Failure Modes in IROC Photon Liver Phantom Irradiations

Purpose: Our purpose was to analyze and classify the patterns of failure for irradiations of the Imaging and Radiation Oncology Core photon liver phantom. Methods and Materials: Imaging and Radiation Oncology Core's anthropomorphic liver phantom simulates multitarget liver disease with respiratory motion. Two hundred forty-nine liver phantom results from 2013 to 2019 were analyzed. Phantom irradiations that failed were categorized by the error attributed to the failure. Phantom results were also compared by demographic data, such as machine type, treatment planning system, motion management technique, number of isocenters, and whether the phantom was a first time or repeat irradiation. Results: The failure rate for the liver phantom was 27%. From the 68 irradiations that did not pass, 5 failure modes were identified. The most common failure mode was localization errors in the direction of motion, with over 50% of failures attributed to this mode. The second-most common failure mode was systematic dose errors. The internal target volume technique performed worse than other motion management techniques. Failure modes were different by the number of isocenters used, with multi-isocenter irradiations having more failure modes in a single phantom irradiation. Conclusions: Motion management techniques and proper alignment of moving targets play a large role in the successful irradiation of the liver phantom. These errors should be examined to ensure accurate patient treatment for liver disease or other sites where multