TY - JOUR
AU - Yukihara, Eduardo G
AB - Abstract Thermoluminescence dosimetry (TLD) has a long history of applications in medicine. However, despite its versatility and sensitivity its use is anecdotally diminishing, at least in part due to the complexity and work intensity of a quality TLD service. The present paper explores the role of TLD in medicine using a common inquiry methodology (5W1H) which systematically asks ‘Who, What, When, Where, Why and How’ to identify what role TLD could and should play in medical applications. INTRODUCTION Thermoluminescence dosimetry (TLD) is based on the ability of appropriately prepared crystals to store part of the energy deposited by ionising radiation. The stored energy, which is proportional to the radiation dose, can then be released by heating the crystal. The light emitted in this way can provide a measure of dose. There are many publications describing the physics and practicalities of TLD in more detail including work of Marko Moscovitch(1–4). Of interest are also three excellent publications on luminescence dosimetry in general covering both TLD and optically stimulated luminescence (OSL) dosimetry(5–7). A bit of history TLD was one of the very first effects due to radiation studied already at the same time of W.C. Roentgen’s discovery of X-rays(8). It has been referenced by H. Becquerel and M. Curie noted it in her doctoral thesis(9). The first comprehensive theory of TL was published by J. Randall and M. Wilkins in 1945(10,11) but the use in medicine can probably be credited to the group in Wisconsin led by F. Daniels who also introduced LiF:Mg,Ti in the 1950s(12), where the lithium fluoride crustal is doped with magnesium and titanium. J. Cameron who joined F. Daniels group further studied the material and TLD100, a material still widely in use today was born(13). A near parallel development occurred in Germany with the use of CaSO4(14). Other important milestones have been the proceedings of the Solid State Dosimetry (SSD) conferences where every 3 years researchers and dosimetry practitioners meet to discuss the latest developments in the field. Since 2001, the SSD conference also features the Marko Moscovitch School in honour of one of the pioneers of SSD. The following paper tries to explore the role TLD has in 2020 for dosimetry in medicine using a ‘Five Ws and How’ approach. 5W1H is a common technique for inquiry going back to ancient Greek philosophy (https://en.wikipedia.org/wiki/Five_Ws). It is based on questioning a topic from many directions asking ‘Who, What, When, Where, Why and How’. In the context of TLD in medicine, we would specifically like to address this in the following six sections: Who would be interested in TLD? What’s not to like about TLD? Why is TLD still relevant? When would we use TLD rather than any other technique? Where shall TLD research effort be directed in order to make it more relevant for medical dosimetry? How to set up a state of the art TLD system? Although this may sound like a somewhat unusual approach, it can be noted that all the typical aspects of a brief review are addressed from the advantages/disadvantages (Sections 2 and 3) to applications (4), technical realisation (6) and future developments (5). WHO WOULD BE INTERESTED IN TLD? TLD is widely used in radiation protection (mostly personnel monitoring)(15), accident dosimetry(16) and dating(17,18). However, also in medical dosimetry TLD has enjoyed continued interest. Figure 1 shows the number of publications listed in the PubMed database with the search terms TLD in any field. This is compared with the mention of OSL covered in a paper by Yukihara and Kron in this special issue(19). Figure 1 Open in new tabDownload slide Number of publications listed in the PubMed database per year containing the search terms ‘TLD’ and ‘OSL’. The data for 2020 are up to 15 October 2020. Compared to search terms such as artificial intelligence (3889 hits in 2019) and immunotherapy (18 704 hits in 2019) these are modest levels of interest but there is a steady growth reflecting mostly wider applications. For medical applications, medical physicists are the most common users of TLD which results in a service that is typically offered on house. As elaborated on in the next section, TLD has a number of complexities that require careful attention and interpretation. Physicists would not only be able to understand the underlying principles but also be willing to invest the time to set up a high quality system as outlined in the final section of this paper. WHAT’S NOT TO LIKE ABOUT TLD? There are several drawbacks which make TLD a difficult proposition for many practitioners: There is a vast variety of TLD materials and physical preparations and it is difficult to select the most appropriate