Thermoluminescent Dosimeters Results Research Articles

Assessment of out-of-field radiation doses for high dose per pulse intraoperative electron beam radiotherapy using TLD-100

PurposeThis research aims to assess the out-of-field surface doses for different combinations of electron energy, applicator size, and distance from the applicator wall of high dose-per-pulse IntraOperative electron Radiation Therapy (IOeRT) using thermoluminescent dosimeters (TLDs). Materials and methodsThe out-of-field doses for various combinations of IOeRT electron beam energy/applicator size were measured using ionometric dosimetry. The calibrated LiF:Mg,Ti dosimeters (TLD-100) were used to make the measurements. TLD results were benchmarked using an Advanced Markus chamber and a polymethyl methacrylate slab phantom. TLDs were read out using a HARSHAW (type 4000) manual TLD reader. ResultsThe TLDs received out-of-field absorbed doses ranging from 0.033 to 0.545 mGy/MU depending on the applicator size, electron energy, and distance from the applicator's outer wall. The mean discrepancies between TLDs and Advanced Markus ion chamber measured data were about 1.82% in the PMMA slab phantom at a depth of maximum dose for each electron energy. ConclusionThe skin surface dose can be significant during IOeRT. The maximum out-of-field doses have occurred at high electron beam energy and closer distances around the applicator wall. TLD can measure the skin's dose level according to its good agreement with the reference Advanced Markus dosimeter. The TLD-100 can be used as a reliable dosimeter for intraoperative radiation dosimetry.

3D in vivo dosimetry of HDR gynecological brachytherapy using micro silica bead TLDs.

PurposeThis study aimed to evaluate the feasibility of defining an in vivo dosimetry (IVD) protocol as a patient‐specific quality assurance (PSQA) using the bead thermoluminescent dosimeters (TLDs) for point and 3D IVD during brachytherapy (BT) of gynecological (GYN) cancer using 60Co high‐dose‐rate (HDR) source.MethodsThe 3D in vivo absorbed dose verification within the rectum and bladder as organs‐at‐risk was performed by bead TLDs for 30 GYN cancer patients. For rectal wall dosimetry, 80 TLDs were placed in axial arrangements around a rectal tube covered with a layer of gel. Ten beads were placed inside the Foley catheter to get the bladder‐absorbed dose. Beads TLDs were localized and defined as control points in the treatment planning system (TPS) using CT images of the patients. Patients were planned and treated using the routine BT protocol. The experimentally obtained absorbed dose map of the rectal wall and the point dose of the bladder were compared to the TPSs predicted absorbed dose at these control points.ResultsRelative difference between TPS and TLDs results were −8.3% ± 19.5% and −7.2% ± 14.6% (1SD) for rectum‐ and bladder‐absorbed dose, respectively. Gamma analysis was used to compare the calculated with the measured absorbed dose maps. Mean gamma passing rates of 84.1%, 90.8%, and 92.5% using the criteria of 3%/2 mm, 3%/3 mm, and 4%/2 mm were obtained, respectively. Eventually, a “considering level” of at least 85% as pass rate with 4%/2‐mm criteria was recommended.ConclusionsA 3D IVD protocol employing bead TLDs was presented to measure absorbed doses delivered to the rectum and bladder during GYN HDR‐BT as a reliable PSQA method. 3D rectal absorbed dose measurements were performed. Differences between experimentally measured and planned absorbed dose maps were presented in the form of a gamma index, which may be used as a warning for corrective action.

64. Skin dose in high dose rate brachytherapy for breast cancers: evaluation by in vivo dosimetry using TLD and MOSFET dosimeters

