MEASUREMENTS OF THE PARASITIC NEUTRON DOSE AT ORGANS FROM MEDICAL LINACS AT DIFFERENT ENERGIES BY USING BUBBLE DETECTORS

MEASUREMENTS OF THE PARASITIC NEUTRON DOSE AT ORGANS FROM MEDICAL LINACS AT DIFFERENT ENERGIES BY... Abstract Conventional linear accelerators (LINACs) for radiotherapy produce fast secondary neutrons due to photonuclear processes. The neutron presence is considered as an extra undesired dose during the radiotherapy treatment, which could cause secondary radio-induced tumors and malfunctions to cardiological implantable devices. It is thus important to measure the neutron dose contribution to patients during radiotherapy, not only at high-energy LINACs, but also at lower energies, near the giant dipole resonance reaction threshold. In this work, the full body neutron dose equivalent has been measured during single-field radiotherapy sessions carried out at different LINAC energies (15, 10 and 6 MV) by using a tissue equivalent (for neutrons) anthropomorphic phantom together with bubble dosemeters. Results have shown that some neutron photoproduction is still present also at lower energies. As a consequence, emitted photoneutrons cannot be ignored and represent a risk contribution for patients undergoing radiotherapy. Medical linear accelerators (LINACs) for radiotherapy produce fast secondary neutrons due to photonuclear (γ, n) reactions. Neutron production is governed by the properties of the giant dipole resonance (GDR) reaction and neutrons are generated when the incident photon energy exceeds the GDR reaction threshold (6–20 MeV), with a mean energy of ~1 MeV and an isotropic angular distribution(1). Most of the neutrons are produced by the interaction of the photon beam on high-Z elements constituting the LINAC head (target, scattering foils, collimators, jaws, etc.), in addition to being produced in the bunker walls, floor and ceiling or directly inside the human body. A patient undergoing a radiotherapy treatment is thus affected by an extra unaccounted neutron dose that could cause secondary radio-induced tumors(2) and malfunctions to cardiological implantable devices(3). Several studies about the photoneutron dose measurements, with different dosimetric system such as bubble detectors, polycarbonate film, activation detectors and thermoluminescance dosemeters, are present in literature(4–6). However, there is sparse information about photoneutrons provided in lower energy LINACs, at 10 and 6 MV, near the GDR reaction threshold. This article describes experimental measurements of the full body parasitic neutron dose equivalent at critical organs, during single-field conformal radiotherapy sessions carried out with a LINAC operating at 15, 10 and 6 MV. The aim is to compare the results in terms of, in-field and out-of-field, neutron photoproduction between different energies with the same irradiation conditions. MATERIALS AND METHODS The LINAC photoneutron production has been investigated during single-field radiotherapy sessions, measuring the neutron dose equivalent in depth in tissue, by using a tissue equivalent (for neutrons) anthropomorphic phantom together with bubble dosemeters. Measurements were performed by using the Elekta Synergy Agility LINAC in photon mode not only at 15 MV, but also at LINAC energies near the GDR reaction threshold, at 10 and 6 MV. The medical LINAC calibration output was carried out by following the International Code of Practice for Dosimetry IAEA TRS398. The relationship of the dose to the number of monitoring units (MU) is 1 cGy = 1 MU at build-up in a water tank, measured with cylindrical chamber (PTW Farmer), with the SSD (source-to-surface distance) equal to 100 cm and the field size of 10 × 10 cm2 at isocenter. In this work, a simplified irradiation geometry was chosen by pointing only one open direct field at the pelvis area of the phantom, with the intent not to reproduce a real clinical treatment but rather to compare the neutron photoproduction at different LINAC energies. The experimental detection system The anthropomorphic phantom, specifically developed for neutron dosimetry, was designed and built by INFN (National Institute of Nuclear Physics) in Trieste following the International Commission on Radiological Protection indications for neutron dosimetric phantoms (ICRP 103)(7), both about organ positions and tissue substitutes (ICRU 44)(8). A new phantom was developed because standard phantoms usually employed in radiotherapy (like RANDO) do not meet all the work needs: to provide suitably large holes to locate bubble dosemeters and to be more tissue equivalent for neutrons than for photons. In fact for neutrons the main interaction with matter is the elastic scattering on light elements (specifically on hydrogen nuclei, that contribute to the absorbed dose up to 97%(8)) constituting the human body. Looking at the mean human upper part body composition, hydrogen is present for 0.10 mass fraction and polyethylene [(C2H4)n], 0.14 