55). The minimum air activation after Siemens operation was 1.55 Bq/LMU due to nitrogen activity. The maximum activity was 11.3 Bq/L MU due to nitrogen and 0.68 Bq/LMU due to oxygen activity after Varian operation(15, 68). Furthermore, Isolan et al.(55) observed that nitrogen activity 13% reduced in 2 min. Saeed et al.(25) in 40 × 40 cm2 field size following Siemens Primus irradiation in 15 MV mode measured nitrogen activity ~836.8 Bq/Gy. Horst et al. simulated air activation around Varian Clinac operating in 18 MV using FLUKA code. Radionuclides of 15O, 13N and 41Ar were simulated and their activities were estimated. They concluded that photonuclear reactions depend on field size, however, the neutron capture reaction (41Ar) was independent of field size(30). Also Masumoto et al.(69) identified activity of 15O, 13N and 11C in 20 MV mode using Monte-Carlo simulation. Furthermore, Juste et al.(62) concluded that dose rate due to 41Ar activation is insignificant. COMPARISON OF INDUCED ACTIVITY BETWEEN DIFFERENT TYPES OF LINACS However, all Linacs have the same basic operating fundamental; they may differ significantly in structure details(3). As mentioned previously different types of Linacs and X-ray beam energies produce various kinds of radioisotopes. By increasing atomic number, cross-section of neutron capture and photonuclear reaction increases(31). Therefore, Linacs produce different radioisotopes and induced dose rate due to diversity among construction details. As reported by several researchers, it seems that different models of Varian Clinac produces the highest induced dose rate, and Siemense Primus installation produces the lowest induced dose rate compared with other types of Linacs(3, 7, 15, 21, 31, 35). Donadile et al. investigated six different installations, related to four manufactures (Varian, Siemens, Elekta and General Electric). Not only they observed presence of neutron in control room for each installation but also Elekta SL20 had the minimum neutron contribution surprisingly(10). For comparison between different types of Linacs, Fischer et al.(3) defined a quantity Aap for spectroscopy investigations as the following equation:   Aap(Δt=0)=exp(λ∇t)n/tlivefε (6)where Aap is the ‘apparent activity’ as specified above, λ is the decay constant of the isotope, ∇t is the time interval beam-off and spectrum acquisition, n is the absolute peak area, tlive is the live measurement time, f is the probability of emission of the observed line and ε is photopeak efficiency of the detector at the energy of the observed line(3). Aap represents the amount of the induced activity of a radionuclide at the time of beam-off, supposing that all radionuclides accumulated at the 1 m intervals of the detector. In this way, it is supposed that whole induced activity accumulated in the Linac target, whilst this assumption definitely would not be true. Their measurements have shown that every Linac has similar behavior in terms of induced activity. However, Siemens Primus exhibited the least induced dose rate after the beam-off, while Varian Clinac 2100 exhibited the highest induced dose rate and as a consequence the highest annual dose to an RTT(3, 45). In Table 3, the diametric characteristics of activated radionuclides including dose rate, apparent activity and annual dose rate to an RTT have been written for each installation. Also, Konefal et al. demonstrated that neutron fluence at the central axis of X-ray beam for Varian Clinac 2300 in 20 MV mode was higher than