192Ir Brachytherapy Sources Research Articles

Comparison of 3D dose distributions for HDR 192Ir brachytherapy sources with normoxic polymer gel dosimetry and treatment planning system

Radiation fluence changes caused by the dosimeter itself and poor spatial resolution may lead to lack of 3-dimensional (3D) information depending on the features of the dosimeter and quality assurance of dose distributions for high–dose rate (HDR) iridium-192 (192Ir) brachytherapy sources is challenging and experimental dosimetry methods used for brachytherapy sources are limited. In this study, we investigated 3D dose distributions of 192Ir brachytherapy sources for irradiation with single and multiple dwell positions using a normoxic gel dosimeter and compared them with treatment planning system (TPS) calculations. For dose calibration purposes, 100-mL gel-containing vials were irradiated at predefined doses and then scanned in an magnetic resonance (MR) imaging unit. Gel phantoms prepared in 2 spherical glasses were irradiated with 192Ir for the calculated dwell positions, and MR scans of the phantoms were obtained. The images were analyzed with MATLAB software. Dose distributions and profiles derived with 1-mm resolution were compared with TPS calculations. Linearity was observed between the delivered dose and the reciprocal of the T2 relaxation time constant of the gel. The x-, y-, and z-axes were defined as the sagittal, coronal, and axial planes, respectively, the sagittal and axial planes were defined parallel to the long axis of the source while the coronal plane was defined horizontally to the long axis of the source. The differences between measured and calculated profile widths of 3-cm source length and point source for 70%, 50%, and 30% isodose lines were evaluated at 3 dose levels using 18 profiles of comparison. The calculations for 3-cm source length revealed a difference of > 3mm in 1 coordinate at 50% profile width on the sagittal plane and 3 coordinates at 70% profile width and 2 coordinates at 50% and 30% profile widths on the axial plane. Calculations on the coronal plane for 3-cm source length showed > 3-mm difference in 1 coordinate at 50% and 70% and 2 coordinates at 30% profile widths. The point source measurements and calculations for 50% profile widths revealed a difference > 3mm in 1 coordinate on the sagittal plane and 2 coordinates on the axial plane. The doses of 3 coordinates on the sagittal plane and 4 coordinates on the axial plane could not be evaluated in 30% profile width because of low doses. There was good agreement between the gel dosimetry and TPS results. Gel dosimetry provides dose distributions in all 3 planes at the same time, which enables us to define the dose distributions in any plane with high resolution. It can be used to obtain 3D dose distributions for HDR 192Ir brachytherapy sources and 3D dose verification of TPS.

Estimation of distance error by fuzzy set theory required for strength determination of HDR (192)Ir brachytherapy sources.

Verification of the strength of high dose rate (HDR) 192Ir brachytherapy sources on receipt from the vendor is an important component of institutional quality assurance program. Either reference air-kerma rate (RAKR) or air-kerma strength (AKS) is the recommended quantity to specify the strength of gamma-emitting brachytherapy sources. The use of Farmer-type cylindrical ionization chamber of sensitive volume 0.6 cm3 is one of the recommended methods for measuring RAKR of HDR 192Ir brachytherapy sources. While using the cylindrical chamber method, it is required to determine the positioning error of the ionization chamber with respect to the source which is called the distance error. An attempt has been made to apply the fuzzy set theory to estimate the subjective uncertainty associated with the distance error. A simplified approach of applying this fuzzy set theory has been proposed in the quantification of uncertainty associated with the distance error. In order to express the uncertainty in the framework of fuzzy sets, the uncertainty index was estimated and was found to be within 2.5%, which further indicates that the possibility of error in measuring such distance may be of this order. It is observed that the relative distance li estimated by analytical method and fuzzy set theoretic approach are consistent with each other. The crisp values of li estimated using analytical method lie within the bounds computed using fuzzy set theory. This indicates that li values estimated using analytical methods are within 2.5% uncertainty. This value of uncertainty in distance measurement should be incorporated in the uncertainty budget, while estimating the expanded uncertainty in HDR 192Ir source strength measurement.