hydrogen mass fraction, is suitable as material for neutron dosimetry. The anthropomorphic phantom is thus made in polyethylene and it approximates dimensions, weight, density and hydrogen content of an average man(9). Inside the phantom several holes, suitable to allocate bubble dosemeters, have been made in correspondence with critical organs in order to measure the neutron dose equivalent in depth in tissue. The phantom external dimensions are: head (22 × 15 × 19 cm3); neck (7 × 9 × 7 cm3); chest (40 × 40 × 15 cm3); abdomen (25 × 40 × 15 cm3); thighs (40 × 25 × 15 cm3); calves (40 × 25 × 12 cm3); and feet 2 × (20 × 10 × 5 cm3). The bubble dosemeters employed are integral passive bubble detectors from Bubble Technology Industries (BTI)(10), sensitive both to fast neutrons (BD-PND) and to thermal neutrons (BDT). These detectors are calibrated by the manufacturer with an Am–Be source, having a fluence-weighted average energy equal to 4.15 MeV and a source strength of 1.13 × 107 n/s(11); the detection sensitivity is then indicated on the label of every bubble dosemeter and a calibration certificate is given together with each detector. The BD-PND detectors provide a good estimate of the dose equivalent for neutrons with energies >100 keV, while BDTs up to 0.4 eV. BTI guarantees optimum dosemeter performance for 3 months after receipt, in which it can be repressurized after use and stored inside the accompanying airtight aluminum storage tube. As stated by the BTI manufactures and shown in several studies(12), the type of the neutron source used in calibration has only a small effect on the sensitivity value; in particular, in this work differences between detectors calibration and the neutron spectrum provided by the LINAC are within the 8%. The BTI detectors have of the order of 10% accuracy, an operational temperature range of 20–37°C, they are not sensitive to gamma radiation and they have an isotropic angular response. They are thus suitable to be employed in mixed radiation fields as in radiotherapy. The experimental method Two different kinds of measurements have been performed: The neutron dose equivalent at the patient surface plane, to estimate the total amount of photoneutrons in the treatment area. The neutron dose equivalent at organs, to measure the full body photoneutron contribution to the patient. Measurements were carried out with the LINAC gantry at 0°, with an open direct photon beam of 10 × 10 cm2 at isocenter. In addition, the patient plane measurements were carried out placing two dosemeters (one for fast and one for thermal neutrons) on the surface of a cubic polyethylene phantom (30 × 30 × 30 cm3) at isocenter, with the SSD equal to 100 cm. While, the neutron dose equivalent measurements at organs were performed by using the anthropomorphic phantom, with the photon beam centered on the pelvis region, having the SSD equal to 92.5 cm and the isocenter at 7.5 cm depth, in correspondence with the prostate position. Eight critical organs were taken into account and two dosemeters (one for fast neutrons and one for thermal neutrons) were placed in each organ position, for a total of 16 detectors employed. In Figure 1 the experimental setup for the second kind of measurements is represented. Figure 1. View largeDownload slide The anthropomorphic phantom undergoing the single-field radiotherapy session. The photon beam is centered on the pelvis region and the isocenter is at 7.5 cm depth, in correspondence with the prostate. The irradiation is done with the LINAC gantry at 0°, with a direct open field of 10 × 10 cm2 at isocenter and with the SSD equal to 92.5 cm. Figure 1. View largeDownload slide The anthropomorphic phantom undergoing the single-field radiotherapy session. The photon beam is centered on the pelvis region and the isocenter is at 7.5 cm depth, in correspondence with the prostate. The irradiation is done with the LINAC gantry at 0°, with a direct open field of 10 × 10 cm2 at isocenter and with the SSD equal to 92.5 cm. RESULTS AND DISCUSSION All results in this article are reported in mSv normalized to 100 MU provided by the LINAC, and typical statistical errors are ~5–10% due to bubble counting in the range of 300–100 bubbles per dosemeter, respectively. The patient surface plane neutron dose equivalent Results of the BTI measurements on the Elekta Synergy Agility LINAC are shown in Figure 2 reporting, in logarithmic scale, both the fast and the thermal neutron dose equivalent at the patient surface plane for nominal x-ray energies of 15, 10 and 6 MV. The measured values are summarized in Table 1. Table 1. Patient surface plane neutron dose equivalent per 100 MU for the LINAC operating at 15, 10 and 6 MV. Irradiation setup conditions: field size of 10 × 10 cm2 at isocenter and SSD of 100 cm. Measurements performed by using BTI detectors.   