Low-energy Brachytherapy Sources Research Articles

Heterogeneous physical phantom for I-125 dose measurements and dose-to-medium determination

In this paper we present a further step in the implementation of a physical phantom designed to generate sets of "true" independent reference data as requested by TG-186, intending to address and mitigate the scarcity of experimental studies on brachytherapy (BT) validation in heterogeneous media. To achieve this, we incorporated well-known heterogeneous materials into the phantom in order to perform measurements of 125I dose distribution. The work aims to experimentally validate Monte Carlo (MC) calculations based on MBDCA and determine the conversion factors from LiF response to absorbed dose in different media, using cavity theory. The physical phantom was adjusted to incorporate tissue equivalent materials, such as: adipose tissue, bone, breast and lung with varying thickness. MC calculations were performed using MCNP6.2 code to calculate the absorbed dose in the LiF and the dose conversion factors (DCF). The proposed heterogeneous phantom associated with the experimental procedure carried out in this work yielded accurate dose data that enabled the conversion of the LiF responses into absorbed dose to medium. The results showed a maximum uncertainty of 6.92 % (k = 1), which may be considered excellent for dosimetry with low-energy BT sources. The presented heterogeneous phantom achieves the required precision in dose evaluations due to its easy reproducibility in the experimental setup. The obtained results support the dose conversion methodology for all evaluated media. The experimental validation of the DCF in different media holds great significance for clinical procedures, as it can be applied to other tissues, including water, which remains a widely utilized reference medium in clinical practice.

EGSnrc-based depth-dependent photon energy response and phantom scatter corrections for low-energy brachytherapy sources.

In the present study, beam quality correction, [Formula: see text], and phantom scatter correction, kphan(r), for low-energy brachytherapy sources, 131Cs, 125I, and 103Pd, are calculated using the Monte Carlo-based EGSnrc code system as a function of the distance along the transverse axis of the source. The solid-state detectors investigated are diamond, LiF, Li2B4O7, Al2O3, and radiochromic films, such as HS, EBT, EBT2, EBT3, RTQA, XRT, and XRQA. The solid phantoms investigated are polystyrene, PMMA, virtual water, solid water, plastic water (LR), A150, RW1, RW3, and WE210. For a given detector and brachytherapy source, [Formula: see text] is independent of distance in the water phantom. Meanwhile, for a given detector, kphan(r) depends on the distance from the source for the investigated solid phantoms. Moreover, the kphan(r) values do not change with the detector type for sources 131Cs, 125I, and 103Pd at all distances. The LR and A150 phantoms are water equivalent for the investigated distances of 1-5cm. The phantoms including solid water, virtual water, and WE210 are not water-equivalent for distances beyond 1cm. Furthermore, PMMA, polystyrene, RW1, and RW3 are not water equivalent.

Mathematical formulation of 125I seed dosimetry parameters and heterogeneity correction in lung permanent implant brachytherapy.

Precise determination of dose distribution around low-energy brachytherapy sources as well as considering tissue heterogeneity is crucial for optimized treatment planning. This study is aimed at determination and mathematically formulation of American Association of Physicists in Medicine Task Group No. 43 (AAPM TG-43) dosimetry parameters of 125I seed (model 6711) and calculation of dose difference caused by neglecting lung heterogeneity in permanent implant brachytherapy. Using MCNPX 2.6.0 code, 125I seed (model 6711) was simulated in a cubic water environment, and its dosimetry parameters mentioned in AAPM TG-43 protocol were obtained. After benchmarking of parameters and comparison with prior studies, mathematical equations were fitted to the data, and a specific set of 125I seeds was simulated on a plane in simulated lung and water environments. Appropriate photon histories were considered to achieve data with maximum accuracy (max error 1%). In the end, isodose curves, profiles, depth dose, and dose difference between lung and water environments were obtained. For 125I seed (model 6711), radial dose function and anisotropy functions were obtained precisely with R2 > 0.99, all in good agreement with previous studies and protocol. In addition, percentage dose difference between inhomogeneous lung and homogenous water environments in a 5 cm distance was calculated and presented as D (r) function with R2 > 0.99. Considering practical difficulties in dose calculations, 125I seed dosimetry parameters and lung heterogeneity corrections can be obtained precisely by MCNPX. Equations presented in this study are recommended to be considered in future studies based on lung permanent implantation.