Monte Carlo Study on Does Enhancement Effect of Various Paramagnetic Nanoshells in Brachytherapy

The dose enhancement effect of various paramagnetic nanoshells irradiated with various brachytherapy sources was calculated. ^(125)I, ^(103)Pd, hypothetical ^(169)Yb, and ^(192)Ir brachytherapy sources were simulated using the MCNPX Monte Carlo code. The dose rate constants and radial dose functions of the sources were calculated and compared with previously published data. Au/Gd, Au/Fe_2O_3, Ag/Dy, and Ag/Mn nanoshells with concentrations of 10, 20, and 30 mg/ml in tumor tissue were simulated. A soft-tissue phantom containing a 1 cm×1 cm×1 cm tumor was loaded with nanoshells. The dose enhancement factor (DEF) was calculated in voxels inside and outside the tumor. The highest average DEFs in tumor tissue were observed with the ^(125)I source for the 30 mg/ml tumor concentration, with DEF values for Au/Gd, Au/Fe_2O_3, Ag/Dy, and Ag/Mn nanoshells of 3.25, 2.82, 2.86, and 2.77, respectively. The calculated dose rate constant and radial dose function of the sources were in good agreement with previously published data. Based on our results, nanoshells can be utilized in brachytherapy to enhance the tumor dose and because of their biocompatible shells, their toxic effects can be avoided. The nanoshells are paramagnetic and can be applied simultaneously in brachytherapy and magnetic resonance imaging. However, clinical application of nanoshells in brachytherapy requires more preclinical studies.

Effect of tissue composition on dose distribution in brachytherapy with various photon emitting sources

PurposeThe aim of this study is to compare the dose in various soft tissues in brachytherapy with photon emitting sources.Material and methods103Pd, 125I, 169Yb, 192Ir brachytherapy sources were simulated with MCNPX Monte Carlo code, and their dose rate constant and radial dose function were compared with the published data. A spherical phantom with 50 cm radius was simulated and the dose at various radial distances in adipose tissue, breast tissue, 4-component soft tissue, brain (grey/white matter), muscle (skeletal), lung tissue, blood (whole), 9-component soft tissue, and water were calculated. The absolute dose and relative dose difference with respect to 9-component soft tissue was obtained for various materials, sources, and distances.ResultsThere was good agreement between the dosimetric parameters of the sources and the published data. Adipose tissue, breast tissue, 4-component soft tissue, and water showed the greatest difference in dose relative to the dose to the 9-component soft tissue. The other soft tissues showed lower dose differences. The dose difference was also higher for 103Pd source than for 125I, 169Yb, and 192Ir sources. Furthermore, greater distances from the source had higher relative dose differences and the effect can be justified due to the change in photon spectrum (softening or hardening) as photons traverse the phantom material.ConclusionsThe ignorance of soft tissue characteristics (density, composition, etc.) by treatment planning systems incorporates a significant error in dose delivery to the patient in brachytherapy with photon sources. The error depends on the type of soft tissue, brachytherapy source, as well as the distance from the source.

Using LiF:Mg,Cu,P TLDs to estimate the absorbed dose to water in liquid water around an192Ir brachytherapy source

The absorbed dose to water is the fundamental reference quantity for brachytherapy treatment planning systems and thermoluminescence dosimeters (TLDs) have been recognized as the most validated detectors for measurement of such a dosimetric descriptor. The detector response in a wide energy spectrum as that of an (192)Ir brachytherapy source as well as the specific measurement medium which surrounds the TLD need to be accounted for when estimating the absorbed dose. This paper develops a methodology based on highly sensitive LiF:Mg,Cu,P TLDs to directly estimate the absorbed dose to water in liquid water around a high dose rate (192)Ir brachytherapy source. Different experimental designs in liquid water and air were constructed to study the response of LiF:Mg,Cu,P TLDs when irradiated in several standard photon beams of the LNE-LNHB (French national metrology laboratory for ionizing radiation). Measurement strategies and Monte Carlo techniques were developed to calibrate the LiF:Mg,Cu,P detectors in the energy interval characteristic of that found when TLDs are immersed in water around an (192)Ir source. Finally, an experimental system was designed to irradiate TLDs at different angles between 1 and 11 cm away from an (192)Ir source in liquid water. Monte Carlo simulations were performed to correct measured results to provide estimates of the absorbed dose to water in water around the (192)Ir source. The dose response dependence of LiF:Mg,Cu,P TLDs with the linear energy transfer of secondary electrons followed the same variations as those of published results. The calibration strategy which used TLDs in air exposed to a standard N-250 ISO x-ray beam and TLDs in water irradiated with a standard (137)Cs beam provided an estimated mean uncertainty of 2.8% (k = 1) in the TLD calibration coefficient for irradiations by the (192)Ir source in water. The 3D TLD measurements performed in liquid water were obtained with a maximum uncertainty of 11% (k = 1) found at 1 cm from the source. Radial dose values in water were compared against published results of the American Association of Physicists in Medicine and the European Society for Radiotherapy and Oncology and no significant differences (maximum value of 3.1%) were found within uncertainties except for one position at 9 cm (5.8%). At this location the background contribution relative to the TLD signal is relatively small and an unexpected experimental fluctuation in the background estimate may have caused such a large discrepancy. This paper shows that reliable measurements with TLDs in complex energy spectra require a study of the detector dose response with the radiation quality and specific calibration methodologies which model accurately the experimental conditions where the detectors will be used. The authors have developed and studied a method with highly sensitive TLDs and contributed to its validation by comparison with results from the literature. This methodology can be used to provide direct estimates of the absorbed dose rate in water for irradiations with HDR (192)Ir brachytherapy sources.

