Ti Thermoluminescent Dosimeters Research Articles

Small field dosimetry employing the thermoluminescence technique using a 3D printed phantom

With the advent of new radiotherapy modalities, it has become necessary to apply relatively small fields that are dynamic or static. The use of these field sizes can cause uncertainty in dosimetry, requiring special attention in small field dosimetry. With these mentioned challenges, it is difficult to select a detector with good performance for dosimetry in small fields. TLDs have advantages because they have characteristics such as high spatial resolution and dose response, they offer a promising opportunity to measure the absorbed dose in a small field. The objective of this study is to evaluate the performance of CaSO4:Dy, LiF:Mg,Ti and μLiF:Mg,Ti thermoluminescent dosimeters for clinical photon beams in small field dosimetry using a 3D printed phantom. For clinical application the 6EX linear accelerator from Varian Medical System and the Multillif collimator from BrainLab were used. The 3D printed phantom was subjected to real treatment conditions. The dynamic arc 3D radiosurgery technique was adopted, with a dose of 7 Gy. To decrease statistical variation, the treatment simulation was repeated three times for each dosimeter. The results obtained demonstrated the viability of TLDs for clinical applications from photon beams to small fields. The values showed agreement in percentage terms below ±5% as recommended by the ICRU. The greatest percentage difference found was 1.2% (LiF:Mg;Ti) in relation to the planning system. All thermoluminescent dosimeters presented an uncertainty relatively low, with good stability and reproducibility in all measurements. The 3D printed phantom showed the possibility of achieving real treatment conditions.

Assessment of energy and angular dependence of LiF:Mg,Ti dosimeters irradiated in the quantity Hp(0.07)

Radiation dosimetry has the purpose of quantifying the dose received by the occupationally exposed individual. The device used in this process is called a dosimeter, the dosimeter can be used in different situations, for example, the dosimeter used to quantify the dose received in the fingers is the ring model dosimeter, for the extremity, which is the focus of this work. In Brazil, we still do not have standards for the calibration of extremity dosimeters, therefore, in this work, the CASEC recommendations were used, adapted for extremity dosimetry. For a dosimeter to be used in its respective routine, it must present results within some pre-established limits in reference standards. For this purpose, energy dependence and angular dependence tests were carried out. To calibrate the LiF:Mg,Ti thermoluminescent dosimeters, a phantom rod was used. The phantom rod has the function of simulating the region of interest, in the case of this work, the fingers. The dosimeters were irradiated in the magnitude Hp(0.07), with the doses and energies recommended by the CASEC standard. The aim of this work is to characterize end dosimeters in the ring model with LiF:Mg,Ti detectors.

Characterization and calibration of thermoluminescent dosimeters of LiF:Mg, Ti in the quantity Hp(0.07)

The extremity dosimeters are devices used to quantifying the radiation dose that the occupationally exposed individual receives in specific regions of the body during the work time. Dosimeter calibration is essential so that the dosimeter response is equivalent to the received dose. Tests such as batch homogeneity and lower detection limit are part of the dosimeter calibration process. The rod phantom simulates the region of interest regarding the interaction with radiation and the scattered dose. The extremity dosimeters used were the LiF:Mg,Ti thermoluminescent dosimeters.

Fading study and readout optimization for routinely use of LiF:Mg,Ti thermoluminescent detectors for personal dosimetry

LiF:Mg,Ti thermoluminescent dosimeters (TLDs) are frequently used for personal dosimetry by individual monitoring services (IMS) all over the world. This study focused on the investigation and accurate description of thermoluminescent signal fading in these dosimeters. The routine readout procedure was performed with the controlled heating of the detector elements according to defined time-temperature profiles (TTPs). The output of this procedure was a temperature dependent light signal, called glow curve. Additionally, the readout optimization was achieved with the simulation and validation of glow curves with different fast readout TTPs. The TLDs were annealed, irradiated and read out with different storing times at an ambient temperature of 25 °C. Six batches of each 100 TLDs were irradiated and read out with different TTPs and compared afterwards. A fading study with several Harshaw TLD-100 dosimeters was performed, in which the results were responses to describe the pre-irradiation fading and the post-irradiation fading with mathematical functions that give a correction factor for the dose calculation.

