A New Approach for Heterogeneity Corrections for Cs-137 Brachytherapy Sources.

BACKGROUND: Most of the current brachytherapy treatment planning systems (TPS) use the TG-43U1 recommendations for dosimetry in water phantom, not considering the heterogeneity effects. OBJECTIVE: The purpose of this study is developing a method for obtaining correction factors for heterogeneity for Cs-137 brachytherapy sources based on pre-calculated MC simulations and interpolation. METHOD: To simulate the effect of phantom heterogeneity on dose distribution around Cs-137 sources, spherical water phantoms were simulated in which there were spherical shells of bone with different thicknesses (0.2cm to 1.8cm with 0.1cm increment) at different distances (from 0.1cm to 10cm, with 0.5cm increment) from the source center. The spherical shells with 0.1cm thickness at different distances from 0.1cm to 10cm were used as tally cells. The doses at these cells were obtained by tally types F6, *F8, and *F4.The results indicate that the percentage differences between the doses in heterogeneity sections with the dose at the same positions inside the homogeneous water phantom vary when the distance of bone section from the source center increases, because of decreasing the average energy of photons reaching the bone layer. Finally, the results of Monte Carlo simulations were used as the input data of MATLAB software, and the percentage dose difference for each new configuration (i.e. different thickness of inhomogenity at different distances from the source) was estimated using the 2D interpolation of MATLAB. RESULTS: According to the results, the algorithm used in this study, is capable of dose estimation with high accuracy. CONCLUSION: The developed method using the results of Monte Carlo simulations and the dose interpolation can be used in treatment planning systems for heterogeneity corrections.

EchoSeed Model 6733 Iodine-125 brachytherapy source: improved dosimetric characterization using the MCNP5 Monte Carlo code.

This study primarily aimed to obtain the dosimetric characteristics of the Model 6733 (125)I seed (EchoSeed) with improved precision and accuracy using a more up-to-date Monte-Carlo code and data (MCNP5) compared to previously published results, including an uncertainty analysis. Its secondary aim was to compare the results obtained using the MCNP5, MCNP4c2, and PTRAN codes for simulation of this low-energy photon-emitting source. The EchoSeed geometry and chemical compositions together with a published (125)I spectrum were used to perform dosimetric characterization of this source as per the updated AAPM TG-43 protocol. These simulations were performed in liquid water material in order to obtain the clinically applicable dosimetric parameters for this source model. Dose rate constants in liquid water, derived from MCNP4c2 and MCNP5 simulations, were found to be 0.993 cGyh(-1) U(-1) (±1.73%) and 0.965 cGyh(-1) U(-1) (±1.68%), respectively. Overall, the MCNP5 derived radial dose and 2D anisotropy functions results were generally closer to the measured data (within ±4%) than MCNP4c and the published data for PTRAN code (Version 7.43), while the opposite was seen for dose rate constant. The generally improved MCNP5 Monte Carlo simulation may be attributed to a more recent and accurate cross-section library. However, some of the data points in the results obtained from the above-mentioned Monte Carlo codes showed no statistically significant differences. Derived dosimetric characteristics in liquid water are provided for clinical applications of this source model.

A novel device for automatic withdrawal and accurate calibration of 99m-technetium radiopharmaceuticals to minimise radiation exposure to nuclear medicine staff and patient.

A Joint Automatic Dispenser Equipment (JADE) has been designed and fabricated for automatic withdrawal and calibration of radiopharmaceutical materials. The thermoluminescent dosemeter procedures have shown a reduction in dose to the technician's hand with this novel dose dispenser system JADE when compared with the manual withdrawal of (99m)Tc. This system helps to increase the precision of calibration and to minimise the radiation dose to the hands and body of the workers. This paper describes the structure of this device, its function and user-friendliness, and its efficacy. The efficacy of this device was determined by measuring the radiation dose delivered to the hands of the nuclear medicine laboratory technician. The user-friendliness of JADE has been examined. The automatic withdrawal and calibration offered by this system reduces the dose to the technician's hand to a level below the maximum permissible dose stipulated by the international protocols. This research will serve as a backbone for future study about the safe use of ionising radiation in medicine.

SU-E-T-320: A New Verification Phantom for GYN Brachytherapy Applicators Using GafChromic - Films.

