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Dosimetric characterization of a new directional low-dose rate brachytherapy source.

ABSTRACT: PURPOSE:CivaTech Oncology Inc. (Durham, NC) has developed a novel low-dose rate (LDR) brachytherapy source called the CivaSheet.TM The source is a planar array of discrete elements ("CivaDots") which are directional in nature. The CivaDot geometry and design are considerably different than conventional LDR cylindrically symmetric sources. Thus, a thorough investigation is required to ascertain the dosimetric characteristics of the source. This work investigates the repeatability and reproducibility of a primary source strength standard for the CivaDot and characterizes the CivaDot dose distribution by performing in-phantom measurements and Monte Carlo (MC) simulations. Existing dosimetric formalisms were adapted to accommodate a directional source, and other distinguishing characteristics including the presence of gold shield x-ray fluorescence were addressed in this investigation. METHODS:Primary air-kerma strength (SK ) measurements of the CivaDots were performed using two free-air chambers namely, the Variable-Aperture Free-Air Chamber (VAFAC) at the University of Wisconsin Medical Radiation Research Center (UWMRRC) and the National Institute of Standards and Technology (NIST) Wide-Angle Free-Air Chamber (WAFAC). An intercomparison of the two free-air chamber measurements was performed along with a comparison of the different assumed CivaDot energy spectra and associated correction factors. Dose distribution measurements of the source were performed in a custom polymethylmethacrylate (PMMA) phantom using GafchromicTM EBT3 film and thermoluminescent dosimeter (TLD) microcubes. Monte Carlo simulations of the source and the measurement setup were performed using MCNP6 radiation transport code. RESULTS:The CivaDot SK was determined using the two free-air chambers for eight sources with an agreement of better than 1.1% for all sources. The NIST measured CivaDot energy spectrum intensity peaks were within 1.8% of the MC-predicted spectrum intensity peaks. The difference in the net source-specific correction factor determined for the CivaDot free-air chamber measurements for the NIST WAFAC and UW VAFAC was 0.7%. The dose-rate constant analog was determined to be 0.555 cGy h-1 U-1 . The average difference observed in the estimated CivaDot dose-rate constant analog using measurements and MCNP6-predicted value (0.558 cGy h-1 U-1 ) was 0.6% ± 2.3% for eight CivaDot sources using EBT3 film, and -2.6% ± 1.7% using TLD microcube measurements. The CivaDot two-dimensional dose-to-water distribution measured in phantom was compared to the corresponding MC predictions at six depths. The observed difference using a pixel-by-pixel subtraction map of the measured and the predicted dose-to-water distribution was generally within 2-3%, with maximum differences up to 5% of the dose prescribed at the depth of 1 cm. CONCLUSION:Primary SK measurements of the CivaDot demonstrated good repeatability and reproducibility of the free-air chamber measurements. Measurements of the CivaDot dose distribution using the EBT3 film stack phantom and its subsequent comparison to Monte Carlo-predicted dose distributions were encouraging, given the overall uncertainties. This work will aid in the eventual realization of a clinically viable dosimetric framework for the CivaSheet based on the CivaDot dose distribution.

SUBMITTER: Aima M

PROVIDER: S-EPMC6548702 | biostudies-literature | 2018 May

REPOSITORIES: biostudies-literature

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Dosimetric characterization of a new directional low-dose rate brachytherapy source.

Aima Manik M DeWerd Larry A LA Mitch Michael G MG Hammer Clifford G CG Culberson Wesley S WS

Medical physics 20180524

<h4>Purpose</h4>CivaTech Oncology Inc. (Durham, NC) has developed a novel low-dose rate (LDR) brachytherapy source called the CivaSheet.<sup>TM</sup> The source is a planar array of discrete elements ("CivaDots") which are directional in nature. The CivaDot geometry and design are considerably different than conventional LDR cylindrically symmetric sources. Thus, a thorough investigation is required to ascertain the dosimetric characteristics of the source. This work investigates the repeatabili ...[more]