Determination of surface dose in pencil beam scanning proton therapy.

Quantification of surface dose within the first few hundred water equivalent µm is challenging. Nevertheless, it is of large interest for the proton therapy community to study dose effects in the skin. The experimental determination is affected by the detector properties, such as the detector volume and material. TheInternational Commission on Radiation Units and Measurements in its report 39 recommends assessing the skin dose at a depth of 0.07mm. The aim of this study is the estimation of the absorbed dose at and around a depth of 70µm. We used various dosimetric approaches in conjunction with proton pencil beam scanning delivery to determine the skin dose in a clinical setting. Five different detectors were tested for determining the surface dose in water: EBT3 and HD-V2 GAFCHROMIC™ radiochromic film, LiF:Mg,Ti thermoluminescent dosimeter, IBA PPC05 plane-parallel ionization chamber, and PTW 23391 extrapolation chamber. The irradiation setup consisted of quasi-monoenergetic scanned proton pencil beams with kinetic energies of 100, 150, and 226.7MeV, respectively. Radiochromic films were placed within a vertical stack and in wedge geometry and were analyzed with FilmQA Pro™ adopting triple channel dosimetry. The extrapolation chamber PTW 23391, which served as a reference in the current work, was used in a conventional ionization chamber setup with a fixed electrode gap of 2mm. Three Kapton® entrance windows with thicknesses of 25, 50, and 75µm were employed. Thermoluminescent dosimeters were provided as powder and were pressed onto a sheet of aluminum. Furthermore,the Monte Carlo code TOol for PArticle Simulation (TOPAS) in version 3.1.p2 was used to model an IBA pencil beam scanning nozzle and score dose to water in a water phantom. The resulting depth dose curves were normalized to their 100% dose at the reference depth of 3cm. We obtained the skin doses with the extrapolation chamber and with TOPAS. For the experimental approach this resulted in 79.7±0.3%, 86.0±0.6%, and 87.1±0.1% for the proton energies 100, 150, and 226.7MeV, respectively. The results for TOPAS were 80.1±0.2% (100MeV), 87.1±0.5% (150MeV), and 86.9±0.4% (226.7MeV), respectively. Based on the experimental results of the skin dose, we provided a clinically relevant surface extrapolation factor for the common measurement methods. This allows the result of the first measurement depth of a detector to be scaled to the dose at the skin depth. Most practical would be the use of the surface extrapolation factor for the PPC05 chamber, due to its direct reading, the wide availability in clinics and the low uncertainties. The calculated factors were 0.986±0.004 for 100MeV, 0.961±0.008 for 150MeV, and 0.963±0.003 for 226.7MeV. In this study, dissimilar experimental approaches were evaluated with respect to measurements at depths close to the surface. The experimental depth dose curves are in good agreement with the simulation with TOPAS Monte Carlo. To the author's knowledge this was the first experimental determination of the skin dose according to the International Commission on Radiation Units and Measurements 39 definition in proton pencil beam scanning.

Pediatric patient and staff dose measurements in barium meal fluoroscopic procedures

Abstract This study investigates patient and staff dose measurements in pediatric barium meal series fluoroscopic procedures. It aims to analyze radiographic techniques, measure the air kerma-area product (PKA), and estimate the staff's eye lens, thyroid and hands equivalent doses. The procedures of 41 patients were studied, and PKA values were calculated using LiF:Mg,Ti thermoluminescent dosimeters (TLDs) positioned at the center of the patient’s upper chest. Furthermore, LiF:Mg,Cu,P TLDs were used to estimate the equivalent doses. The results showed a discrepancy in the radiographic techniques when compared to the European Commission recommendations. Half of the results of the analyzed literature presented lower PKA and dose reference level values than the present study. The staff‘s equivalent doses strongly depends on the distance from the beam. A 55-cm distance can be considered satisfactory. However, a distance decrease of ~20% leads to, at least, two times higher equivalent doses. For eye lenses this dose is significantly greater than the annual limit set by the International Commission on Radiological Protection. In addition, the occupational doses were found to be much higher than in the literature. Changing the used radiographic techniques to the ones recommended by the European Communities, it is expected to achieve lower PKA values ​​and occupational doses.