Collision-kerma conversion between dose-to-tissue and dose-to-water by photon energy-fluence corrections in low-energy brachytherapy

The AAPM TG-43 brachytherapy dosimetry formalism, introduced in 1995, has become a standard for brachytherapy dosimetry worldwide; it implicitly assumes that charged-particle equilibrium (CPE) exists for the determination of absorbed dose to water at different locations, except in the vicinity of the source capsule. Subsequent dosimetry developments, based on Monte Carlo calculations or analytical solutions of transport equations, do not rely on the CPE assumption and determine directly the dose to different tissues. At the time of relating dose to tissue and dose to water, or vice versa, it is usually assumed that the photon fluence in water and in tissues are practically identical, so that the absorbed dose in the two media can be related by their ratio of mass energy-absorption coefficients. In this work, an efficient way to correlate absorbed dose to water and absorbed dose to tissue in brachytherapy calculations at clinically relevant distances for low-energy photon emitting seeds is proposed. A correction is introduced that is based on the ratio of the water-to-tissue photon energy-fluences. State-of-the art Monte Carlo calculations are used to score photon fluence differential in energy in water and in various human tissues (muscle, adipose and bone), which in all cases include a realistic modelling of low-energy brachytherapy sources in order to benchmark the formalism proposed. The energy-fluence based corrections given in this work are able to correlate absorbed dose to tissue and absorbed dose to water with an accuracy better than 0.5% in the most critical cases (e.g. bone tissue).

Low-Energy Brachytherapy Sources for Treating the Pelvic Sidewall

Low-Energy Brachytherapy Sources for Treating the Pelvic Sidewall

Air density dependence of the PTW 34013 ionization chamber for soft X-ray

Introduction It has been proved that some ionization chambers, when measuring low energy brachytherapy or soft X-ray sources, show a dependence with the air density that it is not entirely corrected with the usual pressure/temperature correction factor, k TP . This is due to the chamber dimensions and materials forming it. Purpose To determine if the response of PTW 34013 chamber, used for dose measurements in superficial radiotherapy, is fully corrected with k TP or a residual air density dependence remains. Materials and methods Measurements for 70 and 200 kVp X-ray beams, generated by a MAXISHOT.200 cabinet, have been performed using a pressurized chamber. Raw data have been corrected with k TP and normalized to its value in standard atmospheric conditions (760 mmHg and 20 °C). Results The corrected measurements for both beams still show a linear dependence with the normalized density ρ / ρ 0 , ρ 0 being the density in standard atmospheric conditions. When the air density is below ρ 0 , k TP undercorrects the measurement, and the contrary occurs when ρ > ρ 0 . In our city, with 700 m AMSL, usual atmospheric conditions will cause 1.2% undercorrection for the 70 kV beam and 2.5% for the 200 kV beam. Conclusion Despite its dimensions, the dependence of the PTW 34013 ionization chamber with the air density is not fully corrected with k TP . The weather or the altitude of the place where measurements are carried out will require an additional correction to reach the accuracy level required in radiotherapy. Disclosure We thank to PTW for material support.

SU‐E‐T‐212: Comparison of TG‐43 Dosimetric Parameters of Low and High Energy Brachytherapy Sources Obtained by MCNP Code Versions of 4C, X and 5

Purpose:Different versions of MCNP code are widely used for dosimetry purposes. The purpose of this study is to compare different versions of the MCNP codes in dosimetric evaluation of different brachytherapy sources.Methods:The TG‐43 parameters such as dose rate constant, radial dose function, and anisotropy function of different brachytherapy sources, i.e. Pd‐103, I‐125, Ir‐192, and Cs‐137 were calculated in water phantom. The results obtained by three versions of Monte Carlo codes (MCNP4C, MCNPX, MCNP5) were compared for low and high energy brachytherapy sources. Then the cross section library of MCNP4C code was changed to ENDF/B‐VI release 8 which is used in MCNP5 and MCNPX codes. Finally, the TG‐43 parameters obtained using the MCNP4C‐revised code, were compared with other codes.Results:The results of these investigations indicate that for high energy sources, the differences in TG‐43 parameters between the codes are less than 1% for Ir‐192 and less than 0.5% for Cs‐137. However for low energy sources like I‐125 and Pd‐103, large discrepancies are observed in the g(r) values obtained by MCNP4C and the two other codes. The differences between g(r) values calculated using MCNP4C and MCNP5 at the distance of 6cm were found to be about 17% and 28% for I‐125 and Pd‐103 respectively. The results obtained with MCNP4C‐revised and MCNPX were similar. However, the maximum difference between the results obtained with the MCNP5 and MCNP4C‐revised codes was 2% at 6cm.Conclusion:The results indicate that using MCNP4C code for dosimetry of low energy brachytherapy sources can cause large errors in the results. Therefore it is recommended not to use this code for low energy sources, unless its cross section library is changed. Since the results obtained with MCNP4C‐revised and MCNPX were similar, it is concluded that the difference between MCNP4C and MCNPX is their cross section libraries.