Siemens Primus in 15 MV mode by a factor ~5. Therefore, gamma energy spectra due to induced activity for Varian was about five times higher than Siemens(7). Polaczek-Grelik et al. illustrated that immediately after operations of Siemens in 15 MV mode, Varian Clinac 2100 in 18 MV mode and Varian Clinac 2300 in 20 MV mode, induced dose for two types of Varian Linacs was considerably larger than Siemens. Under the activated Linac head, their results showed that 28Al with 88% abundance and 26.9 μSv/h dose rate, 187W with 31% abundance and 46 μSv/h dose rate, and 187W with 29% abundance and 150.4 μSv/h dose rate are the main sources of activation immediately after operation of foregoing Linacs, respectively(31). On the other hand, after delivering 500 MU with 400 MU/min dose rate by Varian 21EX and Elekta Synergy in 18 MV mode, both installations had similar energy peaks and count rate but the Varian photon fluence rate at the isocenter was twice higher than the fluence which was measured for the Elekta Linac. Whereas both Linacs produced similar radionuclides, as shown in Table 3, this difference could be attributed to variety in the collimation system(6). Konefal et al. measured neutron dose and induced activity around Varian Truebeam in 18 MV mode. 56Mn, 57Ni, 57Co, 187W and 38Cl were also identified like other older Varian models and other Linacs, but 28Al, 62Cu and 64Cu were not identified. Whereas 28Al, 62Cu and 64Cu relatively emit high-energy gamma rays (Table 1), 28Al and 62Cu have approximately high activity owing to their short half-life, it can be predicted that the induced dose rate immediately after operating Varian Truebeam may be lower than other Linacs, leastwise compared with other models of Varian(4). Information about different model of Linacs and radionuclides’ yield is demonstrated in Table 4. Table 3. Activation byproducts, dose rate and apparent activity for four different installation(3). Radionuclide  Elekta (SL-18)  General Electric (Saturne 42F)  Siemense (primus)  Varian (2100 C/D)  Aap (MBq)  Dose rate (μSv/h)  Aap (MBq)  Dose rate (μSv/h)  Aap (MBq)  Dose rate (μSv/h)  Aap (MBq)  Dose rate (μSv/h)  24Na  0.052  0.022  0.060  0.026  0.008  0.0034  0.036  0.0156  28Al  1.85  0.41  4.3  0.95  2.30  0.51  9.79  2.17  51Cr  0.124  0.0005  0.40  0.0017  n.d.    n.d.    54Mn  0.073  0.008  0.13  0.015  0.024  0.0026  0.0058  0.0006  56Mn  0.63  0.145  1.25  0.29  0.131  0.030  0.69  0.160  57Co  0.049  0.0006  0.144  0.0019  0.0132  0.0002  0.012  0.0002  57Ni  0.085  0.022  0.35  0.090  0.260  0.066  0.144  0.037  58Co  0.0113  0.0015  0.037  0.0048  0.005  0.0006  0.025  0.0032  59Fe  0.007  0.001  0.019  0.0028  0.005  0.0007  0.0065  0.001  60Co  0.009  0.0028  0.040  0.0122  0.0079  0.0024  0.0097  0.003  62Cu  1.50  0.226  0.54  0.082  0.59  0.090  3.21  0.48  64Cu  3.78  0.110  n.d.    0.76  0.022  5.4  0.160  65Zn  0.006  0.0004  0.013  0.0009  0.015  0.0011  0.0127  0.0009  82Br  0.059  0.0203  0.009  0.0031  0.091  0.031  0.105  0.0360  99Mo  0.008  0.0003  n.d.    0.008  0.0003  n.d.    122Sb  0.088  0.0061  0.040  0.0028  0.0089  0.0006  0.081  0.0056  124Sb  0.0318  0.008  0.0116  0.003  0.0014  0.0004  0.024  0.0061  184Re  0.077  0.01  n.d.    n.d.    n.d.    187W  0.72  0.053  0.24  0.018  0.134  0.0098  1.09  0.080  196Au  0.044  0.003  n.d.    0.079  0.0056  0.005  0.0004  203Pb  n.d.    0.021  0.001  n.d.    n.d.      