Purpose The aim of this work is to evaluate a novel approach for the verification of the real skin dose in partial breast high dose rate brachytherapy (PB-BRT), quantify the treatment planning system (TPS) overestimation and develop a methodology for routinary in vivo dosimetry (IVD). Methods Currently there is not a general acknowledged constraint for skin dose in PB-BRT. Because of our center’s experience (20 PB-BRT patients treated/year) we achieve a good cosmetic outcome if the TPS calculated skin dose is below 50% of the prescription of 32 Gy (4 Gy/fraction, twice daily). The skin dose was calculated by a TPS which overestimates it assuming a homogeneous water medium and not accounting for the finite patient dimensions. IVD was performed on patients treated with multi-catheter brachytherapy, which involves placement of 9–15 catheters through the breast. Thermoluminescent dosimeters (TLDs) were used in the IVD sessions and they were placed on the skin next to the catheters and the nipple. We are currently implementing MOSFET real time measurements to compare with TLD results and to further improve the estimation of the skin dose. Results The doses measured by TLDs were compared to those calculated by the TPS in specific region of interest (ROI). For each patient two IVD sessions were performed, and the reproducibility of TLD positioning was assessed: 60% of couples give the same value within the experimental error. 77% of measurements show a clear overestimation of skin dose by the TPS. A Gaussian fit of the percentage differences between measured and calculated doses (Fig. 1) yielded an overestimation value of (25.2 ± 7.6)%. For a first group of patients IVD sessions were performed using MOSFET dosimeters, placed on the skin following the previous arrangement of TLDs. The doses measured by MOSFETs were compared to those calculated by the TPS in specific ROIs. Preliminary results show a quantitatively similar overestimation. Conclusions Measurements in PB-BRT are necessary to quantify the real skin dose accurately. A novel approach based on IVD allows us to define dose limits, to achieve not only the tumor control but also a good cosmetic outcome to improve the quality of patient’s life. Download : Download high-res image (131KB) Download : Download full-size image

Dosimetry audit of radiotherapy treatment planning systems.

In radiotherapy Treatment Planning Systems (TPS) various calculation algorithms are used. The accuracy of dose calculations has to be verified. Numerous phantom types, detectors and measurement methodologies are proposed to verify the TPS calculations with dosimetric measurements. A heterogeneous slab phantom has been designed within a Coordinated Research Project (CRP) of the IAEA. The heterogeneous phantom was developed in the frame of the IAEA CRP. The phantom consists of frame slabs made with polystyrene and exchangeable inhomogeneity slabs equivalent to bone or lung tissue. Special inserts allow to position thermoluminescent dosimeters (TLD) capsules within the polystyrene slabs below the bone or lung equivalent slabs and also within the lung equivalent material. Additionally, there are inserts that allow to position films or ionisation chamber in the phantom. Ten Polish radiotherapy centres (of 30 in total) were audited during on-site visits. Six different TPSs and five calculation algorithms were examined in the presence of inhomogeneities. Generally, most of the results from TLD were within 5 % tolerance. Differences between doses calculated by TPSs and measured with TLD did not exceed 4 % for bone and polystyrene equivalent materials. Under the lung equivalent material, on the beam axis the differences were lower than 5 %, whereas inside the lung equivalent material, off the beam axis, in some cases they were of around 7 %. The TLD results were confirmed with the ionisation chamber measurements. The comparison results of the calculations and the measurements allow to detect limitations of TPS calculation algorithms. The audits performed with the use of heterogeneous phantom and TLD seem to be an effective tool for detecting the limitations in the TPS performance or beam configuration errors at audited radiotherapy departments.

Feasibility of lateral dose profile measurements in a small field using TLDs

The purpose of this work was to study the feasibility of lateral dose profile measurements in a small field using thermoluminescent dosimeters (TLDs) and to evaluate the impact of the field size on the absorbed dose ratio factor fmd of LiF and Al2O3 TLDs. The Monte Carlo package BEAM/EGSNRC was used to simulate the lateral dose profile in solid water phantoms (RW3 slab phantom) with various field sizes beyond the build-up region for 6 MV x-rays, and a LiF : Mg, Cu, P (GR-200) dosimeter with dimensions of 0.1 × 0.1 × 0.1 cm3 was used to measure the lateral dose profile under the same conditions as the Monte Carlo simulations. To enable comparisons between dosimeters, Gafchromic® EBT3 films were used. The results indicate that (1) the measured results are in agreement with the simulated results within the uncertainty of the simulation; (2) the values of fmd for Al2O3 and LiF in a 1 × 1 cm2 field are 2.8% and 1.6% less, respectively, than those in a 10 × 10 cm2 field; and (3) within the 80% profile region, the dose differences between TLDs and solid water are less than 1%. In the 80–10% profile region, the TLD results are in agreement with the absorbed doses in the solid water within 1 mm. It is generally acceptable to ignore the impact of field size on the absorbed dose ratio factor fmd when the field sizes are larger than 1 × 1 cm2 for LiF and 2 × 2 cm2 for Al2O3. For 6 MV x-rays, the small GR-200 chip can be used to measure the relative lateral dose profiles of small fields.