Source position verification and dosimetry in HDR brachytherapy using an EPID

Accurate treatment delivery in high dose rate (HDR) brachytherapy requires correct source dwell positions and dwell times to be administered relative to each other and to the surrounding anatomy. Treatment delivery inaccuracies predominantly occur for two reasons: (i) anatomical movement or (ii) as a result of human errors that are usually related to incorrect implementation of the planned treatment. Electronic portal imaging devices (EPIDs) were originally developed for patient position verification in external beam radiotherapy and their application has been extended to provide dosimetric information. The authors have characterized the response of an EPID for use with an (192)Ir brachytherapy source to demonstrate its use as a verification device, providing both source position and dosimetric information. Characterization of the EPID response using an (192)Ir brachytherapy source included investigations of reproducibility, linearity with dose rate, photon energy dependence, and charge build-up effects associated with exposure time and image acquisition time. Source position resolution in three dimensions was determined. To illustrate treatment verification, a simple treatment plan was delivered to a phantom and the measured EPID dose distribution compared with the planned dose. The mean absolute source position error in the plane parallel to the EPID, for dwells measured at 50, 100, and 150 mm source to detector distances (SDD), was determined to be 0.26 mm. The resolution of the z coordinate (perpendicular distance from detector plane) is SDD dependent with 95% confidence intervals of ± 0.1, ± 0.5, and ± 2.0 mm at SDDs of 50, 100, and 150 mm, respectively. The response of the EPID is highly linear to dose rate. The EPID exhibits an over-response to low energy incident photons and this nonlinearity is incorporated into the dose calibration procedure. A distance (spectral) dependent dose rate calibration procedure has been developed. The difference between measured and planned dose is less than 2% for 98.0% of pixels in a two-dimensional plane at an SDD of 100 mm. Our application of EPID dosimetry to HDR brachytherapy provides a quality assurance measure of the geometrical distribution of the delivered dose as well as the source positions, which is not possible with any current HDR brachytherapy verification system.

Measurement of absorbed dose‐to‐water for an HDR 192Ir source with ionization chambers in a sandwich setup

In this study, a dedicated device for ion chamber measurements of absorbed dose-to-water for a Nucletron microSelectron-v2 HDR (192)Ir brachytherapy source is presented. The device uses two ionization chambers in a so-called sandwich assembly. Using this setup and by taking the average reading of the two chambers, any dose error due to difficulties in absolute positioning (centering) of the source in between the chambers is cancelled to first order. The method's accuracy was examined by comparing measurements with absorbed dose-to-water determination based on the AAPM TG-43 protocol. The optimal source-to-chamber distance (SCD) for (192)Ir dosimetry was determined from ion chamber measurements in a water phantom. The (192)Ir source was sandwiched between two Exradin A1SL chambers (0.057 cm(3)) at the optimal SCD separation. The measured ionization was converted to the absorbed dose-to-water using a (60)Co calibration factor and a Monte Carlo-calculated beam quality conversion factor, kQ, for (60)Co to (192)Ir. An uncertainty estimate of the proposed method was determined based on reproducibility of measurements at different institutions for the same type of source. The optimal distance for the A1SL chamber measurements was determined to be 5 cm from the (192)Ir source center, considering the depth dependency of kQ for (60)Co to (192)Ir and the chamber positioning. The absorbed dose to water measured at (5 cm, 90°) on the transverse axis was 1.3% lower than TG-43 values and its reproducibility and overall uncertainty were 0.8% and 1.7%, respectively. The measurement doses at anisotropic points agreed within 1.5% with TG-43 values. The ion chamber measurement of absorbed dose-to-water with a sandwich method for the (192)Ir source provides a more accurate, direct, and reference dose compared to the dose-to-water determination based on air-kerma strength in the TG-43 protocol. Due to the simple but accurate assembly, the sandwich measurement method is useful for daily dose management of (192)Ir sources.