PURPOSE: Brachytherapy plays an important role in radiation therapy a wide range of tumor sits such as vaginal, cervical and endometrial cancers. The purpose of this project was to design, fabricate and verify a new phantom for dosimetric verification at small distances from GYN applicators used with GZP6 cobalt-60 HDR system. METHODS: A new phantom has been designed and fabricated from 90 slabs of 18×16×0.2 cm3 Perspex to accommodate one tandem and two ovoids. The thin layer of the slabs was chosen to place GafChromic films in between the slabs for dosimetry with GZP6 cobalt-60 HDR system. For verification of this device, an assembly composed of a large ovoid size (3cm diameter) and tandem #1 with the least curvature was selected in this study. With this assembly, GafChromic films were exposed using a plan with 500 cGy dose delivery to point "A". The irradiated films were scanned. The responses of the films were converted to dose by calibrating samples of these films using a cobalt-60 teletherapy system in the range of 25 to 800 cGy dose. The measured isodose curves with the films were compared to calculated isodose lines by the treatment planning software. RESULTS: The Result of these investigations indicated differences of up to ± 23 % between the planning and measured dosimetry at different points in GYN implant with cobalt-60 HDR source of GZP6 system. Therefore, this phantom enabled us to confirm the accuracy of radiation delivery to the GYN patients with cobalt-60 HDR source of GZP6 system. CONCLUSIONS: The new phantom design could be utilized for the QA procedure of the GZP6 cobalt-60 HDR system as well as the Ir-192 HDR system to confirmation the accuracy of dose distribution in GYN implants, especially in non-traditional implants. The Radiotherapy Department of Shahid Beheshti University at Shohada hospital sponsored the purchase of the phantom materials and films used in the investigations.

Dosimetric characteristics of the new RadioCoil 103Pd wire line source for use in permanent brachytherapy implants.

Recently, a novel linear brachytherapy source in the form of a coiled wire has become available for use in interstitial implants of various treatment sites such as prostate gland. This source type employs a design completely different from that of most "seed" sources currently on the market, one which improves upon or eliminates several common problems with such sources. Dosimetric characteristics of these sources with active lengths 0.5 cm to 5.0 cm were determined for clinical application. For 0.5 cm and 1.0 cm active length sources, the dose rate constant, radial dose function, and two-dimensional (2D) anisotropy function were experimentally and theoretically determined following the updated AAPM Task Group 43 (TG-43U1) recommendations. Radial dose functions and/or "along-away" matrix functions were also obtained for sources with active lengths 2.0 cm to 5.0 cm. Measurements were performed with LiF thermoluminescent dosimeters in Solid Water phantoms. Measured data was compared to Monte Carlo simulated data in Solid Water utilizing the PTRAN code, version 7.43. After finding the data to be in agreement, Monte Carlo calculations were performed in liquid water to obtain clinically applicable dosimetric data as per TG-43U1 recommendations. The results indicated the dose rate constant of the 0.5 cm long RadioCoil 103Pd source in Solid Water to be 0.641 cGy h(-1) U(-1) when measured, and 0.636 cGy h(-1) U(-1) when simulated by Monte Carlo. The calculated dose rate constant in liquid water was found to be 0.650 cGy h(-1) U(-1). These values are comparable to other commercially available sources. Complete dosimetric data and simulation results are described in this paper. Per TG-43U1, clinical treatment planning systems should utilize the values reported for liquid water.

Experimental determination of the TG-43 dosimetric characteristics of EchoSeed model 6733 I25I brachytherapy source.

Recently an improved design of a 125I brachytherapy source has been introduced for interstitial seed implants, particularly for prostate seed implants. This design improves the in situ ultrasound visualization of the source compared to the conventional seed. In this project, the TG-43 recommended dosimetric characteristics of the new brachytherapy source have been experimentally determined in Solid Water phantom material. The measured dosimetric characteristics of the new source have been compared with data reported in the literature for other source designs. The measured dose rate constant, A, in Solid Water was multiplied by 1.05 to extract the dose rate constant in water. The dose rate constant of the new source in water was found to be 0.99 +/- 8% cGy h(-1) U(-1). The radial dose function was measured at distances between 0.5 and 10 cm using LiF TLDs in Solid Water phantom. The anisotropy function, F(r, theta), was measured at distances of 2, 3, 5, and 7 cm.

Dosimetric characteristics with spatial fractionation using electron grid therapy.