Comparison of doses to the rectum derived from treatment planning system with in-vivo dose values in vaginal vault brachytherapy using cylinder applicators.

PurposeIn-vivo measurements to determine doses to organs-at-risk can be an essential part of brachytherapy quality assurance (QA). This study compares calculated doses to the rectum with measured dose values as a means of QA in vaginal vault brachytherapy using cylinder applicators.Material and methodsAt the Department of Radiotherapy, University College Hospital (UCH), Ibadan, Nigeria, intracavitary brachytherapy (ICBT) was delivered by a GyneSource high-dose-rate (HDR) unit with 60Co. Standard 2D treatment plans were created with HDR basic 2.6 software for prescription doses 5-7 Gy at points 5 mm away from the posterior surface of vaginal cylinder applicators (20, 25, and 30 mm diameters). The LiF:Mg, Ti thermoluminescent dosimeter rods (1 x 6 mm) were irradiated to a dose of 7 Gy on Theratron 60Co machine for calibration purpose prior to clinical use. Measurements in each of 34 insertions involving fourteen patients were performed with 5 TLD-100 rods placed along a re-usable rectal marker positioned in the rectum. The dosimeters were read in Harshaw 3500 TLD reader and compared with doses derived from the treatment planning system (TPS) at 1 cm away from the dose prescription points.ResultsThe mean calculated and measured doses ranged from 2.1-3.8 Gy and 1.2-5.6 Gy with averages of 3.0 ± 0.5 Gy and 3.1 ± 1.1 Gy, respectively, for treatment lengths 2-8 cm along the cylinder-applicators. The mean values correspond to 48.9% and 50.8% of the prescribed doses, respectively. The deviations of the mean in-vivo doses from the TPS values ranged from –1.9 to 2.1 Gy with a p-value of 0.427.ConclusionsThis study was part of efforts to verify rectal dose obtained from the TPS during vaginal vault brachytherapy. There was no significant difference in the dose to the rectum from the two methods of measurements.

Dose response linearity and practical factors influencing minimum detectable dose for various thermoluminescent detector types

The minimum detectable dose (MDD) limit was examined in four different ways for groups of LiF:Mg,Ti thermoluminescent dosimeters, and two ways for CaF2:Dy, CaF2:Tm, CaF2:Mn, and CaSO4:Dy dosimeters. All types were irradiated and read out at dose intervals from 8.8 μGy to 6.6 mGy. Dose response linearity was never lost even for the lowest dose tested. As an ideal MDD, the signal arising from a zero applied dose readout was compared to calibration from true doses, resulting in signal corresponding to 0.04–0.1 μGy. The effects of fading and high ambient radon exposure on the MDD were examined.

Sorting a large set of heavily used LiF:Mg,Ti thermoluminescent detectors into repeatable subsets of similar response

A set of 920 heavily used LiF:Mg,Ti thermoluminescent dosimeters (TLDs) was placed into a polymethyl methacrylate (PMMA) plate attached to a 40×40×15cm3 PMMA phantom and irradiated to 4.52mGy using a 137Cs source. This was repeated three times to determine the mean and standard deviation of each TLD׳s sensitivity. Reader drift was tracked over time with 10 control dosimeters. Two test sets of 100 TLDs were divided into subsets with sensitivities within ±1% of their subset means. All dosimeters were re-irradiated four times to test the TLDs׳ response repeatability and determine the sensitivity uniformity within the subsets. Coefficients of variation revealed that, within a given subset, the dosimeters responded within ±2.5% of their subset mean in all calibrations. The coefficient of variation in any of the 200 TLDs׳ calibrations was below 6% across the four calibrations. The work validates the approach of performing three calibrations to separate heavily used and aged TLDs with overall sensitivity variations of ±25% into subsets that reproducibly respond within ±2.5%.

