Purpose: To calculate gamma results for SRS anthropomorphic phantoms using different thresholding methods and compare to the standard IROC region of interest method.
Methods: The SRS anthropomorphic phantoms have a 1.9 cm target that houses two TLD and radiochromic film in the sagittal and coronal planes. It is mailed to institutions that create a plan treating the center of the target to 30 Gy with target coverage per institutional protocol. They irradiate and return the phantom and submit the plan in DICOM format. TLD are read and a 5%/3mm gamma analysis is done on the film. A newly commissioned in-house software addition includes the ability to use dose thresholding techniques to define the area used in the gamma analysis. Prior versions only allowed the area to be defined by as a rectangle relative to the target and was chosen to be a 2.9 cm square centered on the target. 28 recent SRS phantom results were recalculated using 10%, 20% and 30% of maximum dose thresholds as well as using a 10 Gy limit. Results were compared to the original regions of interest.
Results: Analyzed phantoms had a pass rate of 86% using all methods except the 10 Gy threshold measurement. This method created one more failure making a pass rate of 82%. The average changes in the gamma results were 0% for each percent thresholds and 1% for the 10 Gy threshold with maximum changes of 2%, 3%, 5% and 14% respectively.
Conclusion: The new ability of the software allows more flexibility in setting the area on the film use for gamma analysis. It will be further tested results of other phantoms before our methods are changed.
Funding Support, Disclosures, and Conflict of Interest: This work received funding from the NIH/NCI grants #CA180803