Recently, promising clinical results have been shown in the delivery of palliative treatments using megavoltage photon grid therapy. However, the use of megavoltage photon grid therapy is limited in the treatment of bulky superficial lesions where critical radiosensitive anatomical structures are present beyond tumor volumes. As a result, spatially fractionated electron grid therapy was investigated in this project. Dose distributions of 1.4-cm-thick cerrobend grid blocks were experimentally determined for electron beams ranging from 6 to 20 MeV. These blocks were designed and fabricated at out institution to fit into a 20 x 20-cm(2) electron cone of a commercially available linear accelerator. Beam profiles and percentage depth dose (PDD) curves were measured in Solid Water phantom material using radiographic film, LiF TLD, and ionometric techniques. Open-field PDD curves were compared with those of single holes grid with diameters of 1.5, 2.0, 2.5, 3.0, and 3.5 cm to find the optimum diameter. A 2.5-cm hole diameter was found to be the optimal size for all electron energies between 6 and 20 MeV. The results indicate peak-to-valley ratios decrease with depth and the largest ratio is found at Dmax. Also, the TLD measurements show that the dose under the blocked regions of the grid ranged from 9.7% to 39% of the dose beneath the grid holes, depending on the measurement location and beam energy.

Dosimetric characteristics of the bests double-wall 103Pd brachytherapy source.

103Pd and 125I brachytherapy sources are being used for interstitial implants in tumor sites such as the prostate. Recently, a double-wall 103Pd source has been introduced, which has a design different from that of sources presently on the market. Dosimetric characteristics (dose rate constant, radial dose function, and anisotropy function) of this source were experimentally and theoretically determined following the AAPM Task Group 43 recommendations and were related to the October 10, 2000 revision of the NIST 1999 SK Standard for 103Pd. Measurements were performed in a Solid Water phantom using LiF thermoluminescent dosimeters. For these measurements, slabs of Solid Water phantom material were machined to accommodate the source and LiF TLD chips of dimensions (3.1 x 3.1 x 0.8 mm3) and (1.0 x 1.0 x 1.0 mm3). The TLD chips were surrounded by at least 10 cm of Solid Water phantom material to provide full scattering conditions. The Monte Carlo simulations were performed in Solid Water and liquid water using the PTRAN code. The results of this investigation show an excellent agreement (within 5%) between the measured (0.67+/-8% cGy h(-1) U(-1)) and calculated (to be 0.65+/-3% cGy h(-1) U(-1)) dose rate constant in Solid Water. The Monte Carlo calculated dose rate constant of the Best 103Pd in water was found to be 0.67+/-0.02 cGy h(-1) U(-1). The radial dose function, g(r), of the new 103Pd source was measured at distances ranging from 0.5 and 7 cm using LiF TLD in Solid Water phantom material. Moreover, the radial dose function of the new source was calculated in liquid water and Solid Water at distances ranging from 0.1 to 7 cm using the PTRAN Monte Carlo Code. The anisotropy function, F(r, theta), of the new 103Pd source was also measured in Solid Water and calculated in both Solid Water and water phantom material. From the anisotropy functions, the anisotropy factors, and anisotropy constant were calculated for each medium. The results indicated that the measured anisotropy constant of the Best 103Pd source in Solid Water was 0.89+/-5%. Complete dosimetric data are described in this manuscript.

Dosimetric characteristics of a new 125I brachytherapy source.

125I brachytherapy sources are being used for interstitial implants in tumor sites such as the prostate. Recently, a new 125I source has been introduced, which has a design different from that of other sources presently on the market. Dosimetric characteristics of this source, including dose rate constant, radial dose function, and anisotropy function, were determined experimentally following the AAPM Task Group 43 recommendations. The characteristics were related to the 1999 NIST calibration assigned to this source [SK,99std]. Measurements were performed in a solid water phantom using LiF thermoluminescent dosimeters. For these measurements, slabs of solid water phantom material were machined to accommodate the source and LiF TLD chips of dimensions (3.1 x 3.1 x 0.8 mm3) and (1.0 x 1.0 x 1.0 mm3). The TLD chips were surrounded by at least 10 cm of solid water phantom material to provide full scattering conditions. The results indicated a dose rate constant, lambda, of 0.88 +/- 0.07cGyh(-1)U(-1) for the new 1251 source as compared to 0.98 and 1.04 cGy h(-1)U(-1) for the Nycomed/Amersham model 6711 and 6702 seeds, respectively. Per TG-43, the values reported here represent the dose absorbed by water at 1 cm from the source in a water medium. The radial dose function, g(r), of the new 125I source was measured at distances ranging from 0.5 to 10 cm. The anisotropy function, F(r,theta), of the new 125I source was measured at distances of 2 and 5 cm from the source center. Calculations of anisotropy and radial dose function were also made using a Monte Carlo code. These calculations were made for both solid water and liquid water, the former to validate the Monte Carlo code and the latter to provide results in liquid water for clinical use. All data compared favorably with those from the Nycomed/Amersham models 6711 and 6702 sources.

