Thermoluminescent Dosimetry Measurements Research Articles

Investigation of skin dose of post-mastectomy radiation therapy for the halcyon and tomotherapy treatment machine: Comparison of calculation and in vivo measurements

In treatments such as post-mastectomy radiation therapy (PMRT) for breast cancer patients, skin doses are of great importance in terms of tumor control and side effects. This dosimetric study aimed to measure the skin dose on the chest wall surface for post-mastectomy radiotherapy with Tomotherapy and Halcyon devices using thermoluminescence dosimetry (TLD) and to evaluate the accuracy of the skin dose calculated by treatment planning systems (TPS). Computed tomography (CT) images of the Alderson Rando phantom were obtained. Treatment plans were created with Tomotherapy and Halycon devices for a total of 50 Gy with a fraction dose of 2 Gy to the target volume. The left chest wall was divided into 9 regions for detailed surface dose assessment. 88 dose measurement points were determined on the left chest wall. The phantom was irradiated separately in Halycon and Tomotherapy devices. In vivo, dose measurements were performed with TLD in both devices. TPS and TLD doses were compared for the left chest wall surface. For skin dose, TLD measurements were lower than the dose estimate by Tomotherapy TPS and higher than the dose estimate by Halycon TPS. In addition, the skin dose measurements showed that the skin dose of the Tomotherapy device was higher than that of the Halycon. It is recommended to consider the distance of the treatment device and the accuracy of the TPS dose calculation in skin dose assessment in PMRT.

Investigation of the effect of bolus on skin dose in breast cancer treatment with tomotherapy and LINAC radiotherapy devices

In postmastectomy radiation therapy (PMRT), it is important that the chest wall skin is included in the target volume and receives an adequate dose. This study aims to investigate the effect of bolus on skin dose in breast cancer treatment with Tomotherapy and LINAC radiotherapy devices and to examine the accuracy of the surface dose calculated by treatment planning system (TPS) with thermoluminescence dosimetry (TLD). Female Alderson Rando Phantom's chest wall is virtually divided into 9 regions. Computed tomography (CT) images of the phantom were obtained with a section thickness of 3 mm. Two plans, with and without bolus, were created for each of the helical tomotherapy (HT), intensity-modulated radiotherapy (IMRT) and field-in-field (FiF) techniques. A bolus thickness of 0.5 cm was used for all bolus plans. Doses calculated from TPSs were obtained for 88 predetermined points on the chest wall. After irradiation, doses measured with TLDs were obtained. The effect of bolus use on the chest wall surface was examined. The increase in surface dose due to bolus use was 50.35%, 55.35% and 68.56% for HT, IMRT and FiF techniques in TPS, respectively. This increase in TLD measurements was 58.18%, 30.90% and 46.31% for HT, IMRT and FiF techniques, respectively. The best agreement between TPS and TLD doses for bolus and non-bolus plans was found in the HT technique. The difference between TPS and TLD doses decreased within the three treatment techniques due to bolus use. Since the chest wall skin cannot receive the required dose in IMRT and FiF techniques, bolus use is recommended until acute skin reactions occur. There is no need to use a bolus for the HT technique.

Investigation of Surface Dose Accuracy of Two Dose Calculation Algorithms Using Thermoluminescent Dosimeters

Accurate estimation of the surface dose in radiotherapy is very important in reducing skin reactions. This study aims to evaluate the accuracy of two different treatment planning algorithms in calculating the surface dose in a specially designed phantom using thermoluminescent dosimetry (TLD). In this study, a special phantom was designed for surface dose measurement. The phantom surface consisted of an adhesive bolus for the adhesion of TLDs. 121 TLDs were placed 1 cm apart on the bolus surface. In TPS, irradiation plans were created at different fields and source-surface distances (SSD). Dose calculations were made with Anisotropic Algorithm algorithms (AAA) and Pencil Beam Convolution (PBC) algorithms for all plans. The mean dose was measured for each point. For each of the 4x4, 6x6, 8x8, 10x10, and 12x12 cm2 domains, the TLDs within the domain were approximately 1 cm inward from the edge. To measure the effect of SSD on surface dose, the isocenter point was located at depths of 0 cm, 2.5 cm and 5.0 cm, respectively. The surface dose at each depth was measured with TLDs. The doses calculated by the AAA and PBC algorithms were compared with the doses measured by TLDs. The AAA algorithm overestimates the surface dose by 4% compared to the TLD measurement for the 4x4 field. The surface dose calculation of the PBC algorithm was found to be high when compared to TLD measurements for all SSDs and fields. There was a significant difference between the PBC algorithm dose calculation and TLD measurements in all fields and SSDs (p<0.001). It was observed that the AAA algorithm performed better in calculating the surface dose than the PBC algorithm. AAA and PBC algorithm users are advised to be more careful about surface dose calculation.

