Search

Purpose To evaluate the possibility of utilizing the high-dose rate ( HDR) 169Yb and 60Co sources, in addition to 192Ir, for the treatment of skin malignancies with conical applicators. Methods Monte Carlo ( MC) simulations were used to benchmark the dosimetric parameters of single 169Yb (4140), 60Co (Co0.A86), and 192Ir ( mHDR-V2) brachytherapy sources in a water phantom and compared their results against published data. A standard conical tungsten alloy Leipzig-style applicator (Stand.Appl) was used for determination of the dose distributions at various depths with a single dwell position of the HDR sources. The HDR sources were modeled with its long axis parallel to the treatment plane within the opening section of the applicator. The source-to-surface distance ( SSD) was 1.6 cm, which included a 0.1 cm thick removable plastic end-cap used for clinical applications. The prescription depth was considered to be 0.3 cm in a water phantom following the definitions in the literature for this treatment technique. Dose distributions generated with the Stand.Appl and the 169Yb and 60Co sources have been compared with those of the 192Ir source, for the same geometry. Then, applicator wall thickness for the 60Co source was increased (doubled) in MC simulations in order to minimize the leakage dose and penumbra to levels that were comparable to that from the 192Ir source. For each source-applicator combination, the optimized plastic end-cap dimensions were determined in order to avoid over-dosage to the skin surface. Results The normalized dose profiles at the prescription depth for the 169Yb-Stand.Appl and the 60Co-double-wall applicator were found to be similar to that of the 192Ir-Stand.Appl, with differences < 2.5%. The percentage depth doses ( PDD) for the 192Ir-, 169Yb- and 60Co-Stand.Appl were found to be comparable to the values with the 60Co-double-walled applicator, with differences < 1.7%. The applicator output-factors at the prescription depth were also comparable at 0.309, 0.316, and 0.298 ( cGy/ hU) for the 192Ir-, 169Yb-Stand.Appl, and 60Co-double-wall applicators respectively. The leakage dose around the Stand.Appl for distance > 2 cm from the applicator surface was < 5% for 192Ir, < 1% for 169Yb, and < 18% for 60Co relative to the prescription dose. However, using the double-walled applicator for the 60Co source reduced the leakage dose to around 5% of the prescription dose, which is comparable with that of the 192Ir source. The optimized end-cap thicknesses for the 192Ir-, 169Yb-Stand.Appl, and the 60Co-double-wall applicator were found to be 1.1, 0.6, and 3.7 mm respectively. Conclusions Application of the 169Yb (with Stand.Appl) or the 60Co source (with double-wall applicator) has been evaluated as alternatives to the existing 192Ir source (with Stand.Appl) for the HDR brachytherapy of skin cancer patients. These alternatives enable the clinics that may have 169Yb or 60Co sources instead of the 192Ir source to perform the skin brachytherapy and achieve comparable results. The conical surface applicators must be used with a protective plastic end-cap to eliminate the excess electrons that are created in the source and applicator, in order to avoid skin surface over-dosage. The treatment times for the 60Co source remain to be determined. Additionally, for 169Yb, the source needs to be changed on monthly basis due to its limited half-life. [ABSTRACT FROM AUTHOR]

Since the publication of the 2004 update to the American Association of Physicists in Medicine ( AAPM) Task Group No. 43 Report ( TG-43U1) and its 2007 supplement ( TG-43U1S1), several new low-energy photon-emitting brachytherapy sources have become available. Many of these sources have satisfied the AAPM prerequisites for routine clinical purposes and are posted on the Brachytherapy Source Registry managed jointly by the AAPM and the Imaging and Radiation Oncology Core Houston Quality Assurance Center ( IROC Houston). Given increasingly closer interactions among physicists in North America and Europe, the AAPM and the Groupe Européen de Curiethérapie-European Society for Radiotherapy & Oncology ( GEC- ESTRO) have prepared another supplement containing recommended brachytherapy dosimetry parameters for eleven low-energy photon-emitting brachytherapy sources. The current report presents consensus datasets approved by the AAPM and GEC- ESTRO. The following sources are included: 125I sources ( BEBIG model I25.S17, BEBIG model I25.S17plus, BEBIG model I25.S18, Elekta model 130.002, Oncura model 9011, and Theragenics model AgX100); 103Pd sources (CivaTech Oncology model CS10, IBt model 1031L, IBt model 1032P, and IsoAid model IAPd-103A); and 131Cs (IsoRay Medical model CS-1 Rev2). Observations are included on the behavior of these dosimetry parameters as a function of radionuclide. Recommendations are presented on the selection of dosimetry parameters, such as from societal reports issuing consensus datasets (e.g., TG-43U1, AAPM Report #229), the joint AAPM/ IROC Houston Registry, the GEC- ESTRO website, the Carleton University website, and those included in software releases from vendors of treatment planning systems. Aspects such as timeliness, maintenance, and rigor of these resources are discussed. Links to reference data are provided for radionuclides (radiation spectra and half-lives) and dose scoring materials (compositions and mass densities). The recent literature is examined on photon energy response corrections for thermoluminescent dosimetry of low-energy photon-emitting brachytherapy sources. Depending upon the dosimetry parameters currently used by individual physicists, use of these recommended consensus datasets may result in changes to patient dose calculations. These changes must be carefully evaluated and reviewed with the radiation oncologist prior to their implementation. [ABSTRACT FROM AUTHOR]

