Room: Davidson Ballroom A
Purpose: To reevaluate the criteria used for IROCâ€™s proton prostate phantom.
Methods: The anthropomorphic proton prostate phantom is used for baseline approval for proton center clinical trial participation. The phantom contains a spherical target (prostate) and several OARs (bladder, rectum, and femoral heads), as well as TLD and radiochromic film for dosimetric measurements. When the phantom criteria were first evaluated in 2013, IROC found that the mean TLD to TPS ratio was 0.96 in the PTV, and thus the acceptance criteria for target TLD was set as 0.96 Â±7%. The phantom data were reanalyzed to include all irradiations from 2009 - present.
Results: From 2009-2014, the mean TLD/TPS was 0.96 (Ïƒ 0.02, n=27). From 2015-present, the mean TLD/TPS has increased to 0.99 (Ïƒ 0.02, n=16). The new criteria for point dose agreement between the TLD and the treatment plan is now 1.00 Â±7%.
Conclusion: Why the change? Over time, various proton TPSs have changed how they model scatter. Initially, some TPSs didnâ€™t model aperture scatter for passive and double scatter beams; now they do. Some newer algorithms now calculate angular scattering along the beam direction in a sequential piecewise fashion. This would play a role in the scatter calculated for the common lateral beam configuration used for the prostate phantom, where the beams transverse three different tissue-equivalent plastics as well as water. In addition, Monte Carlo algorithms, which are being used more often for proton dose calculation, model multiple coulomb scattering that pencil beam algorithms donâ€™t properly account for. With the same criteria now being used for both photons and protons, we have more confidence that proton and photon therapy centers deliver comparable physical dose when compared in the multi-institutional clinical trial setting.
Funding Support, Disclosures, and Conflict of Interest: This work was supported by the NCI grant # CA180803.