Room: Stars at Night Ballroom 1
Purpose: To report the commissioning tasks and results for GammaPod, a stereotactic-body-radiation-therapy(SBRT) platform dedicated to breast.
Methods: The new GammaPod device featuring a revamped design of 25 rotating Co-60 sources was commissioned by performing numerous tests addressing machine safety, mechanical and dosimetric specifications. The GammaPod treatment-planning-system(TPS) only calculates and reports dose in breast tissue, posing a challenge in direct TPS dose evaluation via in-water measurements. This was resolved by developing and commissioning an in-house Monte-Carlo dose engine that allows convenient medium switching for dose calculation and verification. For absolute dose calibration, we measured the isocenter dose in an acrylic phantom following the AAPM TG-21 protocol. External auditing service by imaging-and-radiation-oncology-core(IROC) was used for secondary validation. 56 treatment plans featuring different target sizes, locations and breast cup sizes were designed and delivered to water, with absolute doses measured by a micro-ion-chamber, and relative doses evaluated via a diode array.
Results: The GammaPod device passed all safety tests with radiation survey measurements well below the 2mR/hr threshold. Couch motion accuracy was ~0.1 mm, and sagging was <0.8 mm for 300 lbs loading. Radiation and mechanical isocenter congruence tests yielded a deviation <0.4 mm. In-house absolute dose measured by micro-ion-chambers agreed with the IROC TLD measurements to <1% deviation. The plan point-dose measurements in water were within Â±2.5% of the doses calculated by Monte-Carlo in water(range:-2.01% to 2.2%, mean:0.04%). Correspondingly, when switching the medium to breast, the Monte-Carlo-calculated point doses were within Â±1.6% of the TPS-reported doses(range:-1.59% to 1.51%, mean:-0.02%). Relative dosimetry by the diode array yielded >90% pass rates for all plans under the 2%/1mm gamma criteria.
Conclusion: The GammaPod system offers reliable safety features, solid mechanical stability, and high dosimetric accuracy for breast SBRT. Our in-house developed Monte-Carlo engine can be distributed to other centers for commissioning efficiency and inter-institutional comparison.