one. This problem gets bigger when considering that even nominally identical materials from different manufacturers can have significantly different dose response characteristics. TLD typically only yields a point dose. Although the detector can be made suitably small as can be seen in Figure 2, the small size of many TL dosimeters makes them difficult to handle. A related issue is that TL detectors despite their small size do not fulfil Bragg Gray cavity theory conditions in megavoltage radiation beams(20) and are therefore not suitable for reference dosimetry. TLD is a complex technique where small variations of dosimeter preparation matter and tight control of all aspects of the workflow becomes essential. This has three important consequences: (i) TLD is time consuming and requires a dedicated person who pays attention to detail; (ii) The workload and cost is considerable but diminishes as a TLD service processes more measurements(21); (iii) A busy TLD service is typically a better service as standardization and automation become more common. There has been concern about reliability of TLD results and `spurious' readings are sometimes quoted. This is most likely a result of the first two points and it should be possible to achieve a measurement precision of ±2% of dose under normal circumstances. A related problem is that no second readout is possible after the TLD has been evaluated. The dose response of many TLD materials is not linear at doses typical for radiotherapy. For example LiF:Mg,Ti exhibits a supralinear behaviour at doses exceeding about 1Gy(22,23). As supralinearity is a result of both distribution of impurities in the crystal and the microdosimetric dose deposition, it depends on preparation of the material and the radiation quality(24). As recently shown by Ginzburg et al. this also provides scope to optimise the dose response characteristics through clever interventions(25). TLD results are not acquired in real time and it takes typically several hours to obtain a result. Another more subtle problem with TLD is the lack of modernization. The work horse TLD reader in many clinics (Harshaw 5500) is more or less unchanged for the last 30 years and commercially available technology has only recently started to become more modernised with better software. This leads to a sense of using outdated technology despite the fact that TLD research is a thriving field of solid state physics. Figure 2 Open in new tabDownload slide Two typical LiF based TLD detectors: a ‘pinworm’(26) and a 3.5 mm diameter pellet. Figure 3 Open in new tabDownload slide Photo of an anthropomorphic phantom (CIRS Inc., Norfolk VI) of a 1-year-old child. The plugs that can accommodate TLD chips are shown in the insert. Figure 4 Open in new tabDownload slide Holder for three TLDs suitable for in vivo dosimetry. The dome of the holder is designed to give build-up for maximum dose in a 6 MV photon beam from many beam directions. The three dosemeters can either be of the same type for better statistics or of different type for example for mixed field dosimetry using Li-based detectors with different isotopes. All these factors lead many medical physicists to explore different dosimetric techniques but as the next section illustrates there are many advantages of TLD. WHY IS TLD STILL RELEVANT? TLD has many unique features that allow measurements in situations where other dosemeters would be difficult to use: There is a vast variety of TLD materials and physical preparations providing an important choice to optimise the detector for the purpose at hand. Of particular relevance for many applications is that many TLD materials can be considered to be tissue equivalent (e.g. BeO, LiF). TLD materials can feature different sensitivities to different radiation qualities. This allows for measurements in mixed fields for example of photons and neutrons. Many TL detectors are exquisitely sensitive. This facilitates measurements of low doses such as environmental or some medical imaging procedures. It also allows to miniaturise the detectors without losing too much signal. Figure 2 shows a TLD pinworm of 2 mm length and 0.5 mm diameter that has successfully used for brachytherapy dosimetry(26). TL dosemeters do not require cables or any connections to a read out device. This allows them to be embedded into virtually any environment. Being made of a single material without walls or connectors, TLDs typically display minimal variation in response with direction of exposure. TL dosemeters are relatively cheap and with careful handling re-useable for hundreds of times. They are also readily available in many countries. Particularly compared to OSL detectors the insensitivity of TLDs to light is a significant advantage that makes handling easier and the chips