Dose heterogeneity correction for low-energy brachytherapy sources using dual-energy CT images

Permanent seed implant brachytherapy is currently used for adjuvant radiotherapy of early stage prostate and breast cancer patients. The current standard for calculation of dose around brachytherapy sources is based on the AAPM TG-43 formalism, which generates the dose in a homogeneous water medium. Recently, AAPM TG-186 emphasized the importance of accounting for tissue heterogeneities. We have previously reported on a methodology where the absorbed dose in tissue can be obtained by multiplying the dose, calculated by the TG-43 formalism, by an inhomogeneity correction factor (ICF). In this work we make use of dual energy CT (DECT) images to extract ICF parameters. The advantage of DECT over conventional CT is that it eliminates the need for tissue segmentation as well as assignment of population based atomic compositions. DECT images of a heterogeneous phantom were acquired and the dose was calculated using both TG-43 and TG-43 formalisms. The results were compared to experimental measurements using Gafchromic films in the mid-plane of the phantom. For a seed implant configuration of 8 seeds spaced 1.5 cm apart in a cubic structure, the gamma passing score for 2%/2 mm criteria improved from 40.8% to 90.5% when ICF was applied to TG-43 dose distributions.

On the feasibility of polyurethane based 3D dosimeters with optical CT for dosimetric verification of low energy photon brachytherapy seeds.

To investigate the feasibility of and challenges yet to be addressed to measure dose from low energy (effective energy <50 keV) brachytherapy sources (Pd-103, Cs-131, and I-125) using polyurethane based 3D dosimeters with optical CT. The authors' evaluation used the following sources: models 200 (Pd-103), CS-1 Rev2 (Cs-131), and 6711 (I-125). The authors used the Monte Carlo radiation transport code MCNP5, simulations with the ScanSim optical tomography simulation software, and experimental measurements with PRESAGE(®) dosimeters/optical CT to investigate the following: (1) the water equivalency of conventional (density = 1.065 g/cm(3)) and deformable (density = 1.02 g/cm(3)) formulations of polyurethane dosimeters, (2) the scatter conditions necessary to achieve accurate dosimetry for low energy photon seeds, (3) the change in photon energy spectrum within the dosimeter as a function of distance from the source in order to determine potential energy sensitivity effects, (4) the optimal delivered dose to balance optical transmission (per projection) with signal to noise ratio in the reconstructed dose distribution, and (5) the magnitude and characteristics of artifacts due to the presence of a channel in the dosimeter. Monte Carlo simulations were performed using both conventional and deformable dosimeter formulations. For verification, 2.8 Gy at 1 cm was delivered in 92 h using an I-125 source to a PRESAGE(®) dosimeter with conventional formulation and a central channel with 0.0425 cm radius for source placement. The dose distribution was reconstructed with 0.02 and 0.04 cm(3) voxel size using the Duke midsized optical CT scanner (DMOS). While the conventional formulation overattenuates dose from all three sources compared to water, the current deformable formulation has nearly water equivalent attenuation properties for Cs-131 and I-125, while underattenuating for Pd-103. The energy spectrum of each source is relatively stable within the first 5 cm especially for I-125. The inherent assumption of radial symmetry in the TG43 geometry leads to a linear increase in sample points within the 3D dosimeter as a function of distance away from the source, which partially offsets the decreasing signal. Simulations of dose reconstruction using optical CT showed the feasibility of reconstructing dose out to a radius of 10 cm without saturating projection images using an optimal dose and high dynamic range scanning; the simulations also predicted that reconstruction artifacts at the channel surface due to a small discrepancy in refractive index should be negligible. Agreement of the measured with calculated radial dose function for I-125 was within 5% between 0.3 and 2.5 cm from the source, and the median difference of measured from calculated anisotropy function was within 5% between 0.3 and 2.0 cm from the source. 3D dosimetry using polyurethane dosimeters with optical CT looks to be a promising application to verify dosimetric distributions surrounding low energy brachytherapy sources.

