SU‐E‐T‐155: Calibration of Variable Longitudinal Strength 103Pd Brachytherapy Sources

Abstract

Purpose:Brachytherapy sources with variable longitudinal strength (VLS) allow for a customized intensity along the length of the source. These have applications in focal brachytherapy treatments of prostate cancer where dose boosting can be achieved through modulation of intra‐source strengths. This work focused on development of a calibration methodology for VLS sources based on measurements and Monte Carlo (MC) simulations of five 1 cm 1⁰ 3Pd sources each containing four regions of variable 1⁰3Pd strength.Methods:The air‐kerma strengths of the sources were measured with a variable‐aperture free‐air chamber (VAFAC). Source strengths were also measured using a well chamber. The in‐air azimuthal and polar anisotropy of the sources were measured by rotating them in front of a NaI scintillation detector and were calculated with MC simulations. Azimuthal anisotropy results were normalized to their mean intensity values. Polar anisotropy results were normalized to their average transverse axis intensity values. The relative longitudinal strengths of the sources were measured via on‐contact irradiations with radiochromic film, and were calculated with MC simulations.Results:The variable 1⁰3Pd loading of the sources was validated by VAFAC and well chamber measurements. Ratios of VAFAC air‐kerma strengths and well chamber responses were within ±1.3% for all sources. Azimuthal anisotropy results indicated that ≥95% of the normalized values for all sources were within ±1.7% of the mean values. Polar anisotropy results indicated variations within ±0.3% for a ±7.6° angular region with respect to the source transverse axis. Locations and intensities of the 1⁰3Pd regions were validated by radiochromic film measurements and MC simulations.Conclusion:The calibration methodology developed in this work confirms that the VLS sources investigated have a high level of polar uniformity, and that the strength and longitudinal intensity can be verified experimentally and through MC simulations.103Pd sources were provided by CivaTech Oncology, Inc.