can be directly inserted into holders such as the ones shown in Figures 3 and 4 without need to consider additional wrapping. The equipment required to set-up a TLD service is fairly simple: TLD material, a reader and a dedicated annealing oven. Fortunately, much of this equipment is also robust and many old TLD readers are in clinical use. This is related to point 8 in the previous section as there is no sense that one needs the latest technology to get good results. On the downside, this makes the market for TLD equipment small resulting often in relatively high initial investment costs associated to the reader purchase. These features would make TLD a useful technique in circumstances where exposures are unknown in terms of radiation quality, magnitude and direction. Their stand-alone nature also makes them suitable to be carried around as well as embedded in any measurement environment such as an anthropomorphic phantom. A good summary of advantages and disadvantages of TLD in comparison with OSL is provided by P Olko in 2010 (7). WHEN WOULD WE USE TLD RATHER THAN ANY OTHER TECHNIQUE? The advantages of TLD listed in the previous section make it an important technique for several applications in medicine. We would like to highlight different aspects of this in four applications: High sensitivity: dosimetry in low dose diagnostic procedures such as paediatric imaging and screening Many TL dosemeters such as LiF:Mg,Cu,P(27,28) and Al2O3:C(29,30) are sensitive enough to assess environmental radiation doses of a few micro Gy. This makes them also suitable for applications in medicine where low radiation dose is essential such as medical imaging in children or screening procedures where the subject of the investigation is usually a perfectly health person. The most common method to assess doses in paediatric imaging is the use of an anthropomorphic phantom such as the one shown in Figure 3 for a 1-year-old child. The phantom can be taken apart into 28 axial slices each about 25 mm thick. The dosemeters are placed in many locations within the phantom at the same time which allows for assessment of dose in the imaging field as well as scatter dose to sensitive organs such as the thyroid(31–33). Another important field for clinical dosimetry where high sensitivity is required is screening of segments of the population. In these applications, otherwise healthy subjects are exposed to ionising radiation with the objective of identifying persons who may have a very serious disease not yet symptomatic. Cancer is the most common target for image based screening and mammography(34) and CT for lung cancer in high-risk populations(35,36) are successful screening programs. Dose measurements can be done in phantoms or in vivo on patients undergoing the investigation. TLD has the advantage of high sensitivity and easy handling which allowed in 1996 a study of thousands of patients using in vivo dosimetry mapping practice across the USA(37). An important additional advantage of LiF TLDs is that the detector does not affect the diagnostic objective(38). Stand-alone nature: in vivo dosimetry TLD is a ‘passive’ dosimetric technique where no bias or measurement cables are required. Therefore, TL detectors can easily be packaged in small satchels and placed on the skin of patients undergoing imaging or radiotherapy. For the common assessment of incident dose in megavoltage radiotherapy typically build-up is required. In the case of TLDs this can be built into the holder design as shown in Figure 4. Many different detector types are available for in vivo dosimetry. However, there are a few applications such as total skin electron therapy (TSET) where TLD and OSL as high sensitivity passive detectors are the only possible solution for in vivo dosimetry(39–41), in particular if the patient is on a rotating platform as shown in Figure 5(42–44), Rotational TSET is a method to treat cutaneous lymphomas that can spread over the whole body. Figure 5 Open in new tabDownload slide Illustration of total body skin irradiation for cutaneous lymphomas. The patient is standing on a rotating platform at about 3 m distance from the treatment head of a medical linear accelerator in electron mode. Dosimetry is difficult due to lack of applicators, patient shape and dynamic delivery and in vivo dosimetry is usually considered essential, not only to verify the delivered dose but also to identify areas of low dose that must be boosted later. Robustness: dosimetric intercomparisons TL dosemeters—provided packaged appropriately—are robust and largely independent of environmental conditions such as temperature, pressure and moderate light exposure. As such they can easily be mailed out to clinics where ionising radiation is used. This is done in diagnostics as well