PMID: 29797517

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Project description:A novel (169)Yb low dose rate permanent implant brachytherapy source, the GammaClip™, was developed by Source Production & Equipment Co. (New Orleans, LA) which is designed similar to a surgical staple while delivering therapeutic radiation. In this report, the brachytherapy source was characterized in terms of "Dose calculation for photon-emitting brachytherapy sources with average energy higher than 50 keV: Report of the AAPM and ESTRO" by Perez-Calatayud et al. [Med. Phys. 39, 2904-2929 (2012)] using the updated AAPM Task Group Report No. 43 formalism.Monte Carlo calculations were performed using Monte Carlo N-Particle 5, version 1.6 in water and air, the in-air photon spectrum filtered to remove photon energies below 10 keV in accordance with TG-43U1 recommendations and previously reviewed (169)Yb energy cutoff levels [D. C. Medich, M. A. Tries, and J. M. Munro, "Monte Carlo characterization of an Ytterbium-169 high dose rate brachytherapy source with analysis of statistical uncertainty," Med. Phys. 33, 163-172 (2006)]. TG-43U1 dosimetric data, including SK, Ḋ(r,θ), Λ, gL(r), F(r, θ), φan(r), and φan were calculated along with their statistical uncertainties. Since the source is not axially symmetric, an additional set of calculations were performed to assess the resulting axial anisotropy.The brachytherapy source's dose rate constant was calculated to be (1.22±0.03) cGy h(-1) U(-1). The uncertainty in the dose to water calculations, Ḋ(r,θ), was determined to be 2.5%, dominated by the uncertainties in the cross sections. The anisotropy constant, φan, was calculated to be 0.960±0.011 and was obtained by integrating the anisotropy factor between 1 and 10 cm using a weighting factor proportional to r(-2). The radial dose function was calculated at distances between 0.5 and 12 cm, with a maximum value of 1.20 at 5.15±0.03 cm. Radial dose values were fit to a fifth order polynomial and dual exponential regression. Since the source is not axially symmetric, angular Monte Carlo calculations were performed at 1 cm which determined that the maximum azimuthal anisotropy was less than 8%.With a higher photon energy, shorter half-life and higher initial dose rate 169Yb is an interesting alternative to 125I for the treatment of nonsmall cell lung cancer.

Project description:Historically, treatment of malignant surface lesions has been achieved with linear accelerator based electron beams or superficial x-ray beams. Recent developments in the field of brachytherapy now allow for the treatment of surface lesions with specialized conical applicators placed directly on the lesion. Applicators are available for use with high dose rate (HDR)(192)Ir sources, as well as electronic brachytherapy sources. Part I of this paper will discuss the applicators used with electronic brachytherapy sources; Part II will discuss those used with HDR (192)Ir sources. Although the use of these applicators has gained in popularity, the dosimetric characteristics including depth dose and surface dose distributions have not been independently verified. Additionally, there is no recognized method of output verification for quality assurance procedures with applicators like these. Existing dosimetry protocols available from the AAPM bookend the cross-over characteristics of a traditional brachytherapy source (as described by Task Group 43) being implemented as a low-energy superficial x-ray beam (as described by Task Group 61) as observed with the surface applicators of interest.This work aims to create a cohesive method of output verification that can be used to determine the dose at the treatment surface as part of a quality assurance/commissioning process for surface applicators used with HDR electronic brachytherapy sources (Part I) and(192)Ir sources (Part II). Air-kerma rate measurements for the electronic brachytherapy sources were completed with an Attix Free-Air Chamber, as well as several models of small-volume ionization chambers to obtain an air-kerma rate at the treatment surface for each applicator. Correction factors were calculated using MCNP5 and EGSnrc Monte Carlo codes in order to determine an applicator-specific absorbed dose to water at the treatment surface from the measured air-kerma rate. Additionally, relative dose measurements of the surface dose distributions and characteristic depth dose curves were completed in-phantom.Theoretical dose distributions and depth dose curves were generated for each applicator and agreed well with the measured values. A method of output verification was created that allows users to determine the applicator-specific dose to water at the treatment surface based on a measured air-kerma rate.The novel output verification methods described in this work will reduce uncertainties in dose delivery for treatments with these kinds of surface applicators, ultimately improving patient care.