Dosimetric comparison of different radiotherapy techniques to determine the absorbed fetal dose in pregnant patients with left-sided breast cancer

In our study, we aimed to measure the dose exposed by the fetus during three-dimensional conformal radiotherapy (3D-CRT) and intensity-modulated radiotherapy (IMRT) treatment techniques in pregnant patients who had to undergo radiotherapy due to breast cancer with thermoluminescence dosimetry (TLD) and compare these measurements with treatment planning system (TPS). 2.5 mm- thick computed tomography (CT) images of Alderson Rando phantom device were taken for measurements. In order to compare the TPS and TLD readings in PTV, 60 TLDs were placed in the target volume in the Rando phantom. Twenty-four TLDs were placed in the 30th and 31st sections of the phantom, which is likely to be in the volumetric size of the first trimester of pregnancy which is also located 25.84 cm from the lower limit of the PTV. Six MV were used in seven different treatment techniques and the measurements were repeated 5 times for each technique and averaged. Fetal doses for all treatment techniques in TPS were found as 0.00 cGy. However, TLD measurements were measured between 3.36 ± 0.19 cGy and 16.10 ± 2.04. In 3D-CRT treatment technique the fetus received the lowest dose. In addition, for plans made using the IMRT method, it is seen that the MSS-IMRT technique provides better fetal protection compared to the SW-IMRT technique. The 3D-CRT should be the first priority for a pregnant patient who has to undergo left breast RT, but if the desired dose-volume histogram (DVH) cannot be obtained with 3D-CRT, 5 MSS-IMRT technique may be preferred.

Simplified method for determining dose to a non-water phantom through the use of ND,w and IAEA TRS 483 for the GammaPod.

GammaPod, a breast stereotactic radiosurgery device, utilizes 25 rotating Co-60 sources to deliver highly conformal dose distributions. The GammaPod system requires that reference dosimetry be performed in a specific vendor-supplied poly-methylmethacrylate (PMMA) phantom. The nonstandard nature of GammaPod dosimetry, in both the phantom material and machine-specific reference (msr), prohibits use of the American Association of Physicists in Medicine Task Group 51 (TG-51) protocol. This study proposes a practical method using TRS 483 to make the reference dosimetry procedure simpler and to reduce overall uncertainties. The dose to PMMA (DPMMA) is determined under msr conditions using TRS 483 with an Exradin A1SL chamber placed in a PMMA phantom. The conversion factor, which converts from the dose-to-water (Dw) in broad-beam Co-60 reference geometry to DPMMA in the msr small field Co-60 (Qmsr) geometry, is derived using the Monte Carlo simulations and procedure described in TRS 483. The new conversion factor value for an Exradin A1SL chamber is 0.974. When combined with ND,w, DPMMA differs by 0.5% from the TG-21/Nx method and 0.2% from the IROC values. Uncertainty decreased from 2.2% to 1.6%. We successfully implemented TRS 483 reference dosimetry protocols utilizing ND,w for the GammaPod in the PMMA phantom. These results show not only agreement between measurements performed with the previously published method and independent thermoluminescent dosimetry measurements but also reductions in uncertainty. This also provides readers with a pathway to develop their own IAEA TRS 483 factor for any new small field machine that may be developed.

In-vivo dosimetric analysis in total skin electron beam therapy.