as radiation therapy. In diagnostics it is a particularly useful tool for assessment of practice with the objective to improve it. Diagnostic reference levels (DRLs) are an excellent example for this where dose for identical procedures performed in different institutions using different equipment is measured(45,46). If no particular circumstances suggest otherwise, it should be possible for all centres to perform the procedure with the dose of the 75 percentile of dose. This is called the DRL and practitioners who exceed these doses are required to justify the higher dose. DRLs are used for simple measures such as entrance surface dose(47) and CT dose index(48,49) but have recently also been proposed for more complex procedures. Although DRL reporting is most commonly done using parameters recorded by the equipment, an independent verification can easily be done in a clinical scenario using TLDs(47,50). For radiation therapy dosimetric intercomparisons are common in quality audits and for clinical trial credentialing(51–53). Figure 6 shows the simplicity of a TLD based audit performed for many years by the International Atomic Energy Agency (IAEA). Even without any additional material or equipment the irradiation of the TLD capsules filled with LiF powder can be performed in water at a reference depth of 5 cm in low and middle income countries(54,55). Due to faster processing and non-destructive readout, IAEA has recently changed its output audit to OSL dosimetry(56). The verification of output calibration would typically be called a level I audit of external beam radiotherapy. Figure 6 Open in new tabDownload slide Simple set-up for dosimetric intercomparisons used in the IAEA/WHO audit of radiotherapy treatment unit output. Audits can be made more complex to include non-reference conditions(57) or include anthropomorphic phantoms (such as the one in Figure 3) for end to end testing. In these audits, also referred to as level III, the phantom undergoes exactly the same procedures as a patient would from imaging to planning, verification and treatment delivery(53,58). Tissue equivalence, high spatial resolution and independence of incident direction: brachytherapy dosimetry TLD has many features that make it an interesting dosemeter for brachytherapy, the use of radioactive sources implanted in the patient for treatment of malignancies(59). The dosimetric problem in brachytherapy derives from the close proximity of the radioactive sources to the area of interest. Therefore, good spatial resolution is essential. Given the fact that many brachytherapy sources are based on keV gamma rays scatter in tissues plays an important effect. This leads to the requirement of tissue equivalence for photons as well as a response independent on the incidence direction of radiation. The usefulness of TLD can particularly be demonstrated in the case of radioactive eye-plaques which are placed in an operation in direct contact with the eye for treatment of ocular melanomas(60–62). Brachytherapy is also more variable and operator dependent than external beam treatment and the rate of accidents is higher on a per patient basis. All this makes in vivo dosimetry attractive. Of particular interest are here doses to critical structures such as rectum or bladder, which are close to common implants in gynaecology or prostate radiotherapy. Both rectum and bladder allow for relatively easy insertion of radiation detectors(63). As high dose rate brachytherapy often uses many catheters through which a source can step, it is also possible to place TLD rods or pinworms (as in Figure 2) in the catheters while the dose is delivered in a different catheter(26,64). Finally, TLDs have also been used to perform dosimetric intercomparisons and audits for brachytherapy(65,66). WHERE SHALL TLD RESEARCH EFFORT BE DIRECTED IN ORDER TO MAKE IT MORE RELEVANT FOR MEDICAL DOSIMETRY? TLD has a proud history in materials research and several application fields such as dating and retrospective dosimetry. As such there are still many active areas of research and we would like to pick four of them which are particularly of interest for medical applications. Development and standardisation of materials There a many different TLD materials available with Table 1 listing the properties of some of the more common commercially available materials. Table 1 Properties of several commercial TLD materials(1–5,67). . Al2O3:C . BeO . LiF:Mg,Ti . LiF:Mg,C,P . Li2B4O7:Mn . CaSO4:Dy . Density* (g/cm3) 3.95 2.85 2.64 2.64 2.3 2.61 Effective atomic number 11.3 7.2 8.2 8.2 7.4 15.3 Variation in X-ray energy response (30 keV/1.25 MeV) 2.9 0.9 1.7 1.3 0.9 12 Main emission band (nm) 420 370 400 380 600 480, 570 Temperature of main glow peak (C) 190 190, 300 195 210 200 220, 250 Detection