Annual dose to an RTT  1.68 mSv  Annual dose to an RTT  2.3 mSv  Annual dose to an RTT  0.62 mSv  Annual dose to an RTT  2.53 mSv  Radionuclide  Elekta (SL-18)  General Electric (Saturne 42F)  Siemense (primus)  Varian (2100 C/D)  Aap (MBq)  Dose rate (μSv/h)  Aap (MBq)  Dose rate (μSv/h)  Aap (MBq)  Dose rate (μSv/h)  Aap (MBq)  Dose rate (μSv/h)  24Na  0.052  0.022  0.060  0.026  0.008  0.0034  0.036  0.0156  28Al  1.85  0.41  4.3  0.95  2.30  0.51  9.79  2.17  51Cr  0.124  0.0005  0.40  0.0017  n.d.    n.d.    54Mn  0.073  0.008  0.13  0.015  0.024  0.0026  0.0058  0.0006  56Mn  0.63  0.145  1.25  0.29  0.131  0.030  0.69  0.160  57Co  0.049  0.0006  0.144  0.0019  0.0132  0.0002  0.012  0.0002  57Ni  0.085  0.022  0.35  0.090  0.260  0.066  0.144  0.037  58Co  0.0113  0.0015  0.037  0.0048  0.005  0.0006  0.025  0.0032  59Fe  0.007  0.001  0.019  0.0028  0.005  0.0007  0.0065  0.001  60Co  0.009  0.0028  0.040  0.0122  0.0079  0.0024  0.0097  0.003  62Cu  1.50  0.226  0.54  0.082  0.59  0.090  3.21  0.48  64Cu  3.78  0.110  n.d.    0.76  0.022  5.4  0.160  65Zn  0.006  0.0004  0.013  0.0009  0.015  0.0011  0.0127  0.0009  82Br  0.059  0.0203  0.009  0.0031  0.091  0.031  0.105  0.0360  99Mo  0.008  0.0003  n.d.    0.008  0.0003  n.d.    122Sb  0.088  0.0061  0.040  0.0028  0.0089  0.0006  0.081  0.0056  124Sb  0.0318  0.008  0.0116  0.003  0.0014  0.0004  0.024  0.0061  184Re  0.077  0.01  n.d.    n.d.    n.d.    187W  0.72  0.053  0.24  0.018  0.134  0.0098  1.09  0.080  196Au  0.044  0.003  n.d.    0.079  0.0056  0.005  0.0004  203Pb  n.d.    0.021  0.001  n.d.    n.d.      Annual dose to an RTT  1.68 mSv  Annual dose to an RTT  2.3 mSv  Annual dose to an RTT  0.62 mSv  Annual dose to an RTT  2.53 mSv  Table 3. Activation byproducts, dose rate and apparent activity for four different installation(3). Radionuclide  Elekta (SL-18)  General Electric (Saturne 42F)  Siemense (primus)  Varian (2100 C/D)  Aap (MBq)  Dose rate (μSv/h)  Aap (MBq)  Dose rate (μSv/h)  Aap (MBq)  Dose rate (μSv/h)  Aap (MBq)  Dose rate (μSv/h)  24Na  0.052  0.022  0.060  0.026  0.008  0.0034  0.036  0.0156  28Al  1.85  0.41  4.3  0.95  2.30  0.51  9.79  2.17  51Cr  0.124  0.0005  0.40  0.0017  n.d.    n.d.    54Mn  0.073  0.008  0.13  0.015  0.024  0.0026  0.0058  0.0006  56Mn  0.63  0.145  1.25  0.29  0.131  0.030  0.69  0.160  57Co  0.049  0.0006  0.144  0.0019  0.0132  0.0002  0.012  0.0002  57Ni  0.085  0.022  0.35  0.090  0.260  0.066  0.144  0.037  58Co  0.0113  0.0015  0.037  0.0048  0.005  0.0006  0.025  0.0032  59Fe  0.007  0.001  0.019  0.0028  0.005  0.0007  0.0065  0.001  60Co  0.009  0.0028  0.040  0.0122  0.0079  0.0024  0.0097  0.003  62Cu  1.50  0.226  0.54  0.082  0.59  0.090  3.21  0.48  64Cu  3.78  0.110  n.d.    0.76  0.022  5.4  0.160  65Zn  0.006  0.0004  0.013  0.0009  0.015  0.0011  0.0127  0.0009  82Br  0.059  0.0203  0.009  0.0031  0.091  0.031  0.105  0.0360  99Mo  0.008  0.0003  n.d.    0.008  0.0003  n.d.    122Sb  0.088  0.0061  0.040  0.0028  0.0089  0.0006  0.081  0.0056  124Sb  0.0318  0.008  0.0116  0.003  0.0014  0.0004  0.024  0.0061  184Re  0.077  0.01  n.d.    n.d.    n.d.    187W  0.72  0.053  0.24  0.018  0.134  0.0098  1.09  0.080  196Au  0.044  0.003  n.d.    0.079  0.0056  0.005  0.0004  203Pb  n.d.    0.021  0.001  n.d.    n.d.      