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Commissioning of a Noninvasive Breast Stereotactic Body Radiation Therapy Platform: GammaPod

Y Zhang1*, W Lu1 , N Salehi2 , D Parsons1 , R Reynolds1 , S Stojadinovic1 , S Jiang1 , X Gu1 , (1) UT Southwestern Medical Ctr at Dallas, Dallas, TX, (2) Xcision Medical Systems, Columbia, MD


(Monday, 7/15/2019) 7:30 AM - 9:30 AM

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 ±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 ±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.


Commissioning, Co-60, Monte Carlo


TH- External beam- photons: Development (new technology and techniques)

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