Experimental determination of dosimetric characteristics of Best 125I brachytherapy source.

125I brachytherapy sources are being used for interstitial implants in tumor sites such as the prostate. Recently, the Best 125I source became commercially available for interstitial brachytherapy treatment. Dosimetric characteristics (dose rate constant, radial dose function, and anisotropy function) of this source were experimentally determined, following the AAPM Task Group 43 recommendations, and were related to the NIST 1999 calibration assigned to this source. Measurements were performed in Solid Water phantom using LiF thermoluminescent dosimeters. The results indicated a dose rate constant, lambda, of 1.01 +/- 0.08 cGy h(-1) U(-1) for the new source. The radial dose function, g(r), of the new source was measured at distances ranging from 0.5 to 10.0 cm. The anisotropy function, F(r, theta), of the new source was measured at distances of 2, 5, and 7 cm from the source center. These data compare favorably with those from the Nycomed/Amersham Models 6711 and 6702 sources. The anisotropy constant, phi(an), of the Best 125I source was found to be 0.982. Complete dosimetric parameters of the new source are presented in this paper.

Dosimetric characteristics of the Pharma Seed model BT-125-I source.

125I brachytherapy sources are being used with increasing frequency for interstitial implants in tumor sites, especially the prostate. Recently, a new 125I source design has become commercially available for clinical applications. Dosimetric characteristics (i.e., dose rate constant, radial dose function, and anisotropy function) of this source were experimentally and theoretically determined following the AAPM Task Group 43 (TG-43) recommendations and were related to the 1999 NIST calibration assigned to this source [S(k), 99std]. Measurements were performed in a Solid Water phantom using LiF thermoluminescent dosimeters. The measured data were used to validate the Monte Carlo simulations that were performed in Solid Water using the PTRAN code. The Monte Carlo calculations were then performed in liquid water to obtain the dosimetric information for clinical applications in accordance with TG-43 recommendations. The results indicated that the dose rate constant, lambda, of the Pharma Seed model BT-125-I 125I source was 0.90 +/- 0.06 cGy h(-1) U(-1) using thermoluminescent dosimeter (TLD) measurements and 0.92 +/- 0.03 cGy h(-1) U(-1) using Monte Carlo simulations in Solid Water. The calculated value in liquid water was found to be 0.95 +/- 0.03 cGy h(-1) U(-1). The radial dose function, g(r), of the new 125I source was measured at distances ranging from 0.5 to 10 cm using LiF TLD in Solid Water phantom material. The Monte Carlo simulations were performed for distances ranging from 0.1 to 10 cm from the source center in Solid Water and liquid water. The anisotropy function, F(r, theta), was measured at distances of 2, 5, and 7 cm from the source center and calculated at distances of 0.5, 1, 2, 3, 5, and 7 cm from the source center. The anisotropy constant, phi(an), of the Pharma Seed source in water was found to be 0.975. Complete dosimetric data are described in this manuscript. Per TG-43, the values reported in water should be used for clinical treatment planning systems.

Dosimetric characteristics of the InterSource103 palladium brachytherapy source.

103Pd brachytherapy sources are being used for interstitial implants in tumor sites such as the prostate. Recently, the InterSource103 palladium source has been introduced, which has a design different from that of other sources presently on the market. Dosimetric characteristics (i.e., dose rate constant, radial dose function, and anisotropy function) of this source were experimentally and theoretically determined following the AAPM Task Group 43 (TG-43) recommendations and were related to the 1999 NIST calibration assigned to this source [Sk, 99std]. Measurements were performed in a solid water phantom using LiF thermoluminescent dosimeters. The measured data was compared with Monte Carlo simulations performed in solid water using the PTRAN code. The calculations were then performed in liquid water to obtain the dosimetric information for clinical applications as per TG-43 recommendation. The results indicated that the dose rate constant, lambda, of the InterSource103 palladium source was 0.664+/-5% cGy/h/U using TLD measurements and 0.660+/-3% cGy/h/U using Monte Carlo simulations in solid water. The calculated value in liquid water was found to be 0.696 +/- 3 % cGy/h/U. The radial dose function, g(r), of the new 103Pd source was measured at distances ranging from 0.5 to 10 cm using LiF TLD in solid water phantom material. The Monte Carlo simulations were performed at distances ranging from 0.1 to 10 cm from the source center in solid water and liquid water. The anisotropy function, F(r, theta), was measured at distances of 2, 3, 5, and 7 cm from the source center and calculated at distances of 0.5, 1, 2, 3, 5, and 7 cm from the source center. Complete dosimetric data are described in this paper. Per TG-43, the values reported in water should be used for clinical treatment planning systems.