Annual dose to an RTT  1.68 mSv  Annual dose to an RTT  2.3 mSv  Annual dose to an RTT  0.62 mSv  Annual dose to an RTT  2.53 mSv  Radionuclide  Elekta (SL-18)  General Electric (Saturne 42F)  Siemense (primus)  Varian (2100 C/D)  Aap (MBq)  Dose rate (μSv/h)  Aap (MBq)  Dose rate (μSv/h)  Aap (MBq)  Dose rate (μSv/h)  Aap (MBq)  Dose rate (μSv/h)  24Na  0.052  0.022  0.060  0.026  0.008  0.0034  0.036  0.0156  28Al  1.85  0.41  4.3  0.95  2.30  0.51  9.79  2.17  51Cr  0.124  0.0005  0.40  0.0017  n.d.    n.d.    54Mn  0.073  0.008  0.13  0.015  0.024  0.0026  0.0058  0.0006  56Mn  0.63  0.145  1.25  0.29  0.131  0.030  0.69  0.160  57Co  0.049  0.0006  0.144  0.0019  0.0132  0.0002  0.012  0.0002  57Ni  0.085  0.022  0.35  0.090  0.260  0.066  0.144  0.037  58Co  0.0113  0.0015  0.037  0.0048  0.005  0.0006  0.025  0.0032  59Fe  0.007  0.001  0.019  0.0028  0.005  0.0007  0.0065  0.001  60Co  0.009  0.0028  0.040  0.0122  0.0079  0.0024  0.0097  0.003  62Cu  1.50  0.226  0.54  0.082  0.59  0.090  3.21  0.48  64Cu  3.78  0.110  n.d.    0.76  0.022  5.4  0.160  65Zn  0.006  0.0004  0.013  0.0009  0.015  0.0011  0.0127  0.0009  82Br  0.059  0.0203  0.009  0.0031  0.091  0.031  0.105  0.0360  99Mo  0.008  0.0003  n.d.    0.008  0.0003  n.d.    122Sb  0.088  0.0061  0.040  0.0028  0.0089  0.0006  0.081  0.0056  124Sb  0.0318  0.008  0.0116  0.003  0.0014  0.0004  0.024  0.0061  184Re  0.077  0.01  n.d.    n.d.    n.d.    187W  0.72  0.053  0.24  0.018  0.134  0.0098  1.09  0.080  196Au  0.044  0.003  n.d.    0.079  0.0056  0.005  0.0004  203Pb  n.d.    0.021  0.001  n.d.    n.d.      Annual dose to an RTT  1.68 mSv  Annual dose to an RTT  2.3 mSv  Annual dose to an RTT  0.62 mSv  Annual dose to an RTT  2.53 mSv  Table 4. Identified radionuclides for each Linac type. Linac type  Studies  Identified radionuclides  Varian True beam  Konefal et al.(4)  122Sb, 124Sb,131Ba, 82Br, 57Ni, 51Cr,187W, 38Cl, 24Na  Varian Clinac 2100  Wang et al.(22,), Fischer et al.(3), Fischer et al.(26,), Polakzec-Grelik et al.(31,), Janiszewska et al.(20,) and Isolan et al.(55)  54Mn, 56Mn, 57Ni,58Co, 60Co, 179W, 187W, 185mW, 198Au, 196Au, 24Na, 59Fe, 64Cu, 66Cu, 82Br, 124Sb, 122Sb, 182Ta, 51Cr, 203Pb, 65Zn, 28Al, 38Cl, and 139Ba, 131Ba, 207mPb, 37S, and 49Ca  Varian Clinac 2100 DHX  Kalef-Erza et al.(14)  13N, 15O, 24Na, 28Al, 38K,43K, 47Ca, 52V, 53V, 53Fe, 54Mn, 56Mn, 57Mn, 57Ni, 58Co, 62Cu, 64Cu, 63Zn, 65Zn, 122Sb, 182Ta, 183Ta, 185Ta, and 206*Tl  Varian Clinac 2300  Konafal et al. (2007), Konafal et al. (2008), Polakzec-Grelik et al.(31,) and Wang et al.(22)  187W, 62Cu, 28Al, 56Mn, 57Ni, 38Cl, 57Co, and 196Au  Varian Clinac 21EX  Keehan et al. (2015), Wang et al.(22,), Vassiliev et al.(57,) and Rawlinson et al.(51)  187W, 63Zn, 56Mn, 24Na, 62Cu, 28Al, 122Sb, and 49Ca  Varian Clinac 23EX  Israngkul-Na-Ayuthaya et al.(24)  28Al, 24Na, 54Mn and 60Co  Varian Clinac iX  Fujibuchi et al. (2011) and de León-Martínez et al.(13)  116mIn, 64Cu, 24Na, 28Al, 187W, 181W, 198Au, 196Au, 56Mn, 54Mn, 82Br, 122Sb, and 124Sb  CGR Saturne III  Guo et al.(50)  57Ni, 61Co, 63Fe, 56Mn,49Cr,51Cr, 52V, 187W, 62Cu, 203Pb, 111Sn, and 111 mIn  Elekta Precise  Petrovic et al.(53,), Perrin et al.(72) and Juste et al. (2016)  28Al, 187W, 62Cu, 64Cu, 56Mn, 54Mn, 24Na, 65Ni, 121Sb, 55Fe, 59Fe, 45Ca, and 51Cr  Elekta synergy  Keehan et al. (2015) and Bieniasiewicz et al.(28)  187W, 63Zn, 56Mn, 24Na, 82Br and 28Al  Siemens Primus  Konafal et al. (2007), Konafal et al.(7,), Polakzec-Grelik et al.(31,), Fischer et al.(3,) and Ateia et al.(21)  187W, 56Mn, 54Mn, 24Na, 28Al, 57Ni, 38Cl, 57Co, 62Cu, 64Cu, 82Br, 52V, and 196Au  EXL-15SP  Fujibuchi et al. (2011)  187W, 181W, 198Au, 196Au, 56Mn, 82Br, 122Sb, 124Sb, 54Mn, 57Ni,58Co, 60Co, and 65Zn  Linac type  Studies  Identified radionuclides  Varian True beam  Konefal et al.