Fast neutron dose (mSv/100 MU)  Thermal neutron dose (mSv/100 MU)  15 MV  1.98 ± 0.20  0.24 ± 0.02  10 MV  0.91 ± 0.09  0.09 ± 0.01  6 MV  0.07 ± 0.01  0.006 ± 0.001    Fast neutron dose (mSv/100 MU)  Thermal neutron dose (mSv/100 MU)  15 MV  1.98 ± 0.20  0.24 ± 0.02  10 MV  0.91 ± 0.09  0.09 ± 0.01  6 MV  0.07 ± 0.01  0.006 ± 0.001  Table 1. Patient surface plane neutron dose equivalent per 100 MU for the LINAC operating at 15, 10 and 6 MV. Irradiation setup conditions: field size of 10 × 10 cm2 at isocenter and SSD of 100 cm. Measurements performed by using BTI detectors.   Fast neutron dose (mSv/100 MU)  Thermal neutron dose (mSv/100 MU)  15 MV  1.98 ± 0.20  0.24 ± 0.02  10 MV  0.91 ± 0.09  0.09 ± 0.01  6 MV  0.07 ± 0.01  0.006 ± 0.001    Fast neutron dose (mSv/100 MU)  Thermal neutron dose (mSv/100 MU)  15 MV  1.98 ± 0.20  0.24 ± 0.02  10 MV  0.91 ± 0.09  0.09 ± 0.01  6 MV  0.07 ± 0.01  0.006 ± 0.001  Figure 2. View largeDownload slide Fast and thermal neutron dose equivalent at the patient surface plane for a 6, 10 and 15 MV LINAC operating in photon mode. Logarithmic scale. Irradiation setup conditions: photon field size of 10 × 10 cm2 and SSD of 100 cm. BTI measurements. Figure 2. View largeDownload slide Fast and thermal neutron dose equivalent at the patient surface plane for a 6, 10 and 15 MV LINAC operating in photon mode. Logarithmic scale. Irradiation setup conditions: photon field size of 10 × 10 cm2 and SSD of 100 cm. BTI measurements. From the obtained results, two important considerations have been drawn: The neutron dose equivalent is reduced by a factor ~2 and by a factor ~30 of the measured values at 15 MV for data acquired at 10 and 6 MV, respectively. Knowing that medical LINACs produce fast secondary neutrons, it was found that about one-tenth of the fast neutron dose equivalent contribute to the measured value in thermal dosemeters. This value has been so taken into account and excluded in thermal neutron measurements. The measured neutron dose equivalent per unit of 100 MU values at the patient plane compare reasonably well with those measured by other workers using similar detector technology, for LINACs operating at 15 and 10 MV(4, 13). While the photoneutron production in lower energy LINACs, at 6 MV, is usually ignored and only recently measured by Biltekin et al.(14). The parasitic neutron dose equivalent at organs Results of the BTI measurements at organs during the single-field 15 and 10 MV test irradiations are summarized in Figure 3. For each critical organ, the first column represents the total neutron dose equivalent (fast plus thermal), the second one is the fast neutron component, while the third column corresponds to the thermal component. Note that the third column excludes 1/10 of the fast neutron contribution to the measured value in the thermal dosemeter, as explained in the above section. Figure 3. View largeDownload slide Neutron dose equivalent per 100 MU at organs for the LINAC operating at 15 and 10 MV. Photon field centered on the pelvis region. SSD: 92.5 cm; isocenter: 7.5 cm depth in phantom in correspondence with the prostate; field size: 10 × 10 cm2 at isocenter. BTI measurements. Figure 3. View largeDownload slide Neutron dose equivalent per 100 MU at organs for the LINAC operating at 15 and 10 MV. Photon field centered on the pelvis region. SSD: 92.5 cm; isocenter: 7.5 cm depth in phantom in correspondence with the prostate; field size: 10 × 10 cm2 at isocenter. BTI measurements. At nominal x-ray energies of 15, 10 and 6 MV, the in-field recorded values are 2.05 ± 0.2, 0.56 ± 0.06 and 0.031 ± 0.003 mSv/100 MU, respectively. While in the remaining critical organs, the neutron dose equivalent decreases out-of-field and going deep in phantom, due to neutron moderation and absorption in tissue. Notice that in experimental cases like this one described, the last statement could be slightly not accurate. Infact neutrons could be also produced in the bunker walls, floor and ceiling, in a complex and mostly unknown way, giving a contribution to an organ rather than to another one. Only at 6 MV the out-of-field neutron dose equivalent contribution is <0.1% of the photon dose and it may be considered negligible, while photoneutrons are still present in the treatment area. In Figure 4, all the obtained results are plotted together in logarithm scale. It is so possible to notice that the total neutron dose equivalent trend at organs is the same for each simplified direct beam irradiation. Moreover, in the treatment area, in the prostate position, the neutron dose equivalent to the patient is a factor ~4 less from 15 to 10 MV irradiation, and a further factor ~18 less from 10 to 6 MV. This factor increases going far away from the radiation field, but the neutron dose equivalent is not negligible except for the 6 MV irradiation. Figure 4. View largeDownload slide Total neutron dose equivalent per 100 MU at organs provided by a 15, 10 and 6 MV LINAC. Logarithmic scale. Photon field centered on the pelvis region. SSD: 92.5 cm; isocenter: 7.5 cm depth in phantom in correspondence with prostate; field