limit (estimate) <1 uGy 0.1 mGy 10 uGy <1 uGy 0.1 mGy 1 uGy Fading of main peak at room temperature 5–20%/year 5–10% in 3 months <10% per year 5% per year 10% per month 10% per year Comment Also OSL, very high sensitivity Also OSL, good tissue equivalence Most studied, complex glow curve Also OSL, high sensitivity Good tissue equivalence Bone equivalence . Al2O3:C . BeO . LiF:Mg,Ti . LiF:Mg,C,P . Li2B4O7:Mn . CaSO4:Dy . Density* (g/cm3) 3.95 2.85 2.64 2.64 2.3 2.61 Effective atomic number 11.3 7.2 8.2 8.2 7.4 15.3 Variation in X-ray energy response (30 keV/1.25 MeV) 2.9 0.9 1.7 1.3 0.9 12 Main emission band (nm) 420 370 400 380 600 480, 570 Temperature of main glow peak (C) 190 190, 300 195 210 200 220, 250 Detection limit (estimate) <1 uGy 0.1 mGy 10 uGy <1 uGy 0.1 mGy 1 uGy Fading of main peak at room temperature 5–20%/year 5–10% in 3 months <10% per year 5% per year 10% per month 10% per year Comment Also OSL, very high sensitivity Also OSL, good tissue equivalence Most studied, complex glow curve Also OSL, high sensitivity Good tissue equivalence Bone equivalence *Density of the actual detector can vary depending on manufacturing. Open in new tab Table 1 Properties of several commercial TLD materials(1–5,67). . Al2O3:C . BeO . LiF:Mg,Ti . LiF:Mg,C,P . Li2B4O7:Mn . CaSO4:Dy . Density* (g/cm3) 3.95 2.85 2.64 2.64 2.3 2.61 Effective atomic number 11.3 7.2 8.2 8.2 7.4 15.3 Variation in X-ray energy response (30 keV/1.25 MeV) 2.9 0.9 1.7 1.3 0.9 12 Main emission band (nm) 420 370 400 380 600 480, 570 Temperature of main glow peak (C) 190 190, 300 195 210 200 220, 250 Detection limit (estimate) <1 uGy 0.1 mGy 10 uGy <1 uGy 0.1 mGy 1 uGy Fading of main peak at room temperature 5–20%/year 5–10% in 3 months <10% per year 5% per year 10% per month 10% per year Comment Also OSL, very high sensitivity Also OSL, good tissue equivalence Most studied, complex glow curve Also OSL, high sensitivity Good tissue equivalence Bone equivalence . Al2O3:C . BeO . LiF:Mg,Ti . LiF:Mg,C,P . Li2B4O7:Mn . CaSO4:Dy . Density* (g/cm3) 3.95 2.85 2.64 2.64 2.3 2.61 Effective atomic number 11.3 7.2 8.2 8.2 7.4 15.3 Variation in X-ray energy response (30 keV/1.25 MeV) 2.9 0.9 1.7 1.3 0.9 12 Main emission band (nm) 420 370 400 380 600 480, 570 Temperature of main glow peak (C) 190 190, 300 195 210 200 220, 250 Detection limit (estimate) <1 uGy 0.1 mGy 10 uGy <1 uGy 0.1 mGy 1 uGy Fading of main peak at room temperature 5–20%/year 5–10% in 3 months <10% per year 5% per year 10% per month 10% per year Comment Also OSL, very high sensitivity Also OSL, good tissue equivalence Most studied, complex glow curve Also OSL, high sensitivity Good tissue equivalence Bone equivalence *Density of the actual detector can vary depending on manufacturing. Open in new tab The ‘perfect material’ for medical dosimetry does not exist. However, many materials are fairly close, such as LiF:Mg,Cu,P with excellent sensitivity, good tissue equivalence for photons (enhanced by an anomaly in response around 80 keV(24,68,69)) and good response linearity. In practice the number of TLD materials is actually much larger as many manufacturers produce nominally identical materials such as LiF:Mg,Ti which can differ substantially in their dosimetric properties, be it due to different doping levels, different methods to bind the grains together or different surface preparation. This can even vary between different batches of the same manufacturer, as seen in Figure 7. Figure 7 Open in new tabDownload slide MicroCT images with 10 μm spatial resolution of four LiF:Mg,C,P TLD chips (Bruker microCT). All chips were produced by Harshaw Chemical Co (Cleveland) but two are based on the isotope 6Li (TLD 600H) and two on 7Li (TLD 700H). The two different isotopes change the detector sensitivity for neutrons. The microCT analysis also demonstrates that the porosity of the material differs significantly between the different batches. This can affect their response to photons that would otherwise assumed to be identical. (a) and (b) cross-sections through the centre of two TLD 700H chips from the same batch; (c) and (d) cross-sections through the centre of two TLD 600H chips from the same batch. Using modern quality assurance tools (such as the microCT assessment) it should be possible to improve the standardisation of TLD materials. In fields such as medicine, where TL dosemeters are merely tools to obtain an important dosimetric result this could improve reliability and therefore uptake by practitioners. Glow curve analysis Most TLD materials have more than one location where the radiation energy is stored. These traps are dependent on doping levels in the crystal and its (thermal) preparation. An important feature of this is that the energy release is dependent on the energetic depth of the trap