Australasian brachytherapy audit: Results of the ‘end‐to‐end’ dosimetry pilot study

We present the results of a pilot study to test the feasibility of a brachytherapy dosimetry audit. The feasibility study was conducted at seven sites from four Australian states in both public and private centres. A purpose-built cylindrical water phantom was imaged using the local imaging protocol and a treatment plan was generated to deliver 1 Gy to the central (1 of 3) thermoluminescent dosimeter (TLD) from six dwell positions. All centres completed the audit, consisting of three consecutive irradiations, within a 2-h time period, with the exception of one centre that uses a pulsed dose rate brachytherapy unit. All TLD results were within 4.5% of the predicted value, with the exception of one subset where the dwell position step size was incorrectly applied. While the limited data collected in the study demonstrated considerable heterogeneity in clinical practice, the study proved a brachytherapy dosimetry audit to be feasible. Future studies should include verification of source strength using a Standard Dosimetry Laboratory calibrated chamber, a phantom that more closely mimics the clinical situation, a more comprehensive review of safety and quality assurance (QA) procedures including source dwell time and position accuracy, and a review of patient treatment QA procedures such as applicator position verification.

Dosimetry of MammoSite® applicator: Comparison between Monte Carlo simulation, measurements, and treatment planning calculation

To investigate the dosimetric characteristics of accelerated partial breast irradiation technique by MammoSite® applicator using thermoluminescent dosimeter (TLD) and Monte Carlo simulation to comparing them with treatment planning system calculation for planning target volume (PTV) and organs at risk such as skin, lung and chest wall. The Monte Carlo MCNP-5 code was used to simulate dose rate in the PTV that is a MammoSite® balloon with 1 cm margin around it. Experimental dosimetry was carried out within a female-equivalent chest phantom with TLD dosimeter after insertion of 192 Ir source into the MammoSite® applicator. Three dimensional planning (TP) was done for dose delivery to the specific points within the phantom by means of FlexiPlan software. Statistical comparisons were done between TP calculation, Monte Carlo simulation and TLD. Our results showed good agreement for surface doses between simulation and measurement. The mean skin dose for the simulation and TLD result was 61.7% and 56.8% of prescription dose, respectively. The maximum dose to the chest wall for Monte Carlo and TLD were 114.4% and 111.8% of prescription dose, respectively. The maximum dose to the lung for Monte Carlo and TLD results were 28.4% and 27.3% of prescription dose, respectively. Using Monte Carlo simulation and an average female chest phantom, it was possible to demonstrate the accuracy on the calculated dose rate in the PTV of a MammoSite® dose delivery system with 192 Ir HDR sources. The results showed acceptable agreement between simulation, treatment planning, and experimental dosimetry results.

Comparison of TLD calibration methods for dosimetry

For the purpose of dose measurement using a high‐dose rate 192Ir source, four methods of thermoluminescent dosimeter (TLD) calibration were investigated. Three of the four calibration methods used the 192Ir source. Dwell times were calculated to deliver 1 Gy to the TLDs irradiated either in air or water. Dwell time calculations were confirmed by direct measurement using an ionization chamber. The fourth method of calibration used 6 MV photons from a medical linear accelerator, and an energy correction factor was applied to account for the difference in sensitivity of the TLDs in 192Ir and 6 M V. The results of the four TLD calibration methods are presented in terms of the results of a brachytherapy audit where seven Australian centers irradiated three sets of TLDs in a water phantom. The results were in agreement within estimated uncertainties when the TLDs were calibrated with the 192Ir source. Calibrating TLDs in a phantom similar to that used for the audit proved to be the most practical method and provided the greatest confidence in measured dose. When calibrated using 6 MV photons, the TLD results were consistently higher than the 192Ir−calibrated TLDs, suggesting this method does not fully correct for the response of the TLDs when irradiated in the audit phantom.PACS number: 87

SU‐FF‐T‐276: Utilization of Conebeam CT to Accurately Assess TLD Placement for In‐Vivo Dosimetry