(4)  122Sb, 124Sb,131Ba, 82Br, 57Ni, 51Cr,187W, 38Cl, 24Na  Varian Clinac 2100  Wang et al.(22,), Fischer et al.(3), Fischer et al.(26,), Polakzec-Grelik et al.(31,), Janiszewska et al.(20,) and Isolan et al.(55)  54Mn, 56Mn, 57Ni,58Co, 60Co, 179W, 187W, 185mW, 198Au, 196Au, 24Na, 59Fe, 64Cu, 66Cu, 82Br, 124Sb, 122Sb, 182Ta, 51Cr, 203Pb, 65Zn, 28Al, 38Cl, and 139Ba, 131Ba, 207mPb, 37S, and 49Ca  Varian Clinac 2100 DHX  Kalef-Erza et al.(14)  13N, 15O, 24Na, 28Al, 38K,43K, 47Ca, 52V, 53V, 53Fe, 54Mn, 56Mn, 57Mn, 57Ni, 58Co, 62Cu, 64Cu, 63Zn, 65Zn, 122Sb, 182Ta, 183Ta, 185Ta, and 206*Tl  Varian Clinac 2300  Konafal et al. (2007), Konafal et al. (2008), Polakzec-Grelik et al.(31,) and Wang et al.(22)  187W, 62Cu, 28Al, 56Mn, 57Ni, 38Cl, 57Co, and 196Au  Varian Clinac 21EX  Keehan et al. (2015), Wang et al.(22,), Vassiliev et al.(57,) and Rawlinson et al.(51)  187W, 63Zn, 56Mn, 24Na, 62Cu, 28Al, 122Sb, and 49Ca  Varian Clinac 23EX  Israngkul-Na-Ayuthaya et al.(24)  28Al, 24Na, 54Mn and 60Co  Varian Clinac iX  Fujibuchi et al. (2011) and de León-Martínez et al.(13)  116mIn, 64Cu, 24Na, 28Al, 187W, 181W, 198Au, 196Au, 56Mn, 54Mn, 82Br, 122Sb, and 124Sb  CGR Saturne III  Guo et al.(50)  57Ni, 61Co, 63Fe, 56Mn,49Cr,51Cr, 52V, 187W, 62Cu, 203Pb, 111Sn, and 111 mIn  Elekta Precise  Petrovic et al.(53,), Perrin et al.(72) and Juste et al. (2016)  28Al, 187W, 62Cu, 64Cu, 56Mn, 54Mn, 24Na, 65Ni, 121Sb, 55Fe, 59Fe, 45Ca, and 51Cr  Elekta synergy  Keehan et al. (2015) and Bieniasiewicz et al.(28)  187W, 63Zn, 56Mn, 24Na, 82Br and 28Al  Siemens Primus  Konafal et al. (2007), Konafal et al.(7,), Polakzec-Grelik et al.(31,), Fischer et al.(3,) and Ateia et al.(21)  187W, 56Mn, 54Mn, 24Na, 28Al, 57Ni, 38Cl, 57Co, 62Cu, 64Cu, 82Br, 52V, and 196Au  EXL-15SP  Fujibuchi et al. (2011)  187W, 181W, 198Au, 196Au, 56Mn, 82Br, 122Sb, 124Sb, 54Mn, 57Ni,58Co, 60Co, and 65Zn  Table 4. Identified radionuclides for each Linac type. Linac type  Studies  Identified radionuclides  Varian True beam  Konefal et al.(4)  122Sb, 124Sb,131Ba, 82Br, 57Ni, 51Cr,187W, 38Cl, 24Na  Varian Clinac 2100  Wang et al.(22,), Fischer et al.(3), Fischer et al.(26,), Polakzec-Grelik et al.(31,), Janiszewska et al.(20,) and Isolan et al.(55)  54Mn, 56Mn, 57Ni,58Co, 60Co, 179W, 187W, 185mW, 198Au, 196Au, 24Na, 59Fe, 64Cu, 66Cu, 82Br, 124Sb, 122Sb, 182Ta, 51Cr, 203Pb, 65Zn, 28Al, 38Cl, and 139Ba, 131Ba, 207mPb, 37S, and 49Ca  Varian Clinac 2100 DHX  Kalef-Erza et al.(14)  13N, 15O, 24Na, 28Al, 38K,43K, 47Ca, 52V, 53V, 53Fe, 54Mn, 56Mn, 57Mn, 57Ni, 58Co, 62Cu, 64Cu, 63Zn, 65Zn, 122Sb, 182Ta, 183Ta, 185Ta, and 206*Tl  Varian Clinac 2300  Konafal et al. (2007), Konafal et al. (2008), Polakzec-Grelik et al.(31,) and Wang et al.(22)  187W, 62Cu, 28Al, 56Mn, 57Ni, 38Cl, 57Co, and 196Au  Varian Clinac 21EX  Keehan et al. (2015), Wang et al.(22,), Vassiliev et al.(57,) and Rawlinson et al.(51)  187W, 63Zn, 56Mn, 24Na, 62Cu, 28Al, 122Sb, and 49Ca  Varian Clinac 23EX  Israngkul-Na-Ayuthaya et al.(24)  28Al, 24Na, 54Mn and 60Co  Varian Clinac iX  Fujibuchi et al. (2011) and de León-Martínez et al.(13)  116mIn, 64Cu, 24Na, 28Al, 187W, 181W, 198Au, 196Au, 56Mn, 54Mn, 82Br, 122Sb, and 124Sb  CGR Saturne III  Guo et al.(50)  57Ni, 61Co, 63Fe, 56Mn,49Cr,51Cr, 52V, 187W, 62Cu, 203Pb, 111Sn, and 111 mIn  Elekta Precise  Petrovic et al.(53,), Perrin et al.(72) and Juste et al. (2016)  28Al, 187W, 62Cu, 64Cu, 56Mn, 54Mn, 24Na, 65Ni, 121Sb, 55Fe, 59Fe, 45Ca, and 51Cr  Elekta synergy  Keehan et al. (2015) and Bieniasiewicz et al.(28)  187W, 63Zn, 56Mn, 24Na, 82Br and 28Al  Siemens Primus  Konafal et al. (2007), Konafal et al.