size: 10 × 10 cm2 at isocenter. BTI measurements. Figure 4. View largeDownload slide Total neutron dose equivalent per 100 MU at organs provided by a 15, 10 and 6 MV LINAC. Logarithmic scale. Photon field centered on the pelvis region. SSD: 92.5 cm; isocenter: 7.5 cm depth in phantom in correspondence with prostate; field size: 10 × 10 cm2 at isocenter. BTI measurements. CONCLUSIONS The photoneutron production in the Elekta Synergy Agility LINAC operating at 15, 10 and 6 MV has been investigated for an anthropomorphic phantom undergoing single-field radiotherapy sessions. The parasitic neutron dose equivalent has been measured by using bubble detectors both at the patient surface plane and inside critical organs, maintaining the same experimental setup conditions within the two kinds of measurements in order to compare results among the three studied operating LINAC energies. From this work it emerges that a residual neutron photoproduction is also present at energies near the GDR reaction threshold. It is ~20 and ~3% of the measured values at 15 MV for data acquired at 10 and 6 MV, respectively. These values increase during a treatment due to neutron thermalization and absorption in the human body, but the neutron contribution is not negligible at 15 and 10 MV. Although a slight amount of photoneutrons are provided by a 6 MV LINAC, it should not be completely neglected in clinical applications, especially considering other kind of treatments in which the use of multileaf collimators or lead blocks could increase the neutron photoproduction. Further measurements are planned with different treatment geometries, which better reflect clinical radiotherapy treatments, both for 3D conformal radiotherapy and intensity modulated radiotherapy, desirable to confirm results. ACKNOWLEDGEMENTS Authors thank the radiotherapy ward staff of the Maggiore Hospital in Trieste both for making available the LINAC and for technical support. REFERENCES 1 Zanini, A. et al.  . Monte Carlo simulation of the photoneutron field in linac radiotherapy treatments with different collimation systems. Phys. Med. Biol.  49, 571– 582 ( 2004). Google Scholar CrossRef Search ADS PubMed  2 Eaton, B. R., MacDonald, S. M., Yock, T. I. and Tarbell, N. J. Secondary malignancy risk following proton radiation therapy. Front. Oncol.  5, 261 ( 2015). Google Scholar CrossRef Search ADS PubMed  3 Zecchin, M. et al.  . Malfunction of cardiac devices after radiotherapy without direct exposure to ionizing radiation: mechanisms and experimental data. Europace  18( 2), 288– 293 ( 2016). Google Scholar CrossRef Search ADS PubMed  4 Awotwi-Pratt, J. B. and Spyrou, N. M. Measurement of photoneutrons in the output of 15 MV Varian Clinac 2100 C LINAC using bubble detectors. J. Radioanal. Nucl. Chem.  271, 679– 684 ( 2007). Google Scholar CrossRef Search ADS   5 Vanhavere, F., Huyskens, D. and Struelens, L. Peripheral neutron and gamma doses in radiotherapy with a 18 MV linear accelerator. Radiat. Prot. Dosim.  110, 607– 612 ( 2004). Google Scholar CrossRef Search ADS   6 Hashemi, S. M., Bijan, H., Gholamreza, R., Pervaneh, S., Sharafi, A. and Jafarizadeh, M. The effects of field modifier blocks on the fast photoneutron dose equivalent from two high-energy medical linear accelerator. Radiat. Prot. Dosim.  128, 359– 362 ( 2008). Google Scholar CrossRef Search ADS   7 The 2007 Recommendation of the International Commission on Radiological Protection. ICRP Publication 103. Ann. ICRP  37, 2– 4 ( 2007). 8 International Commission on Radiation Units & Measurements. ICRU Report 44, Tissue Substitutes in Radiation Dosimetry and Measurement. 9 Tutt, P. and Adler, D. Anthropometric Data, in New Metric Handbook  ( Oxford: The Architectural Press) ( 1979). 10 Bubble Technology Industries Inc. Instruction Manual for the Bubble Detector  ( Ontario, Canada: Chalk River) ( 1992). 11 Bubble Technology Industries Inc. Calibration certificate letter given together the detectors. 12 Spurny, F. et al.  . Bubble damage neutron detectors response in some reference neutron fields. Radiat. Prot. Dosim.  65, 393– 396 ( 1996). Google Scholar CrossRef Search ADS   13 D’Errico, F., Luszik-Bhadra, M., Nath, R., Siebert, B. R. L. and Wolf, U. Depth dose-equivalent and effective energies of photoneutrons generated by 6–18 MV x-ray beams for radiotherapy. Health Phys.  80, 4– 11 ( 2001). Google Scholar CrossRef Search ADS PubMed  14 Biltekin, F., Yeginer, M. and Ozyigit, G. Evaluation of photoneutron dose measured by bubble detectors in conventional linacs and cyberknife unit: Effective dose and secondary malignancy risk estimation. Technol. Cancer Res. Treat.  15, 560– 565 ( 2016). Google Scholar CrossRef Search ADS PubMed  © The Author(s) 2018. Published by Oxford University Press. All rights reserved. For Permissions, please email: journals.permissions@oup.com http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Radiation Protection Dosimetry Oxford University Press

