Air‐kerma strength determination of a new directional 103Pd source. Issue 12 (1st December 2015)

• Record Type: Journal Article
• Authors: Aima, Manik; Reed, Joshua L.; DeWerd, Larry A.; Culberson, Wesley S.
• Is Part Of: Medical physics. Volume 42:Issue 12(2015)
• Page Start: 7144
• Page End: 7152
• Publication Date: 2015-12-01
• Subjects: brachytherapy -- calibration -- dosimetry -- ion sources -- ionisation chambers -- Monte Carlo methods; Therapeutic applications, including brachytherapy -- Applications -- Gas‐filled counters: ionization chambers, proportional, and avalanche counters -- Standards and calibration -- Charged‐particle beam sources and detectors -- Dosimetry/exposure assessment; Radiation therapy -- Testing or calibrating of apparatus or arrangements provided for in groups G01D1/00 to G01D15/00 -- Measurement of nuclear or x‐radiation -- Calibrating of instruments or apparatus -- Ion beam tubes -- Apparatus for generating ions to be introduced into non‐enclosed gases, e.g. into the atmosphere -- Scintigraphy -- with scintillation detectors -- Ionisation chambers; brachytherapy -- dosimetry -- directional sources -- TG43 -- air‐kerma strength; Gold -- Ionization chambers -- Brachytherapy -- Calibration -- Monte Carlo methods -- Cancer -- Fluorescence spectra -- Anisotropy -- Palladium -- Emission spectra; Medical physics -- Periodicals; Medical physics; Geneeskunde; Natuurkunde; Toepassingen; Biophysics; Periodicals; Periodicals; Electronic journals; 610.153
• DOI: 10.1118/1.4935409
• Languages: English
• ISSNs: 0094-2405

Abstract

Abstract : Purpose: A new directional 103 Pd planar source array called a CivaSheet™ has been developed by CivaTech Oncology, Inc., for potential use in low‐dose‐rate (LDR) brachytherapy treatments. The array consists of multiple individual polymer capsules called CivaDots, containing 103 Pd and a gold shield that attenuates the radiation on one side, thus defining a hot and cold side. This novel source requires new methods to establish a source strength metric. The presence of gold material in such close proximity to the active 103 Pd region causes the source spectrum to be significantly different than the energy spectra of seeds normally used in LDR brachytherapy treatments. In this investigation, the authors perform air‐kerma strength ( S K ) measurements, develop new correction factors for these measurements based on an experimentally verified energy spectrum, and test the robustness of transferring S K to a well‐type ionization chamber. Methods: S K measurements were performed with the variable‐aperture free‐air chamber (VAFAC) at the University of Wisconsin Medical Radiation Research Center. Subsequent measurements were then performed in a well‐type ionization chamber. To realize the quantity S K from a directional source with gold material present, new methods and correction factors were considered. Updated correction factors were calculated using themcnp 6 Monte Carlo code in order to determine S K with the presence of gold fluorescent energy lines. In addition to S K measurements, a low‐energy high‐purity germanium (HPGe) detector was used to experimentally verify the calculated spectrum, a sodium iodide (NaI) scintillating counter was used to verify the azimuthal and polar anisotropy, and a well‐type ionization chamber was used to test the feasibility of disseminating S K values for a directional source within a cylindrically symmetric measurement volume. Results: The UW VAFAC was successfully used to measure the S K of four CivaDots with reproducibilities within 0.3%. Monte Carlo methods were used to calculate the UW VAFAC correction factors and the calculated spectrum emitted from a CivaDot was experimentally verified with HPGe detector measurements. The well‐type ionization chamber showed minimal variation in response (<1.5%) as a function of source positioning angle, indicating that an American Association of Physicists in Medicine (AAPM) Accredited Dosimetry Calibration Laboratory calibrated well chamber would be a suitable device to transfer an S K ‐based calibration to a clinical user. S K per well‐chamber ionization current ratios were consistent among the four dots measured. Additionally, the measurements and predictions of anisotropy show uniform emission within the solid angle of the VAFAC, which demonstrates the robustness of the S K measurement approach. Conclusions: This characterization of a new 103 Pd directional brachytherapy source helps to establish calibration methods that could ultimately be used in the well‐established AAPM Task Group 43 formalism. Monte Carlo methods accurately predict the changes in the energy spectrum caused by the fluorescent x‐rays produced in the gold shield.