and different traps will be emptied using different temperatures following Boltzman’s law. As the readout temperature for the TLD increases traps are emptied and additional ones start to be accessed. This results in a glow curve like the one shown in Figure 8 schematically for LiF:Mg,Ti. Figure 8 Open in new tabDownload slide Schematic drawing of the glow curve for LiF:Mg,Ti. Shown is the emitted light intensity as a function of the applied temperature. As the temperature increases, deeper traps are accessed and emptied. The six peaks shown are typical intensities observed a day after irradiation. The energy depth of the lower peaks is shallow enough to allow already some decay at room temperature. This can be seen from the combined curve as it would be measured directly after the irradiation. The decay of the lower peaks is called fading. Figure 9 Open in new tabDownload slide Plastic sheet with an array of TLD deposits suitable for read-out using a laser scanning process (reproduced from(81) courtesy of Keithley). Glow curves are a unique feature of TLD which can be used for material research and quality assurance. However, it also provides further opportunities to explore the nature of the irradiation as the filling of different traps can depend on the radiation quality. This has been demonstrated for example in mixed field measurements where neutron and gamma irradiation can be distinguished(70–72). Glow curve analysis requires careful read out of the TL material and is typically not possible with automated readers. Figure 10 Open in new tabDownload slide Typical TLD workplace. Shown is an aluminium tray that can house up to 100 TLD chips or pellets, a set of vacuum tweezers to handle the chips and a holder for plugs for an anthropomorphic phantom like the one shown in Figure 3. Figure 11 Open in new tabDownload slide Simplified TLD measurement cycle. There are two parts to the cycle with the calibration establishing an individual sensitivity factor for each detector. T = Temperature. Figure 12 Open in new tabDownload slide Perspex blocks used for irradiation of up to five TLDs to a known dose under standardised conditions. Combination of TL and OSL One of the more interesting aspects of TLD is that OSL, which is discussed in more detail in a different chapter of this special edition(19), is closely related to TLD. Both techniques analyse light emitted after stimulation, one using light and one heat. Not surprisingly, Table 1 also includes the two materials listed in the OSL report by E. Yukihara(19). The combination of two evaluation techniques is possible(73) as TL readout to moderate temperatures can leave the OSL signal unaffected(74). In this context it is interesting to note that readers that can perform dual evaluation have now becoming more widely commercially available (https://www.lexsyg.com/tlosl-reader.html or https://www.nutech.dtu.dk/english/products-and-services/radiation-instruments/tl_osl_reader). Two-dimensional dosimetry Optical stimulation has long been used as the underlying principle of computed radiography (CR)(75,76). Although the materials used for CR imaging are optimised for high sensitivity and not their dosimetric properties, the technique highlights the opportunity to assess both, dose and its distribution using luminescence detectors. In the case of TLD this has been a slow process and although the combination of more than one TLD chips in rows or clusters has been reported(77,78), full two-dimensional dosimetry is rare. TLD sheets such as the one shown in Figure 9 have been proposed more than 20 years ago(79,80), however, they are not commonly used. Compared to OSL and the related CR the advantage would be lack of light sensitivity which could broaden the applications. HOW TO SET UP A STATE OF THE ART TLD SYSTEM? Based on the features noted in the first two sections of this review TLD is often regarded a bit like a black art. However, once it is realised that the quality of a TLD measurement depends on tight control over temperature in every step of the process as well as careful handling of the material it is relatively simple to set-up and maintain a successful TLD system including a dedicated annealing oven. TL dosimetry can make use of different physical forms of the detector material ranging from powder to chips, rods, micro cubes and other configurations(2). TLD crystals are commonly ground to powder and then sintered into common shapes (see e.g. Figures 2 and 7). The TLD powder can also be used by itself or mixed with Teflon to make the detectors more robust(16). An interesting application is the mix or TLD powder with carbon which does not change the dose deposition but limits the emitted light to powder from the very surface of the detectors. These black TLDs have been