Background and purposeThermoluminescent dosimetry (TLD) is an important element of total skin electron beam therapy (TSEBT). In this study, we compare radiation dose distributions to provide data for dose variation across anatomic sites.Materials and methodsRetrospectively collected data on 85 patients with cutaneous lymphoma or leukemia underwent TSEBT were reviewed. Patients were irradiated on two linear accelerators, in one of two positions (standing, n = 77; reclined, n = 8) and 1830 in vivo TLD measurements were obtained for various locations on 76 patients.ResultsThe TLD results showed that the two TSEBT techniques were dosimetrically heterogeneous. At several sites, the dose administered correlated with height, weight, and gender. After the first TLD measurement, fourteen patients (18%) required MU modification, with a mean 10% reduction (range, −25 to +35). Individual TLD results allowed us to customize the boost treatment for each patient. For patients who were evaluated in the standing position, the most common underdosed sites were the axilla, perineum/perianal folds, and soles (each receiving 69%, 20%, and 34% of the prescribed dose, respectively). For patients evaluated in a reclining position, surface dose distribution was more heterogeneous. The sites underdosed most commonly were the axilla and perineum/perianal folds (receiving less than one third of prescribed dose). Significant variables were detected with model building.ConclusionTLD measurements were integral to quality assurance for TSEBT. Dose distribution at several anatomical sites correlated significantly with gender, height, and weight of the treated individual and might be predicted.

Assessment of leakage dose in vivo in patients undergoing radiotherapy for breast cancer.

Background and purposeAccurate quantification of the relatively small radiation doses delivered to untargeted regions during breast irradiation in patients with breast cancer is of increasing clinical interest for the purpose of estimating long-term radiation-related risks. Out-of-field dose calculations from commercial planning systems however may be inaccurate which can impact estimates for long-term risks associated with treatment. This work compares calculated and measured dose out-of-field and explores the application of a correction for leakage radiation.Materials and methodsDose calculations of a Boltzmann transport equation solver, pencil beam-type, and superposition-type algorithms from a commercial treatment planning system (TPS) were compared with in vivo thermoluminescent dosimetry (TLD) measurements conducted out-of-field on the contralateral chest at points corresponding to the thyroid, axilla and contralateral breast of eleven patients undergoing tangential beam radiotherapy for breast cancer.ResultsOverall, the TPS was found to under-estimate doses at points distal to the radiation field edge with a modern linear Boltzmann transport equation solver providing the best estimates. Application of an additive correction for leakage (0.04% of central axis dose) improved correlation between the measured and calculated doses at points greater than 15 cm from the field edge.ConclusionsApplication of a correction for leakage doses within peripheral regions is feasible and could improve accuracy of TPS in estimating out-of-field doses in breast radiotherapy.

Thermoluminescence dosimetry of the dose received by scrotum and testes in radiotherapy of rectal cancer, compared to the point doses calculated by 3D-planning software

PurposeRadiation received by the testes in the course of radiotherapy for rectal cancer may cause oligospermia and azospermia. We sought to determine the dose to the scrotum and testes with thermoluminescence dosimetry (TLD), and compare it to the dose calculated by 3D planning software. MethodsThe TLDs were fixed to the scrotum in six points anteriorly and posteriorly in two fractions of radiotherapy. All patients received a 50–50.4 Gy total dose in prone position with 3D-planning. The average dose of TLD measurements was compared to the average of 6 relevant point doses calculated by the planning software. ResultsThe mean scrotal dose of radiation in 33 patients as measured by TLD was 3.77 Gy (7.5% of the total prescribed dose), and the mean of point doses calculated by the planning software was 4.11 Gy (8.1% of the total dose), with no significant difference. A significant relationship was seen between the position of the inferior edge of the fields and the mean scrotal dose (P = .04). Also body mass index (BMI) was inversely related with the scrotal dose (P = .049). ConclusionWe found a dose of about 4 Gy received by the scrotum and testes from a total prescribed dose of 50 Gy in the radiotherapy of rectal carcinoma patients, with TLD measurements confirming testicular dose estimations by the planning software. This dose could be significantly harmful for spermatogenesis. Thus careful attention to the testicular dose in radiotherapy of rectal cancer for men desiring continued fertility is a necessity.