SU-E-T-102: Measurement of Absorbed Dose-To-Water for An HDR Ir-192 Source with a Farmer Ionization Chamber in a Sandwich Setup

Purpose: This study developed a dedicated device for ion chamber measurements of absorbed dose-to-water for an HDR Ir-192 brachytherapy source. The device uses two Farmer chambers in a so-called sandwich assembly. The accuracy of the sundwich method was examined by comparing chamber measurements with absorbed dose-to-water determination based on the AAPM TG-43 protocol. Methods: A microSelectron-v2 HDR Ir-192 source was modeled with the EGSnrc/egs_chamber code. The accuracy of modeling was confirmed by comparing calculated results for gL(r) and F(r, θ) with those of TG-43. The PTW30013 Framer chamber was also modeled with the egs_chamber code and its beam quality conversion factors, kQ, of Ir-192 to Co-60 were calculated as a function of distance from the Ir-192 source. From calculation results, we developed a dedicated device for the sandwich setup with 1 mm PMMA waterproof sleeves located at both side of 8 cm from the Ir-192 source. The chamber readings measured with the Farmer chambers inserted into the 1 mm PMMA sleeves were converted to the absorbed dose to water using kQ. The average dose of two Farmer chamber measurements with the sandwich setup were compared with the TPS values based on TG-43 for eight Ir-192 sources at four institutes. Results: Calculated gL(r) and F(r, θ) values agreed well with those of TG-43. The calculated kQ values for the PTW30013 Framer chamber ranged from 1.103 to 0.986 at distance of 2∼10 cm from the source and it was 0.992 at distance of 8 cm. The doses to water measured with the sandwich setup were −0.73 +/− 0.28% (n=10) compared to TG-43 values. Conclusion: The dose to water measurement by the Farmer chamber with the sandwich setup is useful for daily dose management for Ir-192 sources.

Perturbation of TG‐43 parameters of the brachytherapy sources under insufficient scattering materials

In the recommendations of Task Group #43 from American Association of Physicists in Medicine (AAPM TG43), methods of brachytherapy source dosimetry are recommended, under full scattering conditions. However, in actual brachytherapy procedures, sources may not be surrounded by full scattering tissue in all directions. Clinical examples include high‐dose‐rate (HDR) brachytherapy of the breast or low‐dose‐rate (LDR) brachytherapy of ocular melanoma using eye plaque treatment with 125I and 103Pd. In this work, the impact of the missing tissue on the TG‐43–recommended dosimetric parameters of different brachytherapy sources was investigated. The impact of missing tissue on the TG‐43–recommended dosimetric parameters of 137Cs, 192Ir, and 103Pd brachytherapy sources was investigated using the MCNP5 Monte Carlo code. These evaluations were performed by placing the sources at different locations inside a 30×30×30 cm3 cubical water phantom and comparing the results with the values of the source located at the center of the phantom, which is in a full scattering condition. The differences between the thickness of the overlying tissues for different source positions and the thickness of the overlying tissue in full scattering condition is referred to as missing tissue. The results of these investigations indicate that values of the radial dose function and 2D anisotropy function vary as a function of the thickness of missing tissue, only in the direction of the missing tissue. These changes for radial dose function were up to 5%, 11%, and 8% for 137Cs, 192Ir, and 103Pd, respectively. No significant changes are observed for the values of the dose rate constants. In this project, we have demonstrated that the TG‐43 dosimetric parameters may only change in the directions of the missing tissue. These results are more practical than the published data by different investigators in which a symmetric effect of the missing tissue on the dosimetric parameters of brachytherapy source are being considered, regardless of the implant geometry in real clinical cases.PACS number: 87.53.JW

Evaluation of Wall Correction Factor of INER's Air-Kerma Primary Standard Chamber and Dose Variation by Source Displacement for HDR192Ir Brachytherapy

