Purpose: GammaPod is an emerging gamma irradiation device dedicated to breast stereotactic radiotherapy. It contains 25 rotary cobalt-60 sources collimated by 15 or 25-mm collimators to deliver highly-conformal dose for boost, partial-breast irradiation, or neo-adjuvant stereotactic radiosurgery. Annual quality assurance of this system using radiochromic films is the current practice, but it is laborious and time-consuming. This study aims to develop a quality assurance program that utilizes the SRS MapCHECK device (Sun Nuclear Corp, Melbourne, FL).
Methods: A custom-designed plastic holder was used to accommodate the SRS MapCHECK embedded in the StereoPHAN™ at the treatment location on the GammaPod™ (Xcision Medical Systems, LLC, Columbia, MD). Single-shot irradiations were used in three preliminary tests: 1). reproducibility of the MapCHECK dose measurement in two consecutive deliveries; 2). dose profile constancy check compared to the baseline measurements; 3). couch motion accuracy by measuring shifts in dose maps with and without couch motions.
Results: Within the radiation field, the dose maps from two consecutive measurements were similar with a mean difference of -0.07% (range: -0.21-0.07%) for the 15-mm collimators, and 0.11% (range: -0.13-0.28%) for the 25-mm collimators. Dose profiles along the y and z directions resembled the basal profiles, all with 100% gamma passing rate at 3%/0.5 mm. The spatial shifts quantified from the two dose maps sensitively detected 0.3-mm couch motions, and accurately delineated couch motions at 21.0 mm.
Conclusion: This pilot study successfully demonstrated the feasibility and reproducibility of quality assurance of the GammaPod™ system using the SRS MapCHECK. The utility of this device for dose profile constancy check and couch motion tests has also been demonstrated. Ongoing studies are being conducted to investigate its utility in quality assurance of patient-specific plans.