Influence of phantom materials on the energy dependence of LiF:Mg,Ti thermoluminescent dosimeters exposed to 20–300 kV narrow x-ray spectra, 137Cs and 60Co photons

LiF:Mg,Ti, are widely used to estimate absorbed-dose received by patients during diagnostic or medical treatment. Conveniently, measurements are usually made in plastic phantoms. However, experimental conditions vary from one group to another and consequently, a lack of consensus data exists for the energy dependence of thermoluminescent (TL) response. This work investigated the energy dependence of TLD-100 TL-response and the effect of irradiating the dosimeters in different phantom materials for a broad range of energy photons in an attempt to understand the parameters that affect the discrepancies reported by various research groups. TLD-100s were exposed to 20–300 kV narrow x-ray spectra, 137Cs and 60Co photons. Measurements were performed in air, PMMA, wt1, polystyrene and TLDS as surrounding material. Total air-kerma values delivered were between 50 and 150 mGy for x-rays and 50 mGy for 137Cs and 60Co beams; each dosimeter was irradiated individually. Relative response, R, defined as the TL-response per air-kerma and relative efficiency, RE, described as the TL-response per absorbed-dose (obtained through Monte Carlo (MC) and analytically) were used to describe the TL-response. Both R and RE are normalized to the responses in a 60Co beam. The results indicate that the use of different phantom materials affects the TL-response and this response varies with energy and material type. MC simulations reproduced qualitatively the experimental data: a) R increases, reaches a maximum at ~25 keV and decreases; b) RE decreases, down to a minimum at ~60 keV, increases to a maximum at ~150 keV and after decreases. Independent of the phantom materials, RE strongly depends on how the absorbed dose is evaluated and the discrepancies between RE evaluated analytically and by MC simulation are around 4% and 18%, dependent on the photon energy. The comparison between our results and that reported in the literature suggests that the discrepancy observed between different research groups appears to be most likely related to supralinearity effect, phantom materials, difference on the energy-spectra and geometry conditions during each experiment rather than parameters such as heating-rate or annealing procedure, which was supported by MC simulation. From the results obtained in this work and the strict analysis performed, we can conclude that for clinical applications of TLD-100, special attention must be taken when published data are used to convert TL calibration curve from 60Co to low-energy photons. Otherwise, this can lead to incorrect results when later used to measure absorbed dose in human tissue.

SU‐F‐19A‐05: Experimental and Monte Carlo Characterization of the 1 Cm CivaString 103Pd Brachytherapy Source

Purpose:To determine the in‐air azimuthal anisotropy and in‐water dose distribution for the 1 cm length of the CivaString 103Pd brachytherapy source through measurements and Monte Carlo (MC) simulations. American Association of Physicists in Medicine Task Group No. 43 (TG‐43) dosimetry parameters were also determined for this source.Methods:The in‐air azimuthal anisotropy of the source was measured with a NaI scintillation detector and simulated with the MCNP5 radiation transport code. Measured and simulated results were normalized to their respective mean values and compared. The TG‐43 dose‐rate constant, line‐source radial dose function, and 2D anisotropy function for this source were determined from LiF:Mg,Ti thermoluminescent dosimeter (TLD) measurements and MC simulations. The impact of 103Pd well‐loading variability on the in‐water dose distribution was investigated using MC simulations by comparing the dose distribution for a source model with four wells of equal strength to that for a source model with strengths increased by 1% for two of the four wells.Results:NaI scintillation detector measurements and MC simulations of the in‐air azimuthal anisotropy showed that ≥95% of the normalized data were within 1.2% of the mean value. TLD measurements and MC simulations of the TG‐43 dose‐rate constant, line‐source radial dose function, and 2D anisotropy function agreed to within the experimental TLD uncertainties (k=2). MC simulations showed that a 1% variability in 103Pd well‐loading resulted in changes of <0.1%, <0.1%, and <0.3% in the TG‐43 dose‐rate constant, radial dose distribution, and polar dose distribution, respectively.Conclusion:The CivaString source has a high degree of azimuthal symmetry as indicated by the NaI scintillation detector measurements and MC simulations of the in‐air azimuthal anisotropy. TG‐43 dosimetry parameters for this source were determined from TLD measurements and MC simulations. 103Pd well‐loading variability results in minimal variations in the in‐water dose distribution according to MC simulations.This work was partially supported by CivaTech Oncology, Inc. through an educational grant for Joshua Reed, John Micka, Wesley Culberson, and Larry DeWerd and through research support for Mark Rivard.