Review of AAPM Task Group No. 43 recommendations on interstitial brachytherapy sources dosimetry. American Association of Physicists in Medicine.

In 1995, the American Association of Physicists in Medicine (AAPM) Task Group No. 43 (TG-43) published its recommendations on the dosimetry of interstitial brachytherapy sources. The report recommended the use of a new dose calculation formalism based on measured quantities. The formalism in modular form permits the computation of doses in two dimensions for 103Pd, 125I, and 192Ir sources. The TG-43 dose calculation formalism introduced new and updated quantities such as air kerma strength, dose rate constant, radial dose function, anisotropy function and anisotropy factor. The dose rate obtained using the TG-43 dose calculation formalism and updated source dosimetry data can be expected to be different from some of the currently used systems by as much as 17%. For the same treatment and implementing the TG-43 dosimetry with point source approximation, the widely prescribed dose of 160 Gy for 125I permanent implants using model 6711 sources changes to 144 Gy. In addition to the dose calculation formalism, TG-43 report also stated that the air kerma strength provided by NIST is estimated to be approximately 7-10% higher than it should be, due to low energy photon contamination for 125I. This difference has not been accounted for in the TG-43 report.

Validation of Monte Carlo dose calculations near 125I sources in the presence of bounded heterogeneities.

PURPOSE: Dose distributions around low energy (< 60 keV) brachytherapy sources, such as 125I, are known to be very sensitive to changes in tissue composition. Available 125I dosimetry data describe the effects of replacing the entire water medium by heterogeneous material. This work extends our knowledge of tissue heterogeneity effects to the domain of bounded tissue heterogeneities, simulating clinical situations. Our goals are three-fold: (a) to experimentally characterize the variation of dose rate as a function of location and dimensions of the heterogeneity, (b) to confirm the accuracy of Monte Carlo dose calculation methods in the presence of bounded tissue heterogeneities, and (c) to use the Monte Carlo method to characterize the dependence of heterogeneity correction factors (HCF) on the irradiation geometry. METHODS AND MATERIALS: Thermoluminescent dosimeters (TLD) were used to measure the deviations from the homogeneous dose distribution of an 125I seed due to cylindrical tissue heterogeneities. A solid water phantom was machined accurately to accommodate the long axis of the heterogeneous cylinder in the transverse plane of a 125I source. Profiles were obtained perpendicular to and along the cylinder axis, in the region downstream of the heterogeneity. Measurements were repeated at the corresponding points in homogeneous solid water. The measured heterogeneity correction factor (HCF) was defined as the ratio of the detector reading in the heterogeneous medium to that in the homogeneous medium at that point. The same ratio was simulated by a Monte Carlo photon transport (MCPT) code, using accurate modeling of the source, phantom, and detector geometry. In addition, Monte Carlo-based parametric studies were performed to identify the dependence of HCF on heterogeneity dimensions and distance from the source. RESULTS: Measured and calculated HCFs reveal excellent agreement (< or = 5% average) over a wide range of materials and geometries. HCFs downstream of 20 mm diameter by 10 mm thick hard bone cylinders vary from 0.12 to 0.30 with respect to distance, while for an inner bone cylinder of the same dimension, it varies from 0.72 to 0.83. For 6 mm diameter by 10 mm thick hard bone and inner bone cylinders, HCF varies 0.27-0.58 and 0.77-0.88, respectively. For lucite, fat, and air, the dependence of HCF on the 3D irradiation geometry was much less pronounced. CONCLUSION: Monte Carlo simulation is a powerful, convenient, and accurate tool for investigating the long neglected area of tissue composition heterogeneity corrections. Simple one dimensional dose calculation models that depend only on the heterogeneity thickness cannot accurately characterize 125I dose distributions in the presence of bone-like heterogeneities.