Project description:Historically, treatment of malignant surface lesions has been achieved with linear accelerator based electron beams or superficial x-ray beams. Recent developments in the field of brachytherapy now allow for the treatment of surface lesions with specialized conical applicators placed directly on the lesion. Applicators are available for use with high dose rate (HDR)(192)Ir sources, as well as electronic brachytherapy sources. Part I of this paper discussed the applicators used with electronic brachytherapy sources. Part II will discuss those used with HDR (192)Ir sources. Although the use of these applicators has gained in popularity, the dosimetric characteristics have not been independently verified. Additionally, there is no recognized method of output verification for quality assurance procedures with applicators like these.This work aims to create a cohesive method of output verification that can be used to determine the dose at the treatment surface as part of a quality assurance/commissioning process for surface applicators used with HDR electronic brachytherapy sources (Part I) and(192)Ir sources (Part II). Air-kerma rate measurements for the (192)Ir sources were completed with several models of small-volume ionization chambers to obtain an air-kerma rate at the treatment surface for each applicator. Correction factors were calculated using MCNP5 and EGSnrc Monte Carlo codes in order to determine an applicator-specific absorbed dose to water at the treatment surface from the measured air-kerma rate. Additionally, relative dose measurements of the surface dose distributions and characteristic depth dose curves were completed in-phantom.Theoretical dose distributions and depth dose curves were generated for each applicator and agreed well with the measured values. A method of output verification was created that allows users to determine the applicator-specific dose to water at the treatment surface based on a measured air-kerma rate.The novel output verification methods described in this work will reduce uncertainties in dose delivery for treatments with these kinds of surface applicators, ultimately improving patient care.

Project description:BACKGROUND:The objective of focal brachytherapy (BT) is to provide effective prostate cancer control for low-risk disease but with reduced genitourinary, gastrointestinal and sexual side effects in a cost-effective way. OBJECTIVE:The aim of this study is to describe a phase II study examining technical and dosimetric feasibility and toxicity, quality of life changes, and local control with post-treatment biopsy outcomes in men with early stage low volume prostate cancer treated with focal iodine-125 seed BT. METHODS:The study design is a prospective, multicenter trial with a planned sample size of 20 patients including men with a minimum age of 60 years, a life expectancy estimated to be greater than 10 years, with low or low-tier intermediate risk prostate cancer, unilateral disease on the biopsy, and a Gleason score of ?3+4 and <25% cores involved. The investigations specific for the study are multi-parametric magnetic resonance imaging (Mp-MRI) baseline, at 20 and 36 months to rule out high grade disease and a transperineal mapping biopsy (baseline and at 36 months) for more accurate patient selection. The hemigland region will receive 144 Gy. Standard normal tissue constraints will be considered as for a whole gland (WG) implant. Dosimetric parameters will be evaluated at day 30 after the implant. Toxicity and quality of life will be evaluated with international validated questionnaires focusing on urinary, rectal, sexual domain, and general health-related quality of life. The patients will complete this assessment at baseline and then approximately every 6 months after the implant up to 10 years. RESULTS:To date, one patient is involved in the trial. He underwent the pre-implant investigations which found bilateral disease. Therefore, a standard seed implant was performed. If the results from this trial provide evidence that the treatment is safe, feasible, and improves toxicity, funding will be sought to conduct a large, multicenter, randomized controlled trial (RCT). CONCLUSIONS:This protocol is designed to show feasibility in delivering hemigland focal therapy with seed BT. It may answer crucial questions and obtain data which will enable downstream decisions on focal low dose rate (LDR) prostate BT. CLINICALTRIAL:Clinicaltrial.gov NCT02643511; https://www.clinicaltrials.gov/ct2/show/NCT02643511 (Archived by Webcite at http://www.webcitation.org/6ghLCzIhY).

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Dosimetric characterization of a new directional low-dose rate brachytherapy source.

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Dosimetric characterization of a new directional low-dose rate brachytherapy source.

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