Experimental and Monte Carlo dosimetric characterization of a 1 cm (103)Pd brachytherapy source.

To determine the in-air azimuthal anisotropy and in-water dose distribution for the 1cm length of a new elongated (103)Pd brachytherapy source through both experimental measurements and Monte Carlo (MC) simulations. Measured and MC-calculated dose distributions were used to determine the American Association of Physicists in Medicine Task Group No. 43 (TG-43) dosimetry parameters for this source. The in-air azimuthal anisotropy of the source was measured with a NaI scintillation detector and was simulated with the MCNP5 radiation transport code. Measured and MC results were normalized to their respective mean values and then compared. The source dose distribution was determined from measurements with LiF:Mg,Ti thermoluminescent dosimeter (TLD) microcubes and MC simulations. TG-43 dosimetry parameters for the source, including the dose-rate constant, Λ, two-dimensional anisotropy function, F(r, θ), and line-source radial dose function, gL(r), were determined from the TLD measurements and MC simulations. NaI scintillation detector measurements and MC simulations of the in-air azimuthal anisotropy of the source showed that ≥95% of the normalized values for each source were within 1.2% of the mean value. TLD measurements and MC simulations of Λ, F(r, θ), and gL(r) agreed to within the associated uncertainties. This new (103)Pd source exhibits a high level of azimuthal symmetry as indicated by the measured and MC-calculated results for the in-air azimuthal anisotropy. TG-43 dosimetry parameters for the source were determined through TLD measurements and MC simulations.

The use of TLDs for brachytherapy dosimetry

Thermoluminescent dosimeters (TLDs) are routinely used to measure the dose around brachytherapy sources due to their small size and high precision. This work presents a concise overview of the use of LiF:Mg,Ti TLDs for brachytherapy dosimetry including the experimental procedures required to achieve high-precision measurements as well as new results regarding the intrinsic energy dependence with some of the commonly used brachytherapy sources. Equations to correct TLD light output to air kerma are outlined and a description of the method to determine the intrinsic energy dependence is presented. For the intrinsic energy dependence investigation, a review of previously published results is presented as well as new experimental results using 125I, 103Pd, 192Ir, and miniature x-ray brachytherapy sources at the University of Wisconsin Medical Radiation Research Center (UWMRRC). The results of these experiments are consistent with previous work and give valuable insight to investigators using TLDs for brachytherapy measurements.

TL response of pairs of 6LiF:Mg,Cu,Si/7LiF:Mg,Cu,Si and TLD-600/TLD-700 to 0.1–12 MeV neutrons

Abstract Neutron dosimetery continues to remain an important and a challenging aspect of radiation protection due to the higher biological effectiveness of neutrons than that of gamma rays and the intricacy in the responses of the detectors. The need for personal dosimetry in mixed fields of neutrons and gamma rays has considerably increased due to the rising number of nuclear facilities, nuclear power plants, medical therapy equipment, accelerators and so on. The most widely used technique for personal dosimetry has been the albedo technique employing pairs of neutron sensitive 6LiF:Mg,Ti (TLD-600) and neutron insensitive 7LiF:Mg,Ti (TLD-700) thermoluminescent dosimeters (TLDs). Off late, LiF:Mg,Cu,Si has emerged as one of the most promising TLD material, having the advantages of high sensitivity, near tissue equivalence to gamma rays, negligible fading on pre and post-irradiation storage, thermal stability for readout and negligible residual signal after the readout. In this study, neutron energy response of indigenously developed 6LiF:Mg,Cu,Si and 7LiF:Mg,Cu,Si TLD pairs was evaluated to neutrons of energy from thermal to 11.6 MeV and was compared with the response of TLD-600 and TLD-700 pairs. The net TL per unit neutron dose for the 6LiF:Mg,Cu,Si/7LiF:Mg,Cu,Si TLD pair was found to be about 10 times of that of the TLD-600/TLD-700 pair. Unlike, TLD-600 and TLD-700, the glow curve structure of 6LiF:Mg,Cu,Si and 7LiF:Mg,Cu,Si remained almost the same for all the irradiations. Thus, 6LiF:Mg,Cu,Si and 7LiF:Mg,Cu,Si TLDs provided a better alternate to TLD-600 and TLD-700 for the dosimetry of mixed fields of neutrons and gamma rays.

