Author: Michael G. Mitch, Csilla I. Szabo-Foster 👨🔬
Affiliation: National Institute of Standards and Technology 🌍
Purpose: The U.S. air-kerma strength standard for low-dose-rate (LDR) low-energy photon-emitting brachytherapy sources provides measurement traceability for calibrations at the AAPM Accredited Dosimetry Calibration Laboratories (ADCLs). This standard ensures accurate dosimetry with calibrated well-type ionization chambers in clinical environments for radioactive seeds used in prostate cancer therapy.
Methods: The Wide-Angle Free-Air Chamber (WAFAC) of the National Institute of Standards and Technology (NIST) has been in use since January 1, 1999. This instrument was designed to directly realize the air kerma of a single brachytherapy seed that emits x-rays with energies up to 40 keV. Recently, the WAFAC was moved to a new laboratory within the modernized Radiation Physics Building at NIST, where substantial upgrades were implemented to improve automation and data acquisition capabilities. Parallel to the instrument update, a new data acquisition and analysis method was developed to mitigate systematics during femtoampere-level current measurements.
Results: The upgrades to the WAFAC have been completed, and for the past few months, test and QA measurements have been performed on the system to demonstrate accurate realization of the air-kerma strength of LDR brachytherapy sources. The new system was successfully validated, showing consistency of measurements taken before and after the upgrades.
Conclusion: The NIST air-kerma strength standard for LDR brachytherapy sources, the WAFAC, is now operational in a new laboratory with an improved data acquisition and analysis system. The upgraded WAFAC and the NIST-traceable calibrations performed by the ADCLs will continue to support research and clinical applications through accurate dosimetry, ultimately leading to improved patient outcomes in prostate cancer therapy.