Comparison of dosimetry methods for panoramic radiography: thermoluminescent dosimeter measurement versus personal computer–based Monte Carlo method calculation

The purpose of this study was to evaluate the patient radiation dose based on panoramic radiography and calculated with personal computer-based Monte Carlo (PCXMC) software compared with thermoluminescent dosimetry (TLD) measurement. We also proposed appropriate input values for dose-determining factors in PCXMC. Tissue-absorbed doses and the effective dose based on panoramic radiography were measured with TLD and with PCXMC under various conditions. The calculated PCXMC doses were compared with those measured with TLD. The effective doses calculated with PCXMC were higher by 9.55% to 51.24% compared with the doses measured with TLD. Reference points on the Y-axis and Z-axis were the sensitive factors when calculating the effective dose. The differences between the highest and the lowest organ doses were 0.32 and 0.10 mGy, respectively, for PCXMC calculation and TLD measurement. The effective dose calculated with PCXMC was generally higher than the dose measured by using TLD, and the absorbed doses varied by organ more severely in the PCXMC calculations than in the TLD measurements. The effective dose obtained from PCXMC calculations was dependent on input values for dose-determining factors. Standard values for each dose-determining factor required for the application of PCXMC to panoramic radiography were suggested in this study.

Performance of two commercial electron beam algorithms over regions close to the lung–mediastinum interface, against Monte Carlo simulation and point dosimetry in virtual and anthropomorphic phantoms

Electron radiotherapy is applied to treat the chest wall close to the mediastinum. The performance of the GGPB and eMC algorithms implemented in the Varian Eclipse™ treatment planning system (TPS) was studied in this region for 9 and 16 MeV beams, against Monte Carlo (MC) simulations, point dosimetry in a water phantom and dose distributions calculated in virtual phantoms. For the 16 MeV beam, the accuracy of these algorithms was also compared over the lung-mediastinum interface region of an anthropomorphic phantom, against MC calculations and thermoluminescence dosimetry (TLD). In the phantom with a lung–equivalent slab the results were generally congruent, the eMC results for the 9 MeV beam slightly overestimating the lung dose, and the GGPB results for the 16 MeV beam underestimating the lung dose. Over the lung–mediastinum interface, for 9 and 16 MeV beams, the GGPB code underestimated the lung dose and overestimated the dose in water close to the lung, compared to the congruent eMC and MC results. In the anthropomorphic phantom, results of TLD measurements and MC and eMC calculations agreed, while the GGPB code underestimated the lung dose. Good agreement between TLD measurements and MC calculations attests to the accuracy of “full” MC simulations as a reference for benchmarking TPS codes. Application of the GGPB code in chest wall radiotherapy may result in significant underestimation of the lung dose and overestimation of dose to the mediastinum, affecting plan optimization over volumes close to the lung–mediastinum interface, such as the lung or heart.

Calculation of organ doses from breast cancer radiotherapy: a Monte Carlo study.

The current study aimed to: a) utilize Monte Carlo simulation methods for the assessment of radiation doses imparted to all organs at risk to develop secondary radiation induced cancer, for patients undergoing radiotherapy for breast cancer; and b) evaluate the effect of breast size on dose to organs outside the irradiation field. A simulated linear accelerator model was generated. The in‐field accuracy of the simulated photon beam properties was verified against percentage depth dose (PDD) and dose profile measurements on an actual water phantom. Off‐axis dose calculations were verified with thermoluminescent dosimetry (TLD) measurements on a humanoid physical phantom. An anthropomorphic mathematical phantom was used to simulate breast cancer radiotherapy with medial and lateral fields. The effect of breast size on the calculated organ dose was investigated. Local differences between measured and calculated PDDs and dose profiles did not exceed 2% for the points at depths beyond the depth of maximum dose and the plateau region of the profile, respectively. For the penumbral regions of the dose profiles, the distance to agreement (DTA) did not exceed 2 mm. The mean difference between calculated out‐of‐field doses and TLD measurements was 11.4%±5.9%. The calculated doses to peripheral organs ranged from 2.32 cGy up to 161.41 cGy depending on breast size and thus the field dimensions applied, as well as the proximity of the organs to the primary beam. An increase to the therapeutic field area by 50% to account for the large breast led to a mean organ dose elevation by up to 85.2% for lateral exposure. The contralateral breast dose ranged between 1.4% and 1.6% of the prescribed dose to the tumor. Breast size affects dose deposition substantially.PACS numbers: 87.10.rt, 87.56.bd, 87.53.Bn, 87.55.K‐, 87.55.ne, 87.56.jf, 87.56.J‐

Gamma-ray thermoluminescence measurements: a record of fallout deposition in Hiroshima?