Calibration of Thermoluminescent Dosimeters for Measuring Effective Dose in Computer Tomography

A method for calibration of LiF: Mg, Ti thermoluminescent dosimeters is presented. The calibrated dosimeters are used in various CT studies involving anthropomorphic phantoms of the human body for estimating the reference levels of effective patient dose. It is shown that application of the method in some cases improves the accuracy of dose estimates by 80% versus conventional calibration in air using standard radiation sources.

Radiation dose reduction by using low dose CT protocol of thorax

Abstract CT makes up to 67% of radiation from medical sources. Therefore emphasizes is especially on the importance of reducing doses and the introduction of procedures with the lowest possible dose received by the patient. The so called low dose CT protocol was studied for some chest diagnosis however it was not investigated for the patients with lymphoproliferative diseases. Lymphoproliferative diseases, including most lymphomas involve a large number of younger and middle age patients i.e. patients in the reproductive period of life. A CT exam of these patients is indispensable method for the diagnosis, and later for long life monitoring of the effectiveness of therapy. The aim of this work was to compare the patient doses between two different CT protocols of thorax in patients with this diagnosis. The entrance surface doses were measured and compared using standard CT protocol of thorax which implies 120 kV and 160 mA conditions and low dose CT protocol using 120 kV and 30 mA conditions while maintaining image quality. The study involved 60 patients. Each patient underwent two different CT thorax protocols during regular follow up of the disease. The doses were measured using LiF:Mg,Ti (TLD-100) thermoluminescent (TL) and GD-352M radiophotoluminescent (RPL) dosimeters on the thyroid, eye lens, sternum and gonads. The results showed that low dose protocol yielded with the reduction of doses by the factor of 1.8–15 on the eye lens, 1.0–9.1 on the thyroid, 2.5–7.0 on sternum and 0.3–12.8 on gonads, respectively. The doses were significantly lower using low dose CT protocol while the image quality for lymph node presentation was satisfactory according to European criteria. Therefore the use of low dose CT protocol as a standard for patients with lymphoproliferative disorders is highly recommended.

Determination of absorbed dose to water around a clinical HDR192Ir source using LiF:Mg,Ti TLDs demonstrates an LET dependence of detector response

Experimental radiation dosimetry with thermoluminescent dosimeters (TLDs), calibrated in a (60)Co or megavoltage (MV) photon beam, is recommended by AAPM TG-43U1for verification of Monte Carlo calculated absorbed doses around brachytherapy sources. However, it has been shown by Carlsson Tedgren et al. [Med. Phys. 38, 5539-5550 (2011)] that for TLDs of LiF:Mg,Ti, detector response was 4% higher in a (137)Cs beam than in a (60)Co one. The aim of this work was to investigate if similar over-response exists when measuring absorbed dose to water around (192)Ir sources, using LiF:Mg,Ti dosimeters calibrated in a 6 MV photon beam. LiF dosimeters were calibrated to measure absorbed dose to water in a 6 MV photon beam and used to measure absorbed dose to water at distances of 3, 5, and 7 cm from a clinical high dose rate (HDR) (192)Ir source in a polymethylmethacrylate (PMMA) phantom. Measured values were compared to values of absorbed dose to water calculated using a treatment planning system (TPS) including corrections for the difference in energy absorption properties between calibration quality and the quality in the users' (192)Ir beam and for the use of a PMMA phantom instead of the water phantom underlying dose calculations in the TPS. Measured absorbed doses to water around the (192)Ir source were overestimated by 5% compared to those calculated by the TPS. Corresponding absorbed doses to water measured in a previous work with lithium formate electron paramagnetic resonance (EPR) dosimeters by Antonovic et al. [Med. Phys. 36, 2236-2247 (2009)], using the same irradiation setup and calibration procedure as in this work, were 2% lower than those calculated by the TPS. The results obtained in the measurements in this work and those obtained using the EPR lithium formate dosimeters were, within the expanded (k = 2) uncertainty, in agreement with the values derived by the TPS. The discrepancy between the results using LiF:Mg,Ti TLDs and the EPR lithium formate dosimeters was, however, statistically significant and in agreement with the difference in relative detector responses found for the two detector systems by Carlsson Tedgren et al. [Med. Phys. 38, 5539-5550 (2011)] and by Adolfsson et al. [Med. Phys. 37, 4946-4959 (2010)]. When calibrated in (60)Co or MV photon beams, correction for the linear energy transfer (LET) dependence of LiF:Mg,Ti detector response will be needed as to measure absorbed doses to water in a (192)Ir beam with highest accuracy. Such corrections will depend on the manufacturing process (MTS-N Poland or Harshaw TLD-100) and details of the annealing and read-out schemes used.