Dependence with air density of the response of the PTW SourceCheck ionization chamber for low energy brachytherapy sources

Air-communicating well ionization chambers are commonly used to assess air kerma strength of sources used in brachytherapy. The signal produced is supposed to be proportional to the air density within the chamber and, therefore, a density-independent air kerma strength is obtained when the measurement is corrected to standard atmospheric conditions using the usual temperature and pressure correction factor. Nevertheless, when assessing low energy sources, the ionization chambers may not fulfill that condition and a residual density dependence still remains after correction. In this work, the authors examined the behavior of the PTW 34051 SourceCheck ionization chamber when measuring the air kerma strength of (125)I seeds. Four different SourceCheck chambers were analyzed. With each one of them, two series of measurements of the air kerma strength for (125)I selectSeed(TM) brachytherapy sources were performed inside a pressure chamber and varying the pressure in a range from 747 to 1040 hPa (560 to 780 mm Hg). The temperature and relative humidity were kept basically constant. An analogous experiment was performed by taking measurements at different altitudes above sea level. Contrary to other well-known ionization chambers, like the HDR1000 PLUS, in which the temperature-pressure correction factor overcorrects the measurements, in the SourceCheck ionization chamber they are undercorrected. At a typical atmospheric situation of 933 hPa (700 mm Hg) and 20 °C, this undercorrection turns out to be 1.5%. Corrected measurements show a residual linear dependence on the density and, as a consequence, an additional density dependent correction must be applied. The slope of this residual linear density dependence is different for each SourceCheck chamber investigated. The results obtained by taking measurements at different altitudes are compatible with those obtained with the pressure chamber. Variations of the altitude and changes in the weather conditions may produce significant density corrections, and that effect should be taken into account. This effect is chamber-dependent, indicating that a specific calibration is necessary for each particular chamber. To our knowledge, this correction has not been considered so far for SourceCheck ionization chambers, but its magnitude cannot be neglected in clinical practice. The atmospheric pressure and temperature at which the chamber was calibrated need to be taken into account, and they should be reported in the calibration certificate. In addition, each institution should analyze the particular response of its SourceCheck ionization chamber and compute the adequate correction factors. In the absence of a suitable pressure chamber, a possibility for this assessment is to take measurements at different altitudes, spanning a wide enough air density range.

A simplified analytical dose calculation algorithm accounting for tissue heterogeneity for low-energy brachytherapy sources

The American Association of Physicists in Medicine Task Group No. 43 (AAPM TG-43) formalism is the standard for seeds brachytherapy dose calculation. But for breast seed implants, Monte Carlo simulations reveal large errors due to tissue heterogeneity. Since TG-43 includes several factors to account for source geometry, anisotropy and strength, we propose an additional correction factor, called the inhomogeneity correction factor (ICF), accounting for tissue heterogeneity for Pd-103 brachytherapy. This correction factor is calculated as a function of the media linear attenuation coefficient and mass energy absorption coefficient, and it is independent of the source internal structure. Ultimately the dose in heterogeneous media can be calculated as a product of dose in water as calculated by TG-43 protocol times the ICF. To validate the ICF methodology, dose absorbed in spherical phantoms with large tissue heterogeneities was compared using the TG-43 formalism corrected for heterogeneity versus Monte Carlo simulations. The agreement between Monte Carlo simulations and the ICF method remained within 5% in soft tissues up to several centimeters from a Pd-103 source. Compared to Monte Carlo, the ICF methods can easily be integrated into a clinical treatment planning system and it does not require the detailed internal structure of the source or the photon phase-space.