In certain Hiroshima neighborhoods, radiation measurements using thermoluminescence dosimetry (TLD) exceed what can be explained by the initial gamma-ray doses and uncertainties from the Dosimetry System 2002 (DS02). This problem was not previously recognized as being isolated to certain parts of that city. The ratio between TLD measurements and DS02 dose calculations for gamma rays appear to grow larger than unity up to more than three with increasing ground range, but closer examination shows the excess TLD dose (0.1, 0.2, or possibly up to 0.8 Gray) is correlated with certain neighborhoods and could be due to radioactive fallout. At Nagasaki, the TLD measurements do not show this same excess, probably because there were no TLD measurements taken more than 800m downwind (eastward) from the Nagasaki hypocenter, so that any small excess TLD dose was masked by larger initial gamma-ray doses of 25-80 Gray in the few downwind samples. The DS02 Report had noted many measurements lower than the DS02 calculation for several Nagasaki TLD samples, independent of ground range. This was explained as being the result of previously unaccounted urban shielding which was observed from Nagasaki pre-bomb aerial photos. However, the Hiroshima excess TLD dose issue was not resolved. If the excess TLD doses at Hiroshima are an indication of fallout, it may be possible to use additional TLD studies to make better estimates of the locations and radiation doses to survivors from the fallout after the bombings at both cities.

SU-E-T-604: Decreasing Subcutaneous Skin Dose in Breast Radiotherapy Using TomoDirect (TD)

Purpose: TD is a newly introduced modality by TomoTherapy (Tomotherapy Inc., Madision, WI) allowing users to plan and deliver a series overlapping radiation fan beams at static beam angles with a moving couch. We observed undesired breast skindosing in whole breast treatment plans with TD. The purpose of this work was to develop a method using a treatment planning subcutaneous avoidance structure (SAS) to lower this subcutaneous skindose (SCSD) in whole breast TD treatment planning. Methods: As a 3DCRT modality, TD only allows PTV coverage to be specified. We placed SAS, circles of approximately 3 mm in diameter, at the subcutaneous region of the breast as a pseudo‐target with slightly lower prescription dose than the BreastPTV. Five left‐sided early breast cancer patients were selected for this study. BreastPTV was defined as breast tissue limited 5mm from the skinsurface. Two skinstructures were contoured, a 5mm and a 2.5mm strip extending from patient surface towards the BreastPTV. Plans with and without SAS were generated for the 5 patients and an anthropomorphic phantom with 6 tangent‐like beams. SAS was optimized to limit the dose to 45 Gy for a Breast TV prescription dose of 50Gy. Dose volume histograms were compared. Subcutaneous dose, with and without SAS, were measured using both Gafchromic EBT2 film and thermoluminescencedosimetry(TLD). Results: With similar BreastPTV coverage and normal tissue sparing, plans without SAS had a V45 in the subcutaneous region of 80–90% whereas plans with SAS had a V45 of 70– 80%, 10% decrease in the 5mm skin strip. For the 2.5 mm skin strip, decreasing in V45 was about 20% in plans with SAS. The results were confirmed with film and TLD measurements, which demonstrated an 8% decrease in a plan with SAS. Conclusions: The SAS planning technique can lower SCSD without compromising PTV coverage using TomoDirect.