Response of LiF:Mg,Ti thermoluminescent dosimeters at photon energies relevant to the dosimetry of brachytherapy (<1 MeV)

High energy photon beams are used in calibrating dosimeters for use in brachytherapy since absorbed dose to water can be determined accurately and with traceability to primary standards in such beams, using calibrated ion chambers and standard dosimetry protocols. For use in brachytherapy, beam quality correction factors are needed, which include corrections for differences in mass energy absorption properties between water and detector as well as variations in detector response (intrinsic efficiency) with radiation quality, caused by variations in the density of ionization (linear energy transfer (LET) -distributions) along the secondary electron tracks. The aim of this work was to investigate experimentally the detector response of LiF:Mg,Ti thermoluminescent dosimeters (TLD) for photon energies below 1 MeV relative to (60)Co and to address discrepancies between the results found in recent publications of detector response. LiF:Mg,Ti dosimeters of formulation MTS-N Poland were irradiated to known values of air kerma free-in-air in x-ray beams at tube voltages 25-250 kV, in (137)Cs- and (60)Co-beams at the Swedish Secondary Standards Dosimetry Laboratory. Conversions from air kerma free-in-air into values of mean absorbed dose in the dosimeters in the actual irradiation geometries were made using EGSnrc Monte Carlo simulations. X-ray energy spectra were measured or calculated for the actual beams. Detector response relative to that for (60)Co was determined at each beam quality. An increase in relative response was seen for all beam qualities ranging from 8% at tube voltage 25 kV (effective energy 13 keV) to 3%-4% at 250 kV (122 keV effective energy) and (137)Cs with a minimum at 80 keV effective energy (tube voltage 180 kV). The variation with effective energy was similar to that reported by Davis et al. [Radiat. Prot. Dosim. 106, 33-43 (2003)] with our values being systematically lower by 2%-4%. Compared to the results by Nunn et al. [Med. Phys. 35, 1861-1869 (2008)], the relative detector response as a function of effective energy differed in both shape and magnitude. This could be explained by the higher maximum read-out temperature (350 °C) used by Nunn et al. [Med. Phys. 35, 1861-1869 (2008)], allowing light emitted from high-temperature peaks with a strong LET dependence to be registered. Use of TLD-100 by Davis et al. [Radiat. Prot. Dosim. 106, 33-43 (2003)] with a stronger super-linear dose response compared to MTS-N was identified as causing the lower relative detector response in this work. Both careful dosimetry and strict protocols for handling the TLDs are required to reach solid experimental data on relative detector response. This work confirms older findings that an over-response relative to (60)Co exists for photon energies below 200-300 keV. Comparison with the results from the literature indicates that using similar protocols for annealing and read-out, dosimeters of different makes (TLD-100, MTS-N) differ in relative detector response. Though universality of the results has not been proven and further investigation is needed, it is anticipated that with the use of strict protocols for annealing and read-out, it will be possible to determine correction factors that can be used to reduce uncertainties in dose measurements around brachytherapy sources at photon energies where primary standards for absorbed dose to water are not available.