SU-E-T-527: A Fast Novel Analytical Model Accounting for Interseed Attenuation Effect in Low Energy Brachytherapy Sources

Purpose: Currently, treatment planning systems make two assumptions that can affect dose calculation; Homogenous media and Radio‐transparency of surrounding seeds. In this project a fast novel analytical model to correct the inter‐seed attenuation (ISA) in multiseed implants for TG‐43U1 LDR brachytherapy have developed. Model sensitivity of the results to examined for multiple seeds of I‐125 and Ir‐192 in different distanc and angles in water phantom. Methods: In this model, dose perturbation (P‐value) and inter‐seed attenuation (ISA‐value) in a multi‐source brachytherapy implant is based on pre Monte Carlo simulated 3D kernels of perturbation and ISA data for single active and single dummy configuration, arranged at different distances and angles relative to each other. The final model results have been examined by a compared for the calculated Perturbation and ISA‐values with full Monte Carlo water simulations (FMCWS). Finally by multiply the model values in TG‐43U1 dose rate for any interest point, the ISA deficiency will be modified. Results: The results show that the perturbation model values at interest point or voxel are equal to multiply perturbation factor of single active‐inactive sources which are placed in straight line from active source to interest point of implant. The accuracy of the model for parallel multi‐source implants was 3.5% and 5% for Ir‐192 and I‐125, respectively. Sensitivity of the results for non‐parallel multi‐source implants was up to 5%. Conclusion: Base on this novel fast approach, one could expedite the processes of dose ISA corrections with the minimum increase of the planning time for standard and loos brachytherapy implants as compared to the FMCWS in TG‐43U1 brachytherapy treatment planning systems. This model is applicable for energy of I‐125 to Ir‐192 sources.

SU-E-T-551: A Novel Analytical Model to Determine Inhomogeneity Correction Factor in TG-43U1 Low Dose Rate Brachytherapy

Purpose: The American Association of Physicists in Medicine (AAPM) Task Group 43 (TG‐43) recommended dosimetric parameters of sources, in homogenous water phantom. However brachytherapy implant volumes are shown large error due to inhomogeneous mediums respect to homogenous water. The goal of this project is to determine a novel fast analytical method to evaluate the impact of the dose Inhomogeneity Correction Factor (DICF). This new method was validated for low (125I) and high (137Cs) energy brachytherapy sources. Methods: The novel analytical DICF is defined as the ratio of dose delivered into heterogeneous media in water to the dose delivered in homogenous water. The DICF is calculated as a function of the media linear attenuation coefficient, mass energy absorption coefficient, water equivalent path length of heterogenic layer, buildup factor coefficients, and media electron density. The actual dose is equal to multiply the TG‐43U1 dose by the DICF of the heterogenic layer in any phantom voxel. This approach was validated using TLD measurement and full Monte Carlo (FMC) simulation techniques. FMC simulations were used for several different air and aluminum heterogenic thickness. The dose data were compared for TG‐43U1 using DICF model and the values, which are directly, obtained using FMC for same Implants. The advantage of this model to FMCS for daily clinical application is the speed of the calculations and ease of the implementations in treatment planning system. Results: For different configurations, the analytical DICF formalism model agrees with FMC simulations up to 4.4% and 3.5% for 125I and 137Cs sources, respectively. The maximum differences of the DICF model in comparison to TLD measurement were 5% for 137CS. Conclusion: This new model can provide inputs for brachytherapy planning software to consider the ICF effect in dose calculations based on TG‐43U1 algorithm and CT images.

SU-E-T-25: Investigating the Impact of Polymer-Coating On the Dosimetric Characteristics of a 103Pd Brachytherapy Source Using Photon Spectroscopy