The aim of the present study was to estimate the wall effect of the self-made spherical graphite-walled cavity chamber with the Monte Carlo method for establishing the air-kerma primary standard of high-dose-rate (HDR) 192Ir brachytherapy sources at the Institute of Nuclear Energy Research (INER, Taiwan). The Monte Carlo method established in this paper was also employed to respectively simulate wall correction factors of the 192Ir air-kerma standard chambers used at the National Institute of Standards and Technology (NIST, USA) and the National Physical Laboratory (NPL, UK) for comparisons and verification. The chamber wall correction calculation results will be incorporated into INER's HDR 192Ir primary standard in the future. For the brachytherapy treatment in the esophagus or in the bronchi, the position of the isotope may have displacement in the cavity. Thus the delivered dose would differ from the prescribed dose in the treatment plan. We also tried assessing dose distribution due to the position displacement of HDR 192Ir brachytherapy source in a phantom with a central cavity by the Monte Carlo method. The calculated results could offer a clinical reference for the brachytherapy within the human organs with cavity.

Optical scanner for 3D radiotherapy polymer gel dosimetry.

Sophisticated techniques employed in radiotherapy for irradiation of tumours require comprehensive dosimetry allowing for precise, high resolution measurements of radiation dose distribution in three dimensions and verification of treatment planning systems. Polymer gel dosimetry has been shown to be a unique technique for three-dimensional high resolution measurements of absorbed radiation dose distributions. If exposed to ionizing radiation, radical polymerisation and crosslinking of monomeric components take place in a 3D polymer gel dosimeter, leading to the formation of large polymeric structures that scatter visible light. This feature allows for optical observation of the effects of the absorbed dose and its distribution. Presently, magnetic resonance imaging is employed the most often for analysis of 3D polymer gel dosimeters. However, much attention is also being given to the development of optical computed tomography since this technique is hoped to serve as a substitute for expensive and not easily available magnetic resonance imaging. The optical scanner presented in this work consists of a laser diode, a scanning system and a signal detector. A 3D polymer gel dosimeter is measured in an immersion liquid in order to reduce deflection of the light from the dosimeter phantom. The very first results were obtained with the newly constructed scanner for PABIGnx 3D polymer gel dosimeter, which was inhomogeneously irradiated with 192Ir brachytherapy source. The results have been contrasted with those for magnetic resonance imaging and are presented in this work together with the description of the optical scanner. Currently, optimization of the optical scanner is performed.

NPL's new absorbed dose standard for the calibration of HDR 192Ir brachytherapy sources

A novel graphite calorimeter for absorbed dose rate measurements close to high dose rate (HDR) 192Ir brachytherapy sources has been designed and built at the UK National Physical Laboratory (NPL). The graphite calorimeter allows a more direct calibration of HDR 192Ir sources in terms of absorbed dose rate to water at a distance of 1 cm, , compared with the current air kerma-based calibration method, where is determined as the product of the measured air kerma strength and the dose rate constant, Λ, resulting in overall standard uncertainties of up to 5%. With the new absorbed dose standard, can be measured directly with standard uncertainties between 0.7% and 1.0%, depending on the operating mode of the calorimeter. A Nucletron microSelectron-v1 Classic HDR 192Ir source was calibrated with the calorimeter in terms of . The same source was also calibrated with NPL's HDR 192Ir air kerma primary standard in terms of reference air kerma rate. Combining both measurements yielded the experimentally determined dose rate constant of the 192Ir source.

Experimental determination of the dose rate constant for selected 125I- and 192Ir-brachytherapy sources

In 2008, the European project ‘T2.J06, Increasing cancer treatment efficacy using 3D brachytherapy’ was launched. One of the main goals of the Joint Research Project was the experimental determination of the dose rate constant Λ to allow the linkage between the air kerma strength or reference air kerma rate currently used for the source characterization and the ‘new’ absorbed dose rate to water with an uncertainty of <3% (k = 1) for some selected brachytherapy sources. The results obtained by five National Metrology Institutes (NMI) for four different types of brachytherapy sources are presented and compared with consensus data published in the literature. A further goal of the project was to develop a calibration chain for the transfer of the new reference quantity to the end user, minimizing the uncertainty. A first direct calibration in terms of absorbed dose rate to water of a secondary standard and the dissemination to the hospitals is presented.