Characteristics of an OSLD in the diagnostic energy range

Optically stimulated luminescence (OSL) dosimetry has been recently introduced in radiation therapy as a potential alternative to the thermoluminescent dosimeter (TLD) system. The aim of this study was to investigate the feasibility of using OSL point dosimeters in the energy range used in diagnostic imaging. NanoDot OSL dosimeters (OSLDs) were used in this study, which started with testing the homogeneity of a new packet of nanoDots. Reproducibility and the effect of optical treatment (bleaching) were then examined, followed by an investigation of the effect of accumulated dose on the OSLD indicated doses. OSLD linearity, angular dependence, and energy dependence were also studied. Furthermore, comparison with LiF:Mg,Ti TLD chips using standard CT dose phantoms at 80 and 120 kVp settings was performed. Batch homogeneity showed a coefficient of variation of <5%. Single-irradiation measurements with bleaching after each OSL readout was found to be associated with a 3.3% reproducibility (one standard deviation measured with a 8 mGy test dose), and no systematic change in OSLDs sensitivity could be noted from measurement to measurement. In contrast, the multiple-irradiation readout without bleaching in between measurements was found to be associated with an uncertainty (using a 6 mGy test dose) that systematically increased with accumulated dose, reaching 42% at 82 mGy. Good linearity was shown by nanoDots under general x-ray, CT, and mammography units with an R2 > 0.99. The angular dependence test showed a drop of approximately 70% in the OSLD response at 90 degrees in mammography (25 kVp). With the general radiography unit, the maximum drop was 40% at 80 kVp and 20% at 120 kVp, and it was only 10% with CT at both 80 and 120 kVp. The energy dependence study showed a range of ion chamber-to-OSLDs ratios between 0.81 and 1.56, at the energies investigated (29-62 keV). A paired t-test for comparing the OSLDs and TLDs showed no significant variation (p > 0.1). OSLDs exhibited good batch homogeneity (<5%) and reproducibility (3.3%), as well as a linear response. In addition, they showed no statistically significant difference with TLDs in CT measurements (p > 0.1). However, high uncertainty (42%) in the dose estimate was found as a result of relatively high accumulated dose. Furthermore, nanoDots showed high angular dependence (up to 70%) in low kVp techniques. Energy dependence of about 60% was found, and correction factors were suggested for the range of energies investigated. Therefore, if angular and energy dependences are taken into consideration and the uncertainty associated with accumulated dose is avoided, OSLDs (nanoDots) can be suitable for use as point dosimeters in diagnostic settings.

Reproducibility of glow peak fading characteristics of thermoluminescent dosimeters

Abstract This study examines the reproducibility of LiF:Mg,Ti pre-irradiation and post-irradiation fading rates. To test post-irradiation fading, 99 calibrated 3.2 × 3.2 × 0.9 mm 3 LiF:Mg,Ti thermoluminescent dosimeters (TLDs) of dimensions were first annealed. The set was then irradiated to an average of 4.4 mGy using a 3.2 × 10 11 Bq 137 Cs source. The irradiated TLDs were allowed to fade at room temperature for 0.5, 1, 2, 3, 4, 8, or 17 d. The TLDs were then read out over a 3.5 h time period using a standard commercial hot planchet TLD reader with nitrogen gas to prevent chemiluminescence. The experiment was then repeated for the other fading times. To test pre-irradiation fading, the TLDs were annealed, then placed in storage for 0.5, 1.6, 3, 4, 8, 18, 27, or 43 d. After that time, the TLDs were irradiated and read out. A computerized glow curve deconvolution program was used to fit the glow curve data to a four-peak first-order Gaussian kinetics model. The areas of peaks 2, 3, 4, and 5 were retrieved in order to be studied alone, as well as normalized to other peaks. The peak areas and peak area ratios were then fit to two or three term exponential decay equations that allowed the determination of the fading rate by calculating their parameters. These parameters were then compared for different TLDs in order to determine if their values followed a Gaussian distribution. Then, a group mean and standard deviation could be used for each value for all TLDs for later use instead of calculating an individual mean for each TLD.