Purpose: Synthetic polymers have been used to coat the surface of existing low-energy brachytherapy sources to improve their fixity in tissue in order to minimize seed migration in the patient. The purpose of this study was to evaluate the effects of the polymer-coating on the photon energy spectra emitted by Pd-103 seeds and to estimate its impact on the dosimetric characteristics of these seeds. Methods: Two Pd-103 AnchorSeeds (Biocompatibles, Inc) and two Pd-103 TheraSeeds (Theragenics Corporation) were used in this study. The AnchorSeed is made from the TheraSeed by coating its surface with bio-absorbable polymeric anchoring rings and ribs. Photon energy spectra emitted by these seeds were measured using a high-resolution, high-purity Germanium (HPGe) photon spectrometer. The effects of polymer coating on the dose rate constant were quantified by comparisons of the coated seed with a non-coated seed. Results: The relative photon energy spectrum emitted by the AnchorSeed was nearly identical to that emitted by the TheraSeed without polymer coating. The dose-rate constants determined from the emitted photon energy spectra for the two seed models were identical to the third digit after the decimal point (0.679 ± 0.037 cGyh-1U-1). Because the basic dosimetric property of a brachytherapy seed is fundamentally determined by the photon characteristics emitted by the seed, effects of the polymer-coating on the relative dosimetry data of TheraSeed is also expected to be minimal. Using a preexisting photon spectrometer, these measurements and calculations were completed in about 10 working hours. Conclusion: The polymer coating used in the AnchorSeed has minimal impact on its dosimetric characteristics compared to the non-coated TheraSeed. We also demonstrate that photon spectroscopy provides a ready tool for the evaluation of impact of various coatings or other material changes in the seed manufacture process.

Calculation of the humidity correction factor in air kerma strength measurement for 125I and 103Pd brachytherapy sources and its uncertainty by Monte Carlo method

Humidity is one of the sources of systematic error in the absolute measurement of air kerma strength of 125I and 103Pd low energy brachytherapy sources by using free air ionization chambers. In this paper, the humidity correction factor and its uncertainty for several environmental conditions have been calculated by applying an indigenous developed uncertainty analysis algorithm programmed in FORTRAN. The results of the analysis showed that the humidity could affect the corrected measured source strength value by about 0.2%.

In vitro radiosensitization by gold nanoparticles during continuous low-dose-rate gamma irradiation with I-125 brachytherapy seeds

This communication reports the first experimental evidence of gold nanoparticle (AuNP) radiosensitization during continuous low-dose-rate (LDR) gamma irradiation with low-energy brachytherapy sources. HeLa cell cultures incubated with and without AuNP were irradiated with an I-125 seed plaque designed to produce a relatively homogeneous dose distribution in the plane of the cell culture slide. Four sets of irradiation experiments were conducted at low-dose rates ranging from 2.1 to 4.5cGy/h. Residual γH2AX was measured 24h after irradiation and used to compare radiation damage to the cells with and without AuNP. The data demonstrate that the biological effect when irradiating in the presence of 0.2mg/ml concentration of AuNP is about 70%–130% greater than without AuNP. Meanwhile, without radiation, the AuNP showed minimal effect on the cancer cells. These findings provide in vitro evidence that AuNP may be employed as radiosensitizers during continuous LDR brachytherapy. From the Clinical EditorIn this basic science paper, the application of gold nanoparticles as radiosensitizing agents for low dose rate gamma radiation therapy is discussed, demonstrating efficacy in cell culture models.

SU-E-T-92: on the Use of High-Sensitivity Thermoluminescent Dosimeters (TLDs) for Dosimetric Characterization of Low-Energy Brachytherapy Sources.

To investigate the utility and accuracy of high-sensitivity TLD for dosimetric characterization of low-energy brachytherapy sources. One hundred high-sensitivity (TLD-100H) and 100 normal-sensitivity (TLD-100) TLDs were used in this study. The TLD-100s were annealed at 400°C for one hour and then kept at room temperature for 45 minutes followed by 80°C heating for 24 hours. To prevent temperature overshot from reducing the sensitivity of TLD-100Hs, a novel thermal reservoir was built, tested, and used to anneal TLD-100H at 240 0C for 15 minutes and then kept at room temperature for 45 minutes followed by 100 0C heating for one hour. These TLDs were then irradiated uniformly in a large cavity Cs-137 irradiator for biomedical research (Shepherd, Mark III) to test their reproducibility and to establish their relative sensitivities. The radial dose function of a Model AgX100 125I source was measured using both types of TLDs in water-equivalent solid phantoms as a test case. The radial dose function measured by the TLD-100H was compared with that measured by TLD-100 to determine its utility in brachytherapy dosimetry characterization. Consistent and accurate annealing of high-sensitivity TLDs was achieved by using a custom-built thermal reservoir system. TLD-100H was found to be about 18 times more sensitive than TLD-100. For a 125I source with a source-strength of 2.7U, the irradiation time for radial dose function characterization up to 7 cm can be cut down from 38 days to 3 days. The radial dose function measured by TLD-100H agreed well (within ±6%) with that measured by TLD-100. A novel thermal reservoir was used for consistent annealing of high-sensitivity TLDs. TLD-100H can significantly shorten the irradiation time needed for a complete characterization of radial dose function. Investigation of TLD-100H for complete brachytherapy source characterization is in progress. Supported in part by NIH grant R01-CA134627.

