Room: Exhibit Hall
Purpose: GammaPod is a stereotactic radiotherapy device that recently received FDA clearance for treating cancerous targets in the breast. It was shown in a Phase 1 clinical trial that large fractional doses can be delivered to targets of up to 5cm in diameter. Grid therapy, or spacial fractionation, a technique in which columns of high dose are interlaced with columns of very low dose, was shown effective in tumor volume reduction. This study takes the grid therapy concept into three dimensions by delivering regularly spaced spots of high dose surrounded by areas of very low dose, possibly providing an option for breast conservation for large tumors currently ineligible for breast conservation therapy.
Methods: The new simulated GammaPod is equipped with 5mm diameter non-coplanar circular beams that span 36 degrees latitudinally from 18 to 53 degrees off the horizontal plane. Two degrees longitudinal intervals were used to simulate rotating beams. Three different breast sizes --10cm, 15cm, and 20cm-- were simulated by three water-equivalent hemisphere targets. 48-224 shots were generated for these targets due to shot spacing of 2 cm in the transverse and sagittal planes and 2.5 cm in the coronal plane. The maximum dose for each individual shot was 20Gy. The peak-to-valley dose ratio and skin dose were analyzed.
Results: The dose profiles showed a reduction from the peak-to-valley dose of 86%-96% in 10cm breast, 80%-96% in 15cm breast, and 80%-92.5% in 20cm breast. The average skin dose was 1.5Gy, 3.2Gy, and 3.7Gy for the 10cm, 15cm, and 20cm breast respectively. Additionally, the maximum doses were 21.6Gy, 23.1Gy, and 24.9Gy.
Conclusion: This shows that the GammaPod-based grid therapy is a viable alternative to conventional grid therapy delivered by Linac using MLC or brass compensator, if the beams can be reduced from their current minimum size of 15mm in diameter.
Funding Support, Disclosures, and Conflict of Interest: Dr Cedric Yu is the founder of Xcision Medical Systems, LLC