SU-E-T-259: Development of a Primary Standard for LDR Brachytherapy

Purpose: The National Research Council initiated a program in 2012 to develop a primary standard to calibrate I-125 and Pd-103 sources used for LDR brachytherapy and disseminate this through calibration services to Canadian users. This will simplify procedures as Canadian cancer centres currently have to ship instruments to the US. Methods: The standard is based on a commercial version of the wide-angle free air chamber (WAFAC) pioneered by NIST. Significant enhancements were implemented to improve signal-to-noise and measurement reproducibility and eliminate electric field effects. Validation of this ionization chamber was then carried out in a low-energy X -ray beam (∼ 31 keV) where the dose rate had been previously established using the existing NRC primary standard free-air chamber. As a final component of this initial testing, measurements were made with a set of I-125 seeds (with air kerma strength traceable to NIST). Results: Excellent agreement of the two NRC free-air chambers was obtained within the combined standard uncertainty of 0.5 %. However, it was found that the WAFAC response is very sensitive to the beam geometry (distance from the source, diameter of the beam-defining aperture, etc) and Monte Carlo calculations, carried out to evaluate these geometry corrections, have confirmed the experimental results. The results for the seed measurements indicated a precision of better than 1 % is achievable for a reasonable acquisition time and the air kerma strength agreed with the manufacturer (NIST-traceable) value within 2 %. Conclusion: The prototype primary standard for LDR brachytherapy has met accuracy target of 3 % for the determination of air kerma strength. Work is ongoing to refine operation of the device and develop the calibration protocol for clinical users, with an anticipated launch of a calibration service in late 2015