Absorbed dose to water distribution measured around an HDR 192Ir brachytherapy source by thermoluminescent dosimeters

The purpose of this work was to develop a procedure to directly estimate the spatial distribution of the absorbed dose rate to water, , around an HDR 192Ir brachytherapy source. The methodology developed was based on Monte Carlo calculations and measurements in air and in water with thermoluminescent detectors. Variations in detector positioning had a significant influence near the brachytherapy source (20% at 1 cm). The method leads to a mean difference of about 7% with the CLRP TG-43 Parameter Database when the absorbed dose to water is characterized along the transverse plane to the source (from 1 cm to about 11 cm). This mean difference, however, is within an uncertainty of 7.7% over all distances. This method therefore can be used to provide direct estimates of the absorbed dose rate to water for HDR brachytherapy source irradiations which are more realistic than those which use other phantom materials. In addition, measurements are indicative of the source geometry and material composition.

Introduction of IPEM code of practice for determination of reference air kerma rate for HDR 192Ir brachytherapy sources based on the NPL air kerma standard

Introduction of IPEM code of practice for determination of reference air kerma rate for HDR 192Ir brachytherapy sources based on the NPL air kerma standard

Polyamide woven fabrics with 2,3,5-triphenyltetrazolium chloride or nitro blue tetrazolium chloride as 2D ionizing radiation dosimeters

Abstract The development of flat woven fabric-based ionizing radiation 2D dosimeters is reported in this work. Polyamide fabric was surface modified with radiation-sensitive 2,3,5-triphenyltetrazolium chloride (TTC) or nitro blue tetrazolium chloride (NBT). These samples responded to gamma radiation of 60 Co through a colour change: red and blue for TTC and NBT, respectively. The tinge intensity was observed to depend on the absorbed dose, which allowed for the reflectance of light ( R ) measurements and calculation of the calibration parameters: dose range, quasi-linear range, threshold dose and sensitivity. Oxygen was shown to be an important factor determining the dose response of the samples. For this reason, a range of additional modifications to the TTC- and NBT-polyamide fabric was proposed which lead to a decrease in the threshold dose and increase in the sensitivity to irradiation of the samples. For instance, a dosimeter made of polyamide fabric modified with 10 g/dm 3 TTC, 0.5 mol/dm 3 tert-butyl alcohol, 7.5% gelatine hydrogel at pH 10 (vacuum packed) showed the lowest dose threshold (50 Gy), dose range up to 2.8 kGy and the highest sensitivity to irradiation (−0.0396%/Gy) among the compositions studied. In consequence, this dosimeter was examined in terms of response to inhomogeneous irradiation from a 192 Ir brachytherapy radiation source. The relative dose distribution profiles across the source's longitudinal axis were calculated. This showed potential of the textile dosimeters for 2D dose distribution measurements; however, further modifications towards improvement of the dosimeter's low dose response can be considered.

SU-E-T-501: Perturbation of TG-43 Parameters of the Brachytherapy Sources under Insufficient Scattering Materials.

According to the TG-43 recommendations of American Association of Physicist in Medicine (AAPM), the dosimetry parameters of brachytherapy sources are obtained in a water phantom with full scattering conditions. However, in many actual clinical treatments the source are not surrounded with sufficient tissue in all directions to provide the full scattering condition. HDR brachytherapy of the breast or 125I eye-plaque treatment of the ocular melanoma are among the treatment sites that fits this category. In this project, the impact of insufficient phantom material surrounding the 137Cs, 192Ir, and 103Pd brachytherapy source on their TG43 dosimetric characteristics has been investigated. In this study, the effect of the insufficient tissue (referred here as missing tissues) around the brachytherapy sources on their TG-43 dosimetric parameters have been investigated using MCNP5 Monte Carlo code. The brachytherapy sources were simulated in different locations inside a cubical water phantom with the dimensions of 30*30*30 cm3 . The variation of the dosimetric parameters of the three sources (i.e. Î>, g(r), and F(r,θ)) were compared to the values from full scattering conditions (i.e. source at the center of the phantom). The results of this study indicate the variation of g(r)and 2D anisotropy function of the brachytherapy sources as a function of the missing tissue thickness. These changes increase by decreasing the energy of the photons emitted by the brachytherapy sources. These differences are mainly due to the lack of full scattering condition for the points near the phantom boundary. Unlike the published data for symmetric, but insufficient phantom material around the source, the impact of asymmetric phantom materials have been evaluated on dosimetric characteristics of the brachytherapy sources.