(7,), Polakzec-Grelik et al.(31,), Fischer et al.(3,) and Ateia et al.(21)  187W, 56Mn, 54Mn, 24Na, 28Al, 57Ni, 38Cl, 57Co, 62Cu, 64Cu, 82Br, 52V, and 196Au  EXL-15SP  Fujibuchi et al. (2011)  187W, 181W, 198Au, 196Au, 56Mn, 82Br, 122Sb, 124Sb, 54Mn, 57Ni,58Co, 60Co, and 65Zn  Linac type  Studies  Identified radionuclides  Varian True beam  Konefal et al.(4)  122Sb, 124Sb,131Ba, 82Br, 57Ni, 51Cr,187W, 38Cl, 24Na  Varian Clinac 2100  Wang et al.(22,), Fischer et al.(3), Fischer et al.(26,), Polakzec-Grelik et al.(31,), Janiszewska et al.(20,) and Isolan et al.(55)  54Mn, 56Mn, 57Ni,58Co, 60Co, 179W, 187W, 185mW, 198Au, 196Au, 24Na, 59Fe, 64Cu, 66Cu, 82Br, 124Sb, 122Sb, 182Ta, 51Cr, 203Pb, 65Zn, 28Al, 38Cl, and 139Ba, 131Ba, 207mPb, 37S, and 49Ca  Varian Clinac 2100 DHX  Kalef-Erza et al.(14)  13N, 15O, 24Na, 28Al, 38K,43K, 47Ca, 52V, 53V, 53Fe, 54Mn, 56Mn, 57Mn, 57Ni, 58Co, 62Cu, 64Cu, 63Zn, 65Zn, 122Sb, 182Ta, 183Ta, 185Ta, and 206*Tl  Varian Clinac 2300  Konafal et al. (2007), Konafal et al. (2008), Polakzec-Grelik et al.(31,) and Wang et al.(22)  187W, 62Cu, 28Al, 56Mn, 57Ni, 38Cl, 57Co, and 196Au  Varian Clinac 21EX  Keehan et al. (2015), Wang et al.(22,), Vassiliev et al.(57,) and Rawlinson et al.(51)  187W, 63Zn, 56Mn, 24Na, 62Cu, 28Al, 122Sb, and 49Ca  Varian Clinac 23EX  Israngkul-Na-Ayuthaya et al.(24)  28Al, 24Na, 54Mn and 60Co  Varian Clinac iX  Fujibuchi et al. (2011) and de León-Martínez et al.(13)  116mIn, 64Cu, 24Na, 28Al, 187W, 181W, 198Au, 196Au, 56Mn, 54Mn, 82Br, 122Sb, and 124Sb  CGR Saturne III  Guo et al.(50)  57Ni, 61Co, 63Fe, 56Mn,49Cr,51Cr, 52V, 187W, 62Cu, 203Pb, 111Sn, and 111 mIn  Elekta Precise  Petrovic et al.(53,), Perrin et al.(72) and Juste et al. (2016)  28Al, 187W, 62Cu, 64Cu, 56Mn, 54Mn, 24Na, 65Ni, 121Sb, 55Fe, 59Fe, 45Ca, and 51Cr  Elekta synergy  Keehan et al. (2015) and Bieniasiewicz et al.(28)  187W, 63Zn, 56Mn, 24Na, 82Br and 28Al  Siemens Primus  Konafal et al. (2007), Konafal et al.(7,), Polakzec-Grelik et al.(31,), Fischer et al.(3,) and Ateia et al.(21)  187W, 56Mn, 54Mn, 24Na, 28Al, 57Ni, 38Cl, 57Co, 62Cu, 64Cu, 82Br, 52V, and 196Au  EXL-15SP  Fujibuchi et al. (2011)  187W, 181W, 198Au, 196Au, 56Mn, 82Br, 122Sb, 124Sb, 54Mn, 57Ni,58Co, 60Co, and 65Zn  INDUCED DOSE RECEIVED BY RTTS According to the linear dose–response model of radiation, even small dose of radiation increases the risk of cancer(12). As demonstrated in an RTT stands beside the treatment table and close to the Linac head for patients set up; this short distance increases the exposure from induced activity(14, 21, 29, 35). Moreover, as RTTs are exposed to persistent low level of radionuclides, more than background level, studies have shown greater possibility for developing cancers(70). Several studies determined the maximum induced dose rate after operation of high-energy Linacs in the first 2 min post-irradiation(3, 6, 7, 12, 26, 53). Thus, with regard to maintain the ‘As Low As Reasonably Achievable’ (ALARA) principle, RTTs should wait for at least 2 min before entrance to the treatment room. The ICRP reported that 10% of occupational annual dose is due to induced activity conservatively(71), but a number of authors estimated 50–54% occupational dose due to induced activity(10, 53). It has been reported by a number of studies that the exposure of an RTT from induced activity is not negligible and depends on many factors ranged between 0.5 and 5 mSv/y(3, 26, 51, 53, 72). A number of studies determined the