MEASUREMENTS OF THE PARASITIC NEUTRON DOSE AT ORGANS FROM MEDICAL LINACS AT DIFFERENT ENERGIES BY USING BUBBLE DETECTORS

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Abstract

Abstract Conventional linear accelerators (LINACs) for radiotherapy produce fast secondary neutrons due to photonuclear processes. The neutron presence is considered as an extra undesired dose during the radiotherapy treatment, which could cause secondary radio-induced tumors and malfunctions to cardiological implantable devices. It is thus important to measure the neutron dose contribution to patients during radiotherapy, not only at high-energy LINACs, but also at lower energies, near the giant dipole resonance reaction threshold. In this work, the full body neutron dose equivalent has been measured during single-field radiotherapy sessions carried out at different LINAC energies (15, 10 and 6 MV) by using a tissue equivalent (for neutrons) anthropomorphic phantom together with bubble dosemeters. Results have shown that some neutron photoproduction is still present also at lower energies. As a consequence, emitted photoneutrons cannot be ignored and represent a risk contribution for patients undergoing radiotherapy. Medical linear accelerators (LINACs) for radiotherapy produce fast secondary neutrons due to photonuclear (γ, n) reactions. Neutron production is governed by the properties of the giant dipole resonance (GDR) reaction and neutrons are generated when the incident photon energy exceeds the GDR reaction threshold (6–20 MeV), with a mean energy of ~1 MeV and an isotropic angular distribution(1). Most of the neutrons are produced by the interaction of the photon beam on high-Z elements constituting the LINAC head (target, scattering foils, collimators, jaws, etc.), in addition to being produced in the bunker walls, floor and ceiling or directly inside the human body. A patient undergoing a radiotherapy treatment is thus affected by an extra unaccounted neutron dose that could cause secondary radio-induced tumors(2) and malfunctions to cardiological implantable devices(3). Several studies about the photoneutron dose measurements, with different dosimetric system such as bubble detectors, polycarbonate film, activation detectors and thermoluminescance dosemeters, are present in literature(4–6). However, there is sparse information about photoneutrons provided in lower energy LINACs, at 10 and 6 MV, near the GDR reaction threshold. This article describes experimental measurements of the full body parasitic neutron dose equivalent at critical organs, during single-field conformal radiotherapy sessions carried out with a LINAC operating at 15, 10 and 6 MV. The aim is to compare the results in terms of, in-field and out-of-field, neutron photoproduction between different energies with the same irradiation conditions. MATERIALS AND METHODS The LINAC photoneutron production has been investigated during single-field radiotherapy sessions, measuring the neutron dose equivalent in depth in tissue, by using a tissue equivalent (for neutrons) anthropomorphic phantom together with bubble dosemeters. Measurements were performed by using the Elekta Synergy Agility LINAC in photon mode not only at 15 MV, but also at LINAC energies near the GDR reaction threshold, at 10 and 6 MV. The medical LINAC calibration output was carried out by following the International Code of Practice for Dosimetry IAEA TRS398. The relationship of the dose to the number of monitoring units (MU) is 1 cGy = 1 MU at build-up in a water tank, measured with cylindrical chamber (PTW Farmer), with the SSD (source-to-surface distance) equal to 100 cm and the field size of 10 × 10 cm2 at isocenter. In this work, a simplified irradiation geometry was chosen by pointing only one open direct field at the pelvis area of the phantom, with the intent not to reproduce a real clinical treatment but rather to compare the neutron photoproduction at different LINAC energies. The experimental detection system The anthropomorphic phantom, specifically developed for neutron dosimetry, was designed and built by INFN (National Institute of Nuclear Physics) in Trieste following the International Commission on Radiological Protection indications for neutron dosimetric phantoms (ICRP 103)(7), both about organ positions and tissue substitutes (ICRU 44)(8). A new phantom was developed because standard phantoms usually employed in radiotherapy (like RANDO) do not meet all the work needs: to provide suitably large holes to locate bubble dosemeters and to be more tissue equivalent for neutrons than for photons. In fact for neutrons the main interaction with matter is the elastic scattering on light elements (specifically on hydrogen nuclei, that contribute to the absorbed dose up to 97%(8)) constituting the human body. Looking at the mean human upper part body composition, hydrogen is present for 0.10 mass fraction and polyethylene [(C2H4)n], 0.14 hydrogen mass fraction, is suitable as material for neutron