Purpose: Assessing the predicted dose at the thermoluminescent dosimeter (TLD) location is one of difficulties when utilizing in‐vivo TLD dosimetry. Accurate dose prediction at the exact TLD location increases confidence in the measured TLD results. Method and Materials: Prior to localization and treatment, TLDs are placed according to physician's direction. A conebeam CT (CBCT) image set, in treatment position and encompassing all TLD locations, is obtained. The patient's treatment is delivered and the TLDs are sent for analysis (University of Wisconsin, Radiation Calibration Laboratory). The acquired CBCT image set is imported into the Eclipse treatment planning computer and the data set is rigidly fused with the initial treatment planning simulation CT data set. The TLDs are visible on the CBCT data set, allowing for visualization of the TLD location on initial planning CT which contains the calculated dose distribution. The calculated dose at the exact location of the TLD is obtained for comparison to the measured TLDs. Thus, the calculated dose at the exact TLD position is obtained for comparison to the measured TLD results. From 2008 – 2009, three patients underwent treatment for scalp lesions, resulting in twelve TLD measurements from the above described technique. Results: All TLD results were corrected for contribution from the CBCT acquisition. The percentage agreement of measured vs. predicted TLD doses was 0.2 ± 7.5 %. Conclusion: These results indicate that CBCT can be used to predict the dose at the exact TLD location during treatment within 10%. Additionally, CBCT removes uncertainty in TLD location on the patient at the time of exposure.

Estimation of organ and effective doses resulting from cone beam CT imaging for radiotherapy treatment planning

In this study, organ doses were measured for various kilovoltage cone beam CT exposures on the Varian Acuity simulator and an alternative method of dose estimation was also assessed. Organ doses were measured by distributing thermoluminescent dosimeters (TLDs) throughout an anthropomorphic phantom, and effective doses were calculated using International Commission on Radiological Protection (ICRP) 60 and ICRP 103 tissue-weighting factors. The ImPACT CT patient dosimetry calculator was also used to estimate doses for comparison with the TLD results. Effective doses of 15.3 mSv (19.4 mSv), 14.3 mSv (9.7 mSv) and 2.8 mSv (3.2 mSv) were calculated from the TLD measurements and ICRP 60 (ICRP 103) weighting factors for breast, pelvis and head acquisitions, respectively. When a 10 cm pencil ionisation chamber was used to measure the CT dose index, the ImPACT calculator was found to provide an adequate estimation of dose when compared with the TLD results. However, the doses for half-fan exposures were found to be overestimated, with the extent of overestimation depending on the radiosensitive organs irradiated. The organ and effective doses reported provide information for justification and optimisation of cone beam CT procedures, and are compared with doses delivered by other imaging devices. The ImPACT calculator may be used to estimate doses from cone beam CT procedures, if the potential for overestimation is acknowledged.

Validation of Metal Oxide Semiconductor Field Effect Transistor Technology for Organ Dose Assessment During CT: Comparison with Thermoluminescent Dosimetry

The purposes of this study were to apply near-real-time dose-measurement technology with metal oxide semiconductor field effect transistors (MOSFETs) to the assessment of organ dose during CT and to validate the method in comparison with the thermoluminescent dosimeter (TLD) method. Dosimetry measurements were performed in two ways, one with TLDs and the other with MOSFETs. Twenty organ locations were selected in an adult anthropomorphic female phantom. High-sensitivity MOSFET dosimeters were used. For the reference standard, TLDs were placed in the same organ locations as the MOSFETs. Both MOSFET and TLD detectors were calibrated with an X-ray beam equivalent in quality to that of a commercial CT scanner (half-value layer, approximately 7 mm Al at 120 kVp). Organ dose was determined with a scan protocol for pulmonary embolus studies on a 4-MDCT scanner. Measurements for selected organ doses and the percentage difference for TLDs and MOSFETs, respectively, were as follows: thyroid (0.34 cGy, 0.31 cGy, -8%), middle lobe of lung (2.4 cGy, 3.0 cGy, +26%), bone marrow of thoracic spine (2.2 cGy, 2.5 cGy, +11%), stomach (1.0 cGy, 0.93 cGy, -6%), liver (2.5 cGy, 2.6 cGy, +6%), and left breast (3.0 cGy, 2.9 cGy, -1%). Bland-Altman analysis showed that the MOSFET results agreed with the TLD results (bias, 0.042). We found good agreement between the results with the MOSFET and TLD methods. Near-real-time CT organ dose assessment not previously feasible with TLDs was achieved with MOSFETs. MOSFET technology can be used for protocol development in the rapidly changing MDCT scanner environment, in which organ dose data are extremely limited.