Dosimetry of eye plaques for ocular tumors presents unique challenges in brachytherapy. The challenges in accurate dosimetry are in part related to the steep dose gradient in the tumor and critical structures that are within millimeters of radioactive sources. In most clinical applications, calculations of dose distributions around eye plaques assume a homogenous water medium and full scatter conditions. Recent Monte Carlo (MC)-based eye-plaque dosimetry simulations have demonstrated that the perturbation effects of heterogeneous materials in eye plaques, including the gold-alloy backing and Silastic insert, can be calculated with reasonable accuracy. Even additional levels of complexity introduced through the use of gold foil 'seed-guides' and custom-designed plaques can be calculated accurately using modern MC techniques. Simulations accounting for the aforementioned complexities indicate dose discrepancies exceeding a factor of ten to selected critical structures compared to conventional dose calculations. Task Group 129 was formed to review the literature; re-examine the current dosimetry calculation formalism; and make recommendations for eye-plaque dosimetry, including evaluation of brachytherapy source dosimetry parameters and heterogeneity correction factors. A literature review identified modern assessments of dose calculations for Collaborative Ocular Melanoma Study (COMS) design plaques, including MC analyses and an intercomparison of treatment planning systems (TPS) detailing differences between homogeneous and heterogeneous plaque calculations using the American Association of Physicists in Medicine (AAPM) TG-43U1 brachytherapy dosimetry formalism and MC techniques. This review identified that a commonly used prescription dose of 85 Gy at 5 mm depth in homogeneous medium delivers about 75 Gy and 69 Gy at the same 5 mm depth for specific 125I and 103Pd sources, respectively, when accounting for COMS plaque heterogeneities. Thus, the adoption of heterogeneous dose calculation methods in clinical practice would result in dose differences >10% and warrant a careful evaluation of the corresponding changes in prescription doses. Doses to normal ocular structures vary with choice of radionuclide, plaque location, and prescription depth, such that further dosimetric evaluations of the adoption of MC-based dosimetry methods are needed. The AAPM and American Brachytherapy Society (ABS) recommend that clinical medical physicists should make concurrent estimates of heterogeneity-corrected delivered dose using the information in this report's tables to prepare for brachytherapy TPS that can account for material heterogeneities and for a transition to heterogeneity-corrected prescriptive goals. It is recommended that brachytherapy TPS vendors include material heterogeneity corrections in their systems and take steps to integrate planned plaque localization and image guidance. In the interim, before the availability of commercial MC-based brachytherapy TPS, it is recommended that clinical medical physicists use the line-source approximation in homogeneous water medium and the 2D AAPM TG-43U1 dosimetry formalism and brachytherapy source dosimetry parameter datasets for treatment planning calculations. Furthermore, this report includes quality management program recommendations for eye-plaque brachytherapy. [ABSTRACT FROM AUTHOR]