presence of neutron in control room during Linac operation(10, 30, 35, 73). Konefal et al. (2007) observed neutron radiation in the operator room and close to the door of treatment room, during 20 MV and 22 MV X-ray beam emission of Varian Clinac 2300. However, the strongest peak close to the door was ~30 times more than in operator consol. Also Horst et al. measured 0.1–0.0001 μSv/Gy in operator console room(30). However, Ho et al. (2012) did not observe the presence of neutron in operator room in 18 MV X-ray beam of Elekta Precise(12). It is worth noting that neutrons exist only when the beam is on, because the half-life of a neutron in air is about microseconds(12, 35). Horst et al. performed mathematical methods as Equations (7) and (8) for inhalation effective dose and external exposure effective dose, respectively(30)  Ėin=CinAn∇V∇V∇t (7)where An/ΔV is the air activation, ΔV/Δt is the breathing rate and Cinis the inhalation conversion coefficient of the specific nuclide, n  Ėsn=CsnAn∇V (8)where An/∇V is the air activation and Csn is the submersion conversion coefficient of the nuclide, n. They estimated inhalation effective dose from air activation 31.3 μSv/y totally. As well Ateia et al.(21) used following equation to calculate realistic dose assessment for an RTT standing 10 min per patient next to treatment table:   E=NdNPf∫t0t1Hdṫ (9)where Nd is the number of working days per year, Np is the number of patients treated per day, f is the fraction of high-energy fields and Ḣ is the measured dose rate, while parameters t0 and t1 define the time interval which the radiotherapy staff spends in the room between two treatments. With 20% treatment of 18 MV and 250 working day, whole-body annual dose and RTT’s hand annual dose was estimated 3.5 and 5 mSv, respectively(53). Ho et al. measured dose rate of induced radioactivity for three treatment approaches. They found maximum dose rate of 7 μSv/h for 18 MV photons, and for the mixed-energy (15 and18 MV) 3DCRT method, dose rate of ~5 μSv/h was estimated. The majority of dose rates due to induced radioactivity were in 2 min post-irradiation, and the minority was in 14.5 min following; also this time approved by Ateia et al.(21). So, Ho et al. suggested optimum entry times between 7 and 11 min depend on treatment method. They calculated occupational annual dose for three approaches and concluded that 15 min delay in the entrance time cause to 0.1 mSv/y and for entrance immediately maximum received dose is 4.9 mSv/h(12). Another researches estimated annual dose between 0.7 and 5 mSv/y(10, 54, 72). Similarly Najafi et al. for Siemens Primus in 15 MV mode estimated annual dose for different treatment approaches. AP-PA and 3fied had maximum induced dose rate. For various entering time annual dose estimated between 0.3 and 5.9 mSv(23). Isragkul et al. proposed at least as much as an Al half-life (more than 2 min) wait before enter to treatment room. With a 5 min delay entrance time annual dose rate reduced to 0.65 μSv/h and after the 1 h, dose rate was <0.001 μSv/h(24). Rawlinson et al.(51,) estimated occupational dose for intensity modulate radiation therapy (IMRT) 17 mSv/y and for conventional treatment 3 mSv/y. However, for 24 TBI patients, annually RTTs received 750 μSv that reduced up to 65 μSv by closing collimator jaws before entering(6). In addition, Petrovic et al.