successfully used to assess dose at the sensitive layer of the skin at 0.07 mm depth(82). Figure 10 shows a ‘typical’ workplace for TLD. As TL dosemeters—like virtually all radiation detectors—should never be touched, tweezers are an essential tool. Vacuum tweezers are preferred as they are less likely to damage the surface of the TLDs. The figure also shows an aluminium tray for storing and annealing the detectors. Aluminium is well suited for this as it has excellent thermal conductivity ensuring all TL detectors share the same thermal history. Not shown in Figure 10 is the TLD reader, the choice of which depends on the material used as well as the application. Evaluation or at least visualisation of glow curves as discussed in Section 5 is a very useful feature as it can identify incorrect thermal contact or other problems during read out. Heating systems are either via contact (‘planchet’) or preferably by inert gas. The temperature/time profile that controls the heating of the TL dosemeter needs to be programmable and must be highly reproducible to ensure all detectors in a batch are sharing the same read-out. Due to its complexity TLD is a relative dosimetric technique. This means dose assessments are typically done in unknown irradiation scenarios by comparison with a known dose delivered to a subset of TLDs form the same batch. The known dose should be delivered using a similar radiation quality as expected in the measurement and should be given at the same time as the unknown exposure to avoid problems with fading illustrated in Figure 7. Figure 11 shows a schematic workflow for a clinical TLD system based on individual dosemeters such as chips, pellets or rods. TL dosemeters are grouped in batches that may share their individual dose response characteristics within a predefined limit (±5% is typical but limits the accuracy of any measurement to this limit). A better method is to determine individual ‘sensitivity factors’ for each detector in calibration runs as shown in the figure on the right. These would be done three times when TLDs are initiated and then at predefined time intervals, typically monthly. According to AAPM TG report 191 the two approaches can be termed ‘high efficiency’ and ‘high accuracy’(5). The sensitivity factors, usually the rolling average of the last three calibration runs, are then used to correct the reading of each detector allowing for direct comparison of the unknown dose in the measurement and the known dose given in the parallel standard irradiation. Figure 12 shows a simple way to organise the standards for irradiation in a megavoltage photon beam. If in vivo dosimetry on patients is performed these blocks can be very quickly irradiated on the same treatment unit between patients. Particularly for TLDs with high sensitivity it is also essential to maintain some TLDs from a batch without any irradiation. This allows for background correction which may be important in particular if low doses are to be measured as in some diagnostic imaging and out of field assessments in radiotherapy. Provided good care is taken with the set-up of a clinical TLD systems, it should be possible to keep the variability of the reading of each TLD chip well within ±2% (1SD). If several detectors are used for standards and measurement (compare Figures 4 and 11) this should allow the determination of an unknown dose with better than 5% accuracy with 95% confidence in most circumstances. FINAL REMARKS TLD has a long history for a large range of dosimetric applications in medicine. Due to the development of other techniques, that address some of TLDs shortcomings, TLD is used in fewer clinics as a routine dosimetric tool. In particular OSL(19,83) covered also in this issue has been of interest as an alternative high sensitivity technique without need for cables. However, tissue equivalence and robustness make TLD still an attractive solution in many scenarios where irradiation conditions are tricky to predict. ACKNOWLEDGEMENTS The support of the Gross Foundation for the high precision dosimetry program at Peter MacCallum Cancer Centre is acknowledged as are the helpful comments of Y. Horowitz on the manuscript. We are also grateful to R Tino for the microCT image acquisitions. References 1. Horowitz , Y. S. General characteristics of TLD materials. In: Thermoluminescence and Thermoluminescent Dosimetry . Horowitz , Y. S., Ed. 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TI - THERMOLUMINESCENCE DOSIMETRY (TLD) IN MEDICINE: FIVE ‘W’S AND ONE HOW
JF - Radiation Protection Dosimetry
DO - 10.1093/rpd/ncaa212
DA - 2020-12-30
UR - https://www.deepdyve.com/lp/oxford-university-press/thermoluminescence-dosimetry-tld-in-medicine-five-w-s-and-one-how-6PuW0HXGx0
SP - 139
EP - 151
VL - 192
IS - 2
DP - DeepDyve
ER -