Purpose: Recommendations of the American Association of Physicists in Medicine (AAPM) and the European Society for Radiotherapy and Oncology (ESTRO) on dose calculations for high-energy (average energy higher than 50 keV) photon-emitting brachytherapy sources are presented, including the physical characteristics of specific 192Ir, 137Cs, and 60Co source models. Methods: This report has been prepared by the High Energy Brachytherapy Source Dosimetry (HEBD) Working Group. This report includes considerations in the application of the TG-43U1 formalism to high-energy photon-emitting sources with particular attention to phantom size effects, interpolation accuracy dependence on dose calculation grid size, and dosimetry parameter dependence on source active length. Results: Consensus datasets for commercially available high-energy photon sources are provided, along with recommended methods for evaluating these datasets. Recommendations on dosimetry characterization methods, mainly using experimental procedures and Monte Carlo, are established and discussed. Also included are methodological recommendations on detector choice, detector energy response characterization and phantom materials, and measurement specification methodology. Uncertainty analyses are discussed and recommendations for high-energy sources without consensus datasets are given. Conclusions: Recommended consensus datasets for high-energy sources have been derived for sources that were commercially available as of January 2010. Data are presented according to the AAPM TG-43U1 formalism, with modified interpolation and extrapolation techniques of the AAPM TG-43U1S1 report for the 2D anisotropy function and radial dose function. [ABSTRACT FROM AUTHOR]

This report addresses uncertainties pertaining to brachytherapy single-source dosimetry preceding clinical use. The International Organization for Standardization (ISO) Guide to the Expression of Uncertainty in Measurement (GUM) and the National Institute of Standards and Technology (NIST) Technical Note 1297 are taken as reference standards for uncertainty formalism. Uncertainties in using detectors to measure or utilizing Monte Carlo methods to estimate brachytherapy dose distributions are provided with discussion of the components intrinsic to the overall dosimetric assessment. Uncertainties provided are based on published observations and cited when available. The uncertainty propagation from the primary calibration standard through transfer to the clinic for air-kerma strength is covered first. Uncertainties in each of the brachytherapy dosimetry parameters of the TG-43 formalism are then explored, ending with transfer to the clinic and recommended approaches. Dosimetric uncertainties during treatment delivery are considered briefly but are not included in the detailed analysis. For low- and high-energy brachytherapy sources of low dose rate and high dose rate, a combined dosimetric uncertainty <5% (k=1) is estimated, which is consistent with prior literature estimates. Recommendations are provided for clinical medical physicists, dosimetry investigators, and source and treatment planning system manufacturers. These recommendations include the use of the GUM and NIST reports, a requirement of constancy of manufacturer source design, dosimetry investigator guidelines, provision of the lowest uncertainty for patient treatment dosimetry, and the establishment of an action level based on dosimetric uncertainty. These recommendations reflect the guidance of the American Association of Physicists in Medicine (AAPM) and the Groupe Européen de Curiethérapie-European Society for Therapeutic Radiology and Oncology (GEC-ESTRO) for their members and may also be used as guidance to manufacturers and regulatory agencies in developing good manufacturing practices for sources used in routine clinical treatments. [ABSTRACT FROM AUTHOR]

Since publication of the 2004 update to the American Association of Physicists in Medicine (AAPM) Task Group No. 43 Report (TG-43U1), several new low-energy photon-emitting brachytherapy sources have become available. Many of these sources have satisfied the AAPM prerequisites for routine clinical use as of January 10, 2005, and are posted on the Joint AAPM/RPC Brachytherapy Seed Registry. Consequently, the AAPM has prepared this supplement to the 2004 AAPM TG-43 update. This paper presents the AAPM-approved consensus datasets for these sources, and includes the following 125I sources: Amersham model 6733, Draximage model LS-1, Implant Sciences model 3500, IBt model 1251L, IsoAid model IAI-125A, Mentor model SL-125/SH-125, and SourceTech Medical model STM1251. The Best Medical model 2335 103Pd source is also included. While the methodology used to determine these data sets is identical to that published in the AAPM TG-43U1 report, additional information and discussion are presented here on some questions that arose since the publication of the TG-43U1 report. Specifically, details of interpolation and extrapolation methods are described further, new methodologies are recommended, and example calculations are provided. Despite these changes, additions, and clarifications, the overall methodology, the procedures for developing consensus data sets, and the dose calculation formalism largely remain the same as in the TG-43U1 report. Thus, the AAPM recommends that the consensus data sets and resultant source-specific dose-rate distributions included in this supplement be adopted by all end users for clinical treatment planning of low-energy photon-emitting brachytherapy sources. Adoption of these recommendations may result in changes to patient dose calculations, and these changes should be carefully evaluated and reviewed with the radiation oncologist prior to implementation of the current protocol. [ABSTRACT FROM AUTHOR]

