Session Item

Radiotherapy facilities participating in national clinical trials in the United States must successfully irradiate credentialing phantoms from the Imaging and Radiation Oncology Core (IROC). This work analyzes trends in institutional performance and failure modes for IROC’s proton liver phantom.

The proton liver phantom includes two moving target volumes. Results of 66 phantom irradiations from 28 institutions between 2015-2020 were retrospectively analyzed. Univariate analysis and random forest models were used to associate irradiation conditions with phantom results. Phantom results included pass/fail classification, average thermoluminescent dosimeter (TLD) ratio of both targets, and percent of pixels passing gamma of both targets. The following categories were evaluated in terms of how they predicted these outcomes: irradiation year, treatment planning system (TPS), TPS algorithm, treatment machine, treatment technique, motion management technique, number of isocenters, and superior-inferior extent (in cm) of the 90% TPS isodose line for both targets. In addition, failures were categorized by failure mode.

The overall average pass rate was only 52%, although the average TLD ratio has slightly improved with time. As the of the treatment field increased the pass rate actually decreased (p < 0.01). Lower pass rates were observed for Mevion machines, scattered irradiation techniques and gating and ITV motion management techniques. Overall, the accuracy of the random forest modeling of the phantom results was approximately 73 ± 14%. The most important predictor was the superior-inferior extent for both targets and irradiation year. Three failure modes dominated the failures of the phantom: 1) systematic underdosing, 2) poor localization in the direction of motion (superior-inferior), and 3) range error. Only 44% of failures has the same failure modes between the two targets.

The proton liver phantom is a particularly challenging phantom with a low pass rate (the lowest among all IROC phantoms). Range uncertainty, motion management, and underdosing are the main culprits of failures of the proton liver phantom. Clinically, careful attention to mutli-target liver proton therapy is needed to ensure optimal patient care.