Po‐Poster ‐ 12: Evaluation of MOSFETS for Gamma Knife® helmet factor measurements

The Gamma Knife ® is calibrated annually using the 18 mm collimator helmet; the dose rates for the other helmets are determined by measuring the radiation output relative to the 18 mm helmet. These relative measurements yield a helmet factor for each collimator helmet, which is incorporated into the treatment planning software, and is validated during annual quality assurance testing. Traditionally, helmet factors have been measured using 1×1×1 mm3 thermoluminescent dosimeters (TLDs). Determining helmet factors using this technique is a challenging process since the TLDs are small and fragile, and their preparation, calibration and readout is very time consuming. They also have a large uncertainty of 2 to 5%. The goal of this research is to investigate the use of metal oxide semiconductor field effect transistor dosimeters (MOSFETs) for Gamma Knife® helmet factor measurements. The MOSFET linearity and reproducibility was first characterized using a teletherapy unit; the linearity and reproducibility were both found to be within 2%. The helmet factors were then measured using TLDs, and again using MOSFETs. The TLD results gave helmet factors within 2.5% of the manufacturer's values; the MOSFETs provided helmet factors within 1.3% of the manufacturer's values, in a fraction of the time it took to generate the TLD results. This investigation confirms that MOSFETs can be used to measure Gamma Knife® helmet factors more quickly and accurately than TLDs.

Measurements and calculations of thermal neutron fluence rate and neutron energy spectra resulting from moderation of 252Cf fast neutrons: applications for neutron capture therapy.

252Cf is a neutron emitting radioisotope which has promise for both standard brachytherapy and neutron capture enhanced brachytherapy. In this study, experimental measurements and calculations were used to determine the thermal neutron fluence rate, phi(th) [n cm(-2) s(-1) mg(-1)], in the vicinity of 252Cf applicator tube (AT) type sources. Results of these measurements were confirmed with Monte Carlo calculations performed in a distributed manner on multiple workstations using MCNP. Three studies were executed: (1) relative phi(th) as a function of distance from a 252Cf AT source in an A-150 tissue equivalent plastic phantom using thermoluminescent dosimeters (TLDs) of varying 6Li/Li enrichment, (2) phi(th) measured with gold foils in a 114 liter water phantom 5 cm from two 252Cf AT sources, and (3) calculations of the impact of phantom material composition (e.g., A-150, water, brain, muscle) on phi(th) from moderated 252Cf fast neutrons. TLD results and Monte Carlo calculations in A-150 of relative phi(th) typically agreed within 1% and at most differed by 3% for distances from 1 to 6 cm. Foil measurements followed the ASTM E 262-86e protocol, and the ratio of activated plain and Cd encased gold foils (7.31) agreed well with the calculated ratio (7.26). Measured phi(th) at 5 cm (1.70+/-0.10 x 10(7) n cm(-2) s(-1) mg(-1)) was 10% greater than that determined using MCNP (1.55+/-0.12 x 10(7) n cm(-2) s(-1) mg(-1)), but was within the combined uncertainties. Compared with A-150 at a distance of 1 cm, phi(th) was 20%, 22%, and 32% less for water, brain, and muscle, respectively; these ratios decreased to 16%, 16%, and 24% less, respectively, at a distance of 5 cm from the source in a 15 cm diameter phantom. Comparisons of these results generally agreed with those in the literature for a value of 2 x 10(7) n cm(-2) s(-1) mg(-1) in water at 3 cm.

Evaluation of dose variation during total skin electron irradiation using thermoluminescent dosimeters

Purpose : To determine acceptable dose variation using thermoluminescent dosimeters (TLD) in the treatment of Mycosis Fungiodes with total electron beam (TSEB) irradiation. Methods and Materials : From 1983 to 1993, 22 patients were treated with total skin electron beam therapy in the standing position. A six-field technique was used to deliver 2 Gy in two days, treating 4 days per week, to a total dose of 35 of 40 Gy using a degraded 9 MeV electron beam. Thermoluminescent dosimeters were placed on several locations of the body and the results recorded. The variations in these readings were analyzed to determine normal dose variation for various body locations during TSEB. Results : The dose to flat surfaces of the body was essentially the same as the dose to the prescription point. The dose to tangential surfaces was within ± 10% of the prescription dose, but the readings showed much more variation (up to 24%). Thin areas of the body showed large deviations from the prescriptions dose along with a large amount of variation in the readings (up to 22%). Special areas of the body, such as the perineum and eyelid, showed large deviations from the prescription dose with very large (up to 40%) variations in the readings. Discussion : The TLD results of this study will be used as a quality assurance check for all new patients treated with TSEB. The results of the TLDs will be compared with this baseline study to determine if the delivered dose is within acceptable ranges. If the TLD results fall outside acceptable limits establised above, then the patient position can be modified or the technique itself evaluated.