dosimetry. The anthropomorphic phantom is thus made in polyethylene and it approximates dimensions, weight, density and hydrogen content of an average man(9). Inside the phantom several holes, suitable to allocate bubble dosemeters, have been made in correspondence with critical organs in order to measure the neutron dose equivalent in depth in tissue. The phantom external dimensions are: head (22 × 15 × 19 cm3); neck (7 × 9 × 7 cm3); chest (40 × 40 × 15 cm3); abdomen (25 × 40 × 15 cm3); thighs (40 × 25 × 15 cm3); calves (40 × 25 × 12 cm3); and feet 2 × (20 × 10 × 5 cm3). The bubble dosemeters employed are integral passive bubble detectors from Bubble Technology Industries (BTI)(10), sensitive both to fast neutrons (BD-PND) and to thermal neutrons (BDT). These detectors are calibrated by the manufacturer with an Am–Be source, having a fluence-weighted average energy equal to 4.15 MeV and a source strength of 1.13 × 107 n/s(11); the detection sensitivity is then indicated on the label of every bubble dosemeter and a calibration certificate is given together with each detector. The BD-PND detectors provide a good estimate of the dose equivalent for neutrons with energies >100 keV, while BDTs up to 0.4 eV. BTI guarantees optimum dosemeter performance for 3 months after receipt, in which it can be repressurized after use and stored inside the accompanying airtight aluminum storage tube. As stated by the BTI manufactures and shown in several studies(12), the type of the neutron source used in calibration has only a small effect on the sensitivity value; in particular, in this work differences between detectors calibration and the neutron spectrum provided by the LINAC are within the 8%. The BTI detectors have of the order of 10% accuracy, an operational temperature range of 20–37°C, they are not sensitive to gamma radiation and they have an isotropic angular response. They are thus suitable to be employed in mixed radiation fields as in radiotherapy. The experimental method Two different kinds of measurements have been performed: The neutron dose equivalent at the patient surface plane, to estimate the total amount of photoneutrons in the treatment area. The neutron dose equivalent at organs, to measure the full body photoneutron contribution to the patient. Measurements were carried out with the LINAC gantry at 0°, with an open direct photon beam of 10 × 10 cm2 at isocenter. In addition, the patient plane measurements were carried out placing two dosemeters (one for fast and one for thermal neutrons) on the surface of a cubic polyethylene phantom (30 × 30 × 30 cm3) at isocenter, with the SSD equal to 100 cm. While, the neutron dose equivalent measurements at organs were performed by using the anthropomorphic phantom, with the photon beam centered on the pelvis region, having the SSD equal to 92.5 cm and the isocenter at 7.5 cm depth, in correspondence with the prostate position. Eight critical organs were taken into account and two dosemeters (one for fast neutrons and one for thermal neutrons) were placed in each organ position, for a total of 16 detectors employed. In Figure 1 the experimental setup for the second kind of measurements is represented. Figure 1. View largeDownload slide The anthropomorphic phantom undergoing the single-field radiotherapy session. The photon beam is centered on the pelvis region and the isocenter is at 7.5 cm depth, in correspondence with the prostate. The irradiation is done with the LINAC gantry at 0°, with a direct open field of 10 × 10 cm2 at isocenter and with the SSD equal to 92.5 cm. Figure 1. View largeDownload slide The anthropomorphic phantom undergoing the single-field radiotherapy session. The photon beam is centered on the pelvis region and the isocenter is at 7.5 cm depth, in correspondence with the prostate. The irradiation is done with the LINAC gantry at 0°, with a direct open field of 10 × 10 cm2 at isocenter and with the SSD equal to 92.5 cm. RESULTS AND DISCUSSION All results in this article are reported in mSv normalized to 100 MU provided by the LINAC, and typical statistical errors are ~5–10% due to bubble counting in the range of 300–100 bubbles per dosemeter, respectively. The patient surface plane neutron dose equivalent Results of the BTI measurements on the Elekta Synergy Agility LINAC are shown in Figure 2 reporting, in logarithmic scale, both the fast and the thermal neutron dose equivalent at the patient surface plane for nominal x-ray energies of 15, 10 and 6 MV. The measured values are summarized in Table 1. Table 1. Patient surface plane neutron dose equivalent per 100 MU for the LINAC operating at 15, 10 and 6 MV. Irradiation setup conditions: field size of 10 × 10 cm2 at isocenter and SSD of 100 cm. Measurements performed by using BTI detectors.   