Quantitative evaluation of radiochromic film response for two-dimensional dosimetry.

Radiochromic film (RCF) is attractive as a thin, high resolution, 2D planar dosimeter. We have studied the uniformity, linearity, and reproducibility of a commercially supplied RCF system (model MD-55). Forty 12 cm long strips of RCF were exposed to uniform doses of 6 MV x rays. Optical density (OD) distributions were measured by a helium-neon scanning laser (633 nm) 2D densitometer and also with a manual densitometer. All film strips showed 8%-15% variations in OD values independent of densitometry technique which are evidently due to nonuniform dispersal of the sensor medium. A double exposure technique was developed to solve this problem. The film is first exposed to a uniform beam, which defines a pixel-by-pixel nonuniformity correction matrix. The film is then exposed to the unknown dose distribution, rescanned, and the net OD at each pixel corrected for nonuniformity. The double exposure technique reduces OD/unit dose variation to a 2%-5% random fluctuation. RCF response was found to deviate significantly from linearity at low doses (40% change in net OD/Gy from 1 to 30 Gy); a finding not previously reported. To study the tradeoff between statistical noise and spatial resolution, OD was averaged over blocks of adjacent 50 microns pixels (ranging from 1 x 1 to 10 x 10 pixels). Reproducibility, defined as the standard deviation of repeated single-pixel measurements on separate film pieces, was 2% at 30 Gy for a resolution of 0.25 mm. With careful correction for nonlinearity and nonuniformity, RCF is a promising quantitative 2D dosimeter for radiation oncology applications.

Dosimetric characteristics of a new high-intensity 192Ir source for remote afterloading.

A new high-intensity 192Ir source has recently become commercially available for remote afterloading brachytherapy treatment. The dosimetric characteristics (dose rate constant, radial dose function, and anisotropy function) of this source were experimentally determined through the application of AAPM Task Group 43 recommendations. Complete dosimetric data are presented in this manuscript.

Dosimetric characteristics of an improved radiochromic film.

Recently, a new model of radiochromic film has been developed for medical applications to provide a higher sensitivity and better uniformity of response than existing models (i.e., MD-55). Dosimetric characteristics including sensitivity, linearity, reproducibility, uniformity, and dependence on energy and time have been studied experimentally. The characteristics of the new films were compared with those of model MD-55. For these investigations, the two films were exposed to ionizing radiation in the dose range from 1-72 Gy, using gamma-rays from a 60Co teletherapy unit and 6- and 18-MV x rays from a linear accelerator. The response of the exposed film was measured with a helium-neon laser densitometer. The results indicated that the sensitivity of the improved film was about 40% greater than that of MD-55 film. Moreover, the response of the improved film was found to be uniform within 4% only in one direction of the film. The orthogonal direction indicated a nonuniformity of up to 15%, similar to that of model MD-55. Less than 5% energy dependence in the megavoltage photon range was observed for the new film. Complete dosimetric characteristics of the new film are presented.

Measurement and calculation of heterogeneity correction factors for an Ir-192 high dose-rate brachytherapy source behind tungsten alloy and steel shields.

Shields made of high atomic number material are commonly used in vaginal applicators with high dose-rate (HDR) 192Ir remotely afterloaded brachytherapy sources. However little data is available for the dose distribution around such shields. Heterogeneity correction factors (HCFs) are defined as the ratio of the dose to a point with the heterogeneity (shield) in place, divided by the dose to the same point with no heterogeneity. Using thermoluminescent dosimeters (TLDs) in solid water phantom we have measured the HCFs behind 6 and 20 mm diam tungsten alloy disks, 4 and 2 mm thick and a 4 mm thick steel disk, positioned 15 mm from the source. For each measurement point, the heterogeneity correction factors were also inferred from Monte Carlo simulations, which accurately modeled the experimental geometry. The agreement between measured and calculated HCFs on the average was within 6%. Tungsten alloy disks resulted in about two times greater dose reduction in water (HCF approximately 0.4, for 20 x 4 mm disk) than for a steel disk with the same dimensions (HCF approximately 0.85). Reducing the disk diameter to 6 mm increased the dose transmission up to about 25%. Increasing the source-to-detector distance from 4 to 7 cm caused a change in HCF from 2% to more than 20%, depending on disk material and diameter. The detector artifact effects arising from the finite size and different composition of the TLD chips were determined.