Electron Arc Therapy for Bilateral Chest Wall Irradiation: Treatment Planning and Dosimetric Study

Aims The treatment of patients with synchronous bilateral breast cancer is a challenge. We present a report of dosimetric data of patients with bilateral chest walls as the target treated with electron arc therapy. Materials and methods Ten consecutive patients who had undergone electron arc therapy to the bilateral chest wall for breast cancer were analysed. After positioning and immobilisation, patients underwent computed tomography scans from the neck to the upper abdomen. Electron arc plans were generated using the PLATO RTS (V1.8.2 Nucletron) treatment planning system. Electron energy was chosen depending upon the depth and thickness of the planning target volume (PTV). For all patients, the arc angle ranged between 80 and 280° (start angle 80°, stop angle 280°). The homogeneity index, coverage index and doses to organs at risk were evaluated. The patient-specific output factor and thermoluminescence dosimetry (TLD) measurements were carried out for all patients. The total planned dose to the PTV was 50 Gy/25 fractions/5 weeks. Results The mean PTV (± standard deviation) was 568.9 (±116) cm 3. The mean PTV coverage was 89 (±5.8)% of the prescribed dose. For the right lung, the mean values of D 1 and D 10 were 46 (±7.6) and 30 (±9) Gy, respectively. For the left lung, the mean values of D 1 and D 10 were 45 (±7) and 27 (±8) Gy, respectively. For the heart, the mean values of D 1, D 5 and D 10 were 21 (±15), 13.5 (±12) and 9 (±9) Gy, respectively. The mean values of TLD at various pre-specified locations on the chest wall surface were 1.84, 1.82, 1.82, 1.89 and 1.78 Gy, respectively Conclusion The electron arc technique for treating the bilateral chest wall is a feasible and pragmatic technique. This technique has the twin advantages of adequate coverage of the target volume and sparing of adjacent normal structures. However, compared with other techniques, it needs a firm quality assurance protocol for dosimetry and treatment delivery.

Laser assisted thermoluminescence dosimetry using temperature controlled linear heating

We have developed a non-contact laser–fiberoptic system for thermoluminescence dosimetry (TLD) measurements. The system is based on: (i) linear heating of a TLD sample by a CO 2 laser beam; (ii) measuring the sample's temperature, using infrared thermometry and (iii) using a feedback loop, for controlling the heating rate. The infrared thermometry was carried out, using infrared transmitting AgClBr fibers. The system made it possible to obtain linear heating at slow rates (e.g. 5 °C/s), or fast rates (up to 60 °C/s), or even to obtain non-linear heating. Measurements on LiF TLD samples revealed excellent linearity between the doses, to which the samples had been exposed, and the amplitudes of the measured glow curve peaks, over a wide range of doses. Our results were highly reproducible, which clearly demonstrated the potential of laser heated thermoluminescence for accurate dosimetry.

Thermoluminescence dosimetry measurements of brachytherapy sources in liquid water

Radiation therapy dose measurements are customarily performed in liquid water. The characterization of brachytherapy sources is, however, generally based on measurements made with thermoluminescence dosimeters (TLDs), for which contact with water may lead to erroneous readings. Consequently, most dosimetry parameters reported in the literature have been based on measurements in water-equivalent plastics, such as Solid Water. These previous reports employed a correction factor to transfer the dose measurements from a plastic phantom to liquid water. The correction factor most often was based on Monte Carlo calculations. The process of measuring in a water-equivalent plastic phantom whose exact composition may be different from published specifications, then correcting the results to a water medium leads to increased uncertainty in the results. A system has been designed to enable measurements with TLDs in liquid water. This system, which includes jigs to support water-tight capsules of lithium fluoride in configurations suitable for measuring several dosimetric parameters, was used to determine the correction factor from water-equivalent plastic to water. Measurements of several 125I and 131Cs prostate brachytherapy sources in liquid water and in a Solid Water phantom demonstrated a correction factor of 1.039 +/- 0.005 at 1 cm distance. These measurements are in good agreement with a published value of this correction factor for an 125I source.