Fast neutron dose (mSv/100 MU)  Thermal neutron dose (mSv/100 MU)  15 MV  1.98 ± 0.20  0.24 ± 0.02  10 MV  0.91 ± 0.09  0.09 ± 0.01  6 MV  0.07 ± 0.01  0.006 ± 0.001    Fast neutron dose (mSv/100 MU)  Thermal neutron dose (mSv/100 MU)  15 MV  1.98 ± 0.20  0.24 ± 0.02  10 MV  0.91 ± 0.09  0.09 ± 0.01  6 MV  0.07 ± 0.01  0.006 ± 0.001  Table 1. Patient surface plane neutron dose equivalent per 100 MU for the LINAC operating at 15, 10 and 6 MV. Irradiation setup conditions: field size of 10 × 10 cm2 at isocenter and SSD of 100 cm. Measurements performed by using BTI detectors.   Fast neutron dose (mSv/100 MU)  Thermal neutron dose (mSv/100 MU)  15 MV  1.98 ± 0.20  0.24 ± 0.02  10 MV  0.91 ± 0.09  0.09 ± 0.01  6 MV  0.07 ± 0.01  0.006 ± 0.001    Fast neutron dose (mSv/100 MU)  Thermal neutron dose (mSv/100 MU)  15 MV  1.98 ± 0.20  0.24 ± 0.02  10 MV  0.91 ± 0.09  0.09 ± 0.01  6 MV  0.07 ± 0.01  0.006 ± 0.001  Figure 2. View largeDownload slide Fast and thermal neutron dose equivalent at the patient surface plane for a 6, 10 and 15 MV LINAC operating in photon mode. Logarithmic scale. Irradiation setup conditions: photon field size of 10 × 10 cm2 and SSD of 100 cm. BTI measurements. Figure 2. View largeDownload slide Fast and thermal neutron dose equivalent at the patient surface plane for a 6, 10 and 15 MV LINAC operating in photon mode. Logarithmic scale. Irradiation setup conditions: photon field size of 10 × 10 cm2 and SSD of 100 cm. BTI measurements. From the obtained results, two important considerations have been drawn: The neutron dose equivalent is reduced by a factor ~2 and by a factor ~30 of the measured values at 15 MV for data acquired at 10 and 6 MV, respectively. Knowing that medical LINACs produce fast secondary neutrons, it was found that about one-tenth of the fast neutron dose equivalent contribute to the measured value in thermal dosemeters. This value has been so taken into account and excluded in thermal neutron measurements. The measured neutron dose equivalent per unit of 100 MU values at the patient plane compare reasonably well with those measured by other workers using similar detector technology, for LINACs operating at 15 and 10 MV(4, 13). While the photoneutron production in lower energy LINACs, at 6 MV, is usually ignored and only recently measured by Biltekin et al.(14). The parasitic neutron dose equivalent at organs Results of the BTI measurements at organs during the single-field 15 and 10 MV test irradiations are summarized in Figure 3. For each critical organ, the first column represents the total neutron dose equivalent (fast plus thermal), the second one is the fast neutron component, while the third column corresponds to the thermal component. Note that the third column excludes 1/10 of the fast neutron contribution to the measured value in the thermal dosemeter, as explained in the above section. Figure 3. View largeDownload slide Neutron dose equivalent per 100 MU at organs for the LINAC operating at 15 and 10 MV. Photon field centered on the pelvis region. SSD: 92.5 cm; isocenter: 7.5 cm depth in phantom in correspondence with the prostate; field size: 10 × 10 cm2 at isocenter. BTI measurements. Figure 3. View largeDownload slide Neutron dose equivalent per 100 MU at organs for the LINAC operating at 15 and 10 MV. Photon field centered on the pelvis region. SSD: 92.5 cm; isocenter: 7.5 cm depth in phantom in correspondence with the prostate; field size: 10 × 10 cm2 at isocenter. BTI measurements. At nominal x-ray energies of 15, 10 and 6 MV, the in-field recorded values are 2.05 ± 0.2, 0.56 ± 0.06 and 0.031 ± 0.003 mSv/100 MU, respectively. While in the remaining critical organs, the neutron dose equivalent decreases out-of-field and going deep in phantom, due to neutron moderation and absorption in tissue. Notice that in experimental cases like this one described, the last statement could be slightly not accurate. Infact neutrons could be also produced in the bunker walls, floor and ceiling, in a complex and mostly unknown way, giving a contribution to an organ rather than to another one. Only at 6 MV the out-of-field neutron dose equivalent contribution is <0.1% of the photon dose and it may be considered negligible, while photoneutrons are still present in the treatment area. In Figure 4, all the obtained results are plotted together in logarithm scale. It is so possible to notice that the total neutron dose equivalent trend at organs is the same for each simplified direct beam irradiation. Moreover, in the treatment area, in the prostate position, the neutron dose equivalent to the patient is a factor ~4 less from 15 to 10 MV irradiation, and a further factor ~18 less from 10 to 6 MV. This factor increases going far away from the radiation field, but the neutron dose equivalent is not negligible except for the 6 MV irradiation. Figure 4. View largeDownload slide Total neutron dose equivalent per 100 MU at organs provided by a 15, 10 and 6 MV LINAC. Logarithmic scale. Photon field centered on the pelvis region. SSD: 92.5 cm; isocenter: 7.5 cm depth in phantom in correspondence with prostate; field size: 10 × 10 cm2 at isocenter. BTI measurements. Figure 