Influence of trace elements in human tissue in low-energy photon brachytherapy dosimetry**This work was part of an invited presentation at the ‘International Workshop on Recent Advances in Monte Carlo Techniques for Radiation Therapy’, held in Montreal, June 8–10, 2011.

The aim of this paper is to determine the dosimetric impact of trace elements in human tissues for low-energy photon sources used in brachytherapy. Monte Carlo dose calculations were used to investigate the dosimetric effect of trace elements present in normal or cancerous human tissues. The effect of individual traces (atomic number Z = 11–30) was studied in soft tissue irradiated by low-energy brachytherapy sources. Three other tissue types (prostate, adipose and mammary gland) were also simulated with varying trace concentrations to quantify the contribution of each trace to the dose distribution. The dose differences between cancerous and healthy prostate tissues were calculated in single- and multi-source geometries. The presence of traces in a tissue produces a difference in the dose distribution that is dependent on Z and the concentration of the trace. Low-Z traces (Na) have a negligible effect (<0.3%) in all tissues, while higher Z (K) had a larger effect (>3%). There is a potentially significant difference in the dose distribution between cancerous and healthy prostate tissues (4%) and even larger if compared to the trace-free composition (15%) in both single- and multi-sourced geometries. Trace elements have a non-negligible (up to 8% in prostate D90) effect on the dose in tissues irradiated with low-energy photon sources. This study underlines the need for further investigation into accurate determination of the trace composition of tissues associated with low-energy brachytherapy. Alternatively, trace elements could be incorporated as a source of uncertainty in dose calculations.

Calculation of photon scattering and transmission correction factors for a free air ionization chamber at Nuclear Science and Technology Research Institute in Iran

In primary standard dosimetry laboratories, the air kerma strength of low energy brachytherapy sources 125I and 103Pd is measured by standard free air ionization chambers. At present, one of these free air ionization chambers is constructed in Radiation Application Research School, in Nuclear Science and Technology Research Institute in Iran. The transmission and scattering correction factors are needed for the absolute determination of air kerma strength.Transmission and scattering correction factors were calculated for a monoenergetic point isotropic source with energies from 5keV to 100keV for ionization chamber with 4cm and 6cm aperture radii by using the MCNP 4C code. The calculated scattering correction factors for 125I and 103Pd brachytherapy sources are 0.9987 and 0.9992, respectively and determined transmission correction is one for our free air ionization chamber of aperture radius 4cm.

Experimental characterization of the dosimetric properties of a newly designed I-Seed model AgX100 125I interstitial brachytherapy source

PurposeTo measure the dosimetric properties of the Model AgX100 125I source for interstitial brachytherapy. Methods and MaterialsThe photon energy spectrum emitted by the AgX100 source was measured using a high-resolution germanium spectrometer customized for low-energy brachytherapy source spectrometry. The dose distribution around the source was measured using the 1×1×1mm3 lithium fluoride thermoluminescent dosimeters in water-equivalent solid phantoms. The dosimetric parameters needed for dose calculation using the American Association of Physicists in Medicine Task Group No. 43 (TG-43) formalism were determined and compared with the results of a Monte Carlo simulation by an independent research group and with the TG-43 consensus values of the well-established model 6711 source. ResultsIt was found that (1) the photon energy spectrum emitted by the AgX100 source was nearly identical to that emitted by the model 6711, (2) the dose-rate constant determined by the photon spectrometry technique (0.957±0.037cGy·h−1·U−1) and by the thermoluminescent dosimeter technique (0.995±0.066cGy·h−1·U−1) was within 1.5% of the corresponding values determined for the model 6711 source, and (3) the radial dose function and the anisotropy function of the AgX100 source were also found to be similar to the consensus data established for the model 6711 source in the TG-43 update report. ConclusionsA comprehensive dosimetric characterization has been carried out for the model AgX100 125I source. The American Association of Physicists in Medicine TG-43 dosimetry parameters for this source has been determined from the experimental data.