Purpose: To develop and implement a calibration procedure utilizing the TG-21 and IAEA TRS 483 protocols for the GammaPod, an FDA cleared novel dedicated prone breast stereotactic radiosurgery (SRS) device developed at the University of Maryland Medical Center. This work was done to create a standard calibration method for future GammaPod sites.
Methods: An absolute dose calibration was performed using an Exradin A1SL thimble chamber and a CDX 2000B electrometer. Measurements were performed in a polymethyl methacrylate breast-mimicking phantom (PMMA) and a water-filled breast cup. Validation was performed using the thermoluminescent dosimeter remote monitoring service from the Imaging and Radiation Oncology Core (IROC) calibration laboratory. TG-21 assumes that chamber response for these small fields sizes are unity compared to a 10x10 field size. However with the release of TRS 483 that is not necessarily correct depending on the field size and the medium used. The Monte Carlo calculation and the measurements in the both of the water and the PMMA phantom have been used to determine the kQ factor of plastic to water in the machine specific reference (kQ).
Results: TG-21 in the acrylic phantom agreed within 1% and 2% with the IROC measurements for the 25- and 15-mm collimators; and < 2% and, in most cases, within 1%. with the Monte-Carlo-based treatment planning system for acrylic, water, and breast-density water. All agreed within 2%. TRS 483, which considers the kQ factor of plastic to water in the machine specific reference, showed a 0.6% difference to TG-21.
Conclusion: We successfully implemented the first GammaPod calibration procedure based on the TG-21 and IAEA TRS 483 protocols. The results between the TG-21 and TRS 483 match within the measurement error for the chamber used in this study.