Abstract: In recent years, double-layer radiochromic films (MD-55-2) also known as new improved Gafchromic Films (NMD-55), have been used for measuring dose distributions of radiation fields. It is reported that the response of radiochromic film is affected by the scanning densitometry systems used to read the film. To quantify the response of MD-55-2 films, two sheets (12.5 cm × 12.5 cm) from different batches, were irradiated to 900 cGy and 2000 cGy, using uniform flat photon beams. The films were sent to 5 institutions for response evaluation using 5 different densitometry systems with narrow band light source centered at nominal 633 to 665 nm, which is near the major absorption peaks of the film sensitive emulsion. The dose response curve was established for each dens tometer. The one-dimensional film responses were obtained for specified directions. A set of fiducial marks, located at ∼1.5 cm from the film edges, was used for identification of scan direction and alignment. The local fluctuations were assessed by comparing the film response with the mean response and its relative (percentage) standard deviation (RSD) in the region of interest. The regional non-uniformity was measured by examining the difference between the maximum and minimum responses within the region of interest. Our data indicates that the RSD, as obtained by the 5 institutions, varied from 2.4% to 5.8% for the film irradiated at low (∼900 cGy) dose and from 1.2% to 4.3% for the film irradiated at the higher dose (∼2000 cGy). The regional non-uniformity was also improved with increased dose and was less in longitudinal direction of the film that is parallel to the direction of coating application. Data comparison for regional non-uniformity indicates that the film responses were affected not only by the wavelength of analyzing source, but also by other instrumentation factors such as step size and sampling size. High-resolution scanners may also have more noise that should not be attributed to film non-uniformity. [Copyright &y& Elsevier]

Recently a new design of a 125I brachytherapy source was introduced for interstitial seed implants, particularly for prostate seed implants. This new source is the Best® Model 2301 brachytherapy source. Due to the differences in source design and manufacturing process from one new source to the next, their dosimetric parameters should be determined according to the AAPM TG-43 guidelines. As per AAPM recommendation (Med. Phys. 25 (12) (1998) 2269), it is required to perform the seed dosimetry using at least one experimental study and one Monte Carlo simulation, preferably done by two separate investigators. Other investigators have experimentally determined the dosimetric parameters of this new source. In this project, the Monte Carlo simulated dosimetric parameters of the Best® Model 2301 125I source have been provided. The results of this evaluation indicate the value of dose rate constant of 1.01±3% cGy h−1 U−1 in liquid water, which is in good agreement with 1.02±8% cGy h−1 U−1 reported by Nath and Yue, 2002. The anisotropy constant was found to be 0.98 in liquid water. [Copyright &y& Elsevier]

Abstract Objectives Grid therapy is a radiation therapy technique for the treatment of bulky tumours. The high dose gradients and non-uniformity of dose distributions within the target lead to a challenge in the dosimetry of the Grid radiation fields. The aim of this study is to perform a precise EBT3 Gafchromic film dosimetry for Grid therapy fields using a commercially available flatbed colour scanner. Methods In this project, samples of the EBT3 Gafchromic films are exposed to Grid radiation fields. The irradiated EBT3 films were read using a flatbed Microtek scanner. The responses of these films (i.e. films from the same batch) as functions of the absorbed dose values are calibrated by irradiation under a fixed standard technique (i.e. 10 × 10 cm2 filed, 100 cm SSD, and depth of the maximum dose). These films are also read with the same scanner using the red, green, and blue channels. Four different approaches were used to evaluate film dosimetry for the Grid therapy applications: 1) single channel film dosimetry method (SCM), 2) dual channel film dosimetry method (DCM), 3) linearized dose-response curve method (LRCM), and 4) triple channel film dosimetry method (TCM). A dose of 20 Gy was delivered to the point along the central axis of the grid hole at the depth of maximum dose (dmax) for a 20 × 20 cm2 Grid field size. Beam profiles and percentage depth dose distributions of the Grid radiation have been measured in water-equivalent phantom material, using EBT3 films. The accuracy of the relative and the absolute dosimetry of the films were examined by comparison of the TLD measured data with the Monte Carlo simulated values. Results The results of these investigations show that for a gamma index criterion of 5%/3 mm, the agreements between the MC calculations dose profiles and the SCM, DCM, LRCM, and TCM film dosimetry approaches the passing rates of 91%, 92%, 95%, and 95%, respectively. A much closer agreement was observed for using a linearized dose-response curve and triple-channel methods. Conclusions Selection of an appropriate methodology in Gafchromic film dosimetry may lead to an accurate dose-response in a high dose gradient radiation field such as Grid therapy. [ABSTRACT FROM AUTHOR]