(53) reported 30% reduction in dose rate as a result of closing jaws before entrance. Furthermore, Donadile et al. observed a reduction by a factor 10 after closing collimator jaws, compared to complete opening(10, 21, 26). Wang et al. reported that residual activity was constant during the weekend and increase from Saturday to Monday and in the morning, dose rate was at minimum level and stable. Moreover, 65% of residual activity was due to 187W activity(22). Kalef-erza et al.(14) reported reduction in hand dose by factor 4 as a result of using of 1 cm PMMA between wedge and staff hand. OCCUPATIONAL RADIATION EXPOSURE REDUCTION STRATEGIES Use beams with energy ≤10 MV if feasible. As mentioned before the increase in beam energy result in more photonuclear, and neutron capture reactions(4, 22, 27, 29, 61). Avoid or decrease using of supplies with high photon and neutron activation cross-sections in Linac construction where possible(9). Schedule high-energy treatments in the second part of a working day(3). Consider delay entrance to treatment room 2–5 min at least(12, 23). to minimize the dose of RTT’s hand, reduce the use of physical wedges, compensators and blocks(13, 14, 50, 57). Following high-energy beam treatment, before entering move dynamic wedges into the beam axis(9). Reduce the use of MLCs where possible or Close the collimator jaws before entering into the treatment room, as it leads to more than 30% reduction of induced dose rate(3, 6, 10, 21, 53). Using 1 cm of PMMA shield between wedge and RTTs’ hands(14, 50). Register pregnant staff to lower-energy photon beams if possible(9). Move patients with gantry angle 0°(14). Never expose portal imager direct to 18 MV beam(22). Remove all unnecessary stuff from treatment room during high-energy beam emission. CONCLUSION After operating high-energy photon beam, Linac head and other components in the vicinity of treatment room become activated due to photonuclear, neutron capture and electronuclear reactions. However, induced radioactivity after 10 MV photon beam operation and electron beam operation is negligible. The dose rate from activation products is increased by the number of MU delivered, field size, photon energy and application of beam modifying accessories. Half-life of activation products is ranged between 1 min up to more than a year; classified into three groups: short-lived, medium-lived, and long-lived radionuclides. Short-lived radionuclides mostly contribute to occupational radiation exposure, and others accumulate weekly and yearly. The most significant short-lived radionuclides are 28Al and 62Cu that have the highest activation in the first minutes after Linac operation. In addition, target and MLC are the most important sources of induced radioactivity in the Linac head. Finally, closing jaws before RTTs entrance to the treatment room will lead to significant reduction in occupational radiation exposure. Annual dose to an RTT depends on working situation, number of high-energy treatment and entrance time to treatment room ranged between 0.1 and 5 mSv. RTTs should wait at least 2 min after irradiation, while optimum entry time depends on photon beam energy and is ranged between 7 and 11 min post-irradiation. 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TI - A REVIEW ON THE RADIATION THERAPY TECHNOLOGIST RECEIVED DOSE FROM INDUCED ACTIVATION IN HIGH-ENERGY MEDICAL LINEAR ACCELERATORS
JF - Radiation Protection Dosimetry
DO - 10.1093/rpd/ncx292
DA - 2017-12-22
UR - https://www.deepdyve.com/lp/oxford-university-press/a-review-on-the-radiation-therapy-technologist-received-dose-from-DM0lDBsFYH
SP - 1
VL - Advance Article
IS - 
DP - DeepDyve
ER -