Dosimetric verification of a high dose rate brachytherapy treatment planning system in homogeneous and heterogeneous media

ObjectivesTo verify the dosimetric accuracy of treatment plans in high dose rate (HDR) brachytherapy by using Gafchromic EBT2 film and to demonstrate the adequacy of dose calculations of a commercial treatment planning system (TPS) in a heterogeneous medium. MethodsAbsorbed doses at chosen points in anatomically different tissue equivalent phantoms were measured using Gafchromic EBT2 film. In one case, tandem ovoid brachytherapy was performed in a homogeneous cervix phantom, whereas in the other, organ heterogeneities were introduced in a phantom to replicate the upper thorax for esophageal brachytherapy treatment. A commercially available TPS was used to perform treatment planning in each case and the EBT2 films were irradiated with the HDR Ir-192 brachytherapy source. ResultsFilm measurements in the cervix phantom were found to agree with the TPS calculated values within 3% in the clinically relevant volume. In the thorax phantom, the presence of surrounding heterogeneities was not seen to affect the dose distribution in the volume being treated, whereas, a little dose perturbation was observed at the lung surface. Doses to the spinal cord and to the sternum bone were overestimated and underestimated by 14.6% and 16.5% respectively by the TPS relative to the film measurements. At the trachea wall facing the esophagus, a dose reduction of 10% was noticed in the measurements. ConclusionsThe dose calculation accuracy of the TPS was confirmed in homogeneous medium, whereas, it was proved inadequate to produce correct dosimetric results in conditions of tissue heterogeneity.

Using matrix summation method for three dimensional dose calculation in brachytherapy

AimThe purpose of this study is to calculate radiation dose around a brachytherapy source in a water phantom for different seed locations or rotation the sources by the matrix summation method. BackgroundMonte Carlo based codes like MCNP are widely used for performing radiation transport calculations and dose evaluation in brachytherapy. But for complicated situations, like using more than one source, moving or rotating the source, the routine Monte Carlo method for dose calculation needs a long time running. Materials and methodsThe MCNPX code has been used to calculate radiation dose around a 192Ir brachytherapy source and saved in a 3D matrix. Then, we used this matrix to evaluate the absorbed dose in any point due to some sources or a source which shifted or rotated in some places by the matrix summation method. ResultsThree dimensional (3D) dose results and isodose curves were presented for 192Ir source in a water cube phantom shifted for 10 steps and rotated for 45 and 90° based on the matrix summation method. Also, we applied this method for some arrays of sources. ConclusionThe matrix summation method can be used for 3D dose calculations for any brachytherapy source which has moved or rotated. This simple method is very fast compared to routine Monte Carlo based methods. In addition, it can be applied for dose optimization study.

Comparison of air‐kerma strength determinations for HDR 192Ir sources

To perform a comparison of the interim air-kerma strength standard for high dose rate (HDR) (192)Ir brachytherapy sources maintained by the University of Wisconsin Accredited Dosimetry Calibration Laboratory (UWADCL) with measurements of the various source models using modified techniques from the literature. The current interim standard was established by Goetsch et al. in 1991 and has remained unchanged to date. The improved, laser-aligned seven-distance apparatus of the University of Wisconsin Medical Radiation Research Center (UWMRRC) was used to perform air-kerma strength measurements of five different HDR (192)Ir source models. The results of these measurements were compared with those from well chambers traceable to the original standard. Alternative methodologies for interpolating the (192)Ir air-kerma calibration coefficient from the NIST air-kerma standards at (137)Cs and 250 kVp x rays (M250) were investigated and intercompared. As part of the interpolation method comparison, the Monte Carlo code EGSnrc was used to calculate updated values of A(wall) for the Exradin A3 chamber used for air-kerma strength measurements. The effects of air attenuation and scatter, room scatter, as well as the solution method were investigated in detail. The average measurements when using the inverse N(K) interpolation method for the Classic Nucletron, Nucletron microSelectron, VariSource VS2000, GammaMed Plus, and Flexisource were found to be 0.47%, -0.10%, -1.13%, -0.20%, and 0.89% different than the existing standard, respectively. A further investigation of the differences observed between the sources was performed using MCNP5 Monte Carlo simulations of each source model inside a full model of an HDR 1000 Plus well chamber. Although the differences between the source models were found to be statistically significant, the equally weighted average difference between the seven-distance measurements and the well chambers was 0.01%, confirming that it is not necessary to update the current standard maintained at the UWADCL.