4. View largeDownload slide Total neutron dose equivalent per 100 MU at organs provided by a 15, 10 and 6 MV LINAC. Logarithmic scale. Photon field centered on the pelvis region. SSD: 92.5 cm; isocenter: 7.5 cm depth in phantom in correspondence with prostate; field size: 10 × 10 cm2 at isocenter. BTI measurements. CONCLUSIONS The photoneutron production in the Elekta Synergy Agility LINAC operating at 15, 10 and 6 MV has been investigated for an anthropomorphic phantom undergoing single-field radiotherapy sessions. The parasitic neutron dose equivalent has been measured by using bubble detectors both at the patient surface plane and inside critical organs, maintaining the same experimental setup conditions within the two kinds of measurements in order to compare results among the three studied operating LINAC energies. From this work it emerges that a residual neutron photoproduction is also present at energies near the GDR reaction threshold. It is ~20 and ~3% of the measured values at 15 MV for data acquired at 10 and 6 MV, respectively. These values increase during a treatment due to neutron thermalization and absorption in the human body, but the neutron contribution is not negligible at 15 and 10 MV. Although a slight amount of photoneutrons are provided by a 6 MV LINAC, it should not be completely neglected in clinical applications, especially considering other kind of treatments in which the use of multileaf collimators or lead blocks could increase the neutron photoproduction. Further measurements are planned with different treatment geometries, which better reflect clinical radiotherapy treatments, both for 3D conformal radiotherapy and intensity modulated radiotherapy, desirable to confirm results. ACKNOWLEDGEMENTS Authors thank the radiotherapy ward staff of the Maggiore Hospital in Trieste both for making available the LINAC and for technical support. REFERENCES 1 Zanini, A. et al.  . Monte Carlo simulation of the photoneutron field in linac radiotherapy treatments with different collimation systems. Phys. Med. Biol.  49, 571– 582 ( 2004). Google Scholar CrossRef Search ADS PubMed  2 Eaton, B. R., MacDonald, S. M., Yock, T. I. and Tarbell, N. J. Secondary malignancy risk following proton radiation therapy. Front. Oncol.  5, 261 ( 2015). Google Scholar CrossRef Search ADS PubMed  3 Zecchin, M. et al.  . Malfunction of cardiac devices after radiotherapy without direct exposure to ionizing radiation: mechanisms and experimental data. Europace  18( 2), 288– 293 ( 2016). Google Scholar CrossRef Search ADS PubMed  4 Awotwi-Pratt, J. B. and Spyrou, N. M. Measurement of photoneutrons in the output of 15 MV Varian Clinac 2100 C LINAC using bubble detectors. J. Radioanal. Nucl. Chem.  271, 679– 684 ( 2007). Google Scholar CrossRef Search ADS   5 Vanhavere, F., Huyskens, D. and Struelens, L. Peripheral neutron and gamma doses in radiotherapy with a 18 MV linear accelerator. Radiat. Prot. Dosim.  110, 607– 612 ( 2004). Google Scholar CrossRef Search ADS   6 Hashemi, S. M., Bijan, H., Gholamreza, R., Pervaneh, S., Sharafi, A. and Jafarizadeh, M. The effects of field modifier blocks on the fast photoneutron dose equivalent from two high-energy medical linear accelerator. Radiat. Prot. Dosim.  128, 359– 362 ( 2008). Google Scholar CrossRef Search ADS   7 The 2007 Recommendation of the International Commission on Radiological Protection. ICRP Publication 103. Ann. ICRP  37, 2– 4 ( 2007). 8 International Commission on Radiation Units & Measurements. ICRU Report 44, Tissue Substitutes in Radiation Dosimetry and Measurement. 9 Tutt, P. and Adler, D. Anthropometric Data, in New Metric Handbook  ( Oxford: The Architectural Press) ( 1979). 10 Bubble Technology Industries Inc. Instruction Manual for the Bubble Detector  ( Ontario, Canada: Chalk River) ( 1992). 11 Bubble Technology Industries Inc. Calibration certificate letter given together the detectors. 12 Spurny, F. et al.  . Bubble damage neutron detectors response in some reference neutron fields. Radiat. Prot. Dosim.  65, 393– 396 ( 1996). Google Scholar CrossRef Search ADS   13 D’Errico, F., Luszik-Bhadra, M., Nath, R., Siebert, B. R. L. and Wolf, U. Depth dose-equivalent and effective energies of photoneutrons generated by 6–18 MV x-ray beams for radiotherapy. Health Phys.  80, 4– 11 ( 2001). Google Scholar CrossRef Search ADS PubMed  14 Biltekin, F., Yeginer, M. and Ozyigit, G. Evaluation of photoneutron dose measured by bubble detectors in conventional linacs and cyberknife unit: Effective dose and secondary malignancy risk estimation. Technol. Cancer Res. Treat.  15, 560– 565 ( 2016). Google Scholar CrossRef Search ADS PubMed  © The Author(s) 2018. Published by Oxford University Press. All rights reserved. For Permissions, please email: journals.permissions@oup.com

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Radiation Protection DosimetryOxford University Press

Published: Jan 18, 2018

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