Phantom measurements and computed estimates of breast dose with radiotherapy for Hodgkin’s lymphoma: Dose reduction with the use of the involved field*

The risk of breast cancer following radiotherapy for Hodgkin's lymphoma appears to be dose related. In this study we compared breast dose in an anthropomorphic phantom for conventional 'mantle'; upper mediastinal/bilateral neck (minimantle) and unilateral neck fields, and evaluated the accuracy of computer planned dose estimates for out-of-field doses. For each field, computer-planned breast dose (CPD) estimates were compared with thermoluminescence dosimetry measurements in five locations within 'breast tissue'. CPD were also compared with ion chamber measurements in a slab phantom. Measured dose and CPD were within 20% of each other up to approximately 10 cm from the field edge. Beyond 10 cm, the CPD underestimated dose by a factor of 2 or more. The mini-mantle reduced the breast dose by a factor of approximately 10 compared with the mantle treatment. Treating the neck field lowered the breast dose by a further 50% or more. Modern involved-field radiotherapy for lymphoma substantially reduces breast dose compared with mantle fields. Computer dosimetery underestimated dose at larger distances from the field. This needs to be considered if computer dosimetery is used to estimate breast dose and, by extrapolation, breast cancer risk.

Breast radiation dose estimates compared to measured doses from involved field radiotherapy (IFRT) or mantle fields for early stage Hodgkin lymphoma (ESHL)

18503 Background Breast cancer risk after mantle irradiation has prompted concern regarding the use of IFRT for ESHL. As breast cancer risk is radiation dose related, reliable estimates of breast radiation dose from IFRT might aid individual patient risk assessment. However, estimates of breast dose from radiation scatter/leakage beyond the involved field, using standard computer algorithms, are of uncertain accuracy. The aims of this study were to compare computer estimates with measured breast dose, and to compare breast dose from mantle and neck IFRT. Methods A phantom with breast tissue equivalent was used to determine breast dose using computerized treatment planning (CMS XiO), and to perform measurements within the phantom breast tissue using thermoluminescent dosimetry (TLD). These measurements were made for mantle fields and (neck) IFRT. Results Computer estimates and TLD measurements of breast dose in centigray (cGY) are shown for IFRT and mantle treatment, for a prescribed dose of 3000cGy, using 6MV X-rays from a Varian 21eX linear accelerator. Conclusions Breast dose from neck IFRT was lower than from mantle treatment by an order of magnitude, but being well outside the treatment field, was not accurately estimated by our computer algorithm. Work in progress will examine breast dose from mediastinal IFRT and involved node irradiation. Modifications to the planning algorithm may improve accuracy of dose estimates at a distance from the primary field, allowing routine breast dose estimates for patients. Ultimately, utilisation of dose-risk data may allow risk modelling and risk-benefit analysis of IFRT for ESHL in individual patients. [Table: see text] No significant financial relationships to disclose.

Multiple‐estimate Monte Carlo calculation of the dose rate constant for a cesium‐131 interstitial brachytherapy seed

The purpose of this study was to calculate a more accurate dose rate constant for the 131Cs (model CS-1, IsoRay Medical, Inc., Richland, WA) interstitial brachytherapy seed. Previous measurements of the dose rate constant for this seed have been reported by others with incongruity. Recent direct measurements by thermoluminescence dosimetry and by gamma-ray spectroscopy were about 15% greater than earlier thermoluminescence dosimetry measurements. Therefore, we set about to calculate independent values by a Monte Carlo approach that combined three estimates as a consistency check, and to quantify the computational uncertainty. The calculated dose rate constant for the 131Cs seed was 1.040 cGy h(-1) U(-1) for an ionization chamber model and 1.032 cGy h(-1) U(-1) for a circular ring model. A formal value of 2.2% uncertainty was calculated for both values. The range of our multiestimate values were from 1.032 to 1.061 cGy h(-1) U(-1). We also modeled three 125I seeds with known dose rate constants to test the accuracy of this study's approach.

2894

At institutions offering TSEB, it is often customary to determine the actual radiation dose at multiple sites of patients with thermoluminescent dosimetry (TLD). These measurements facilitate adjustments to the patient position and orientation to achieve dose uniformity and help determine the need for boost treatment. This practice is time consuming and sometimes uncomfortable for the patient, and requires considerable resources. This report describes statistics of TLD results for a large population of patients treated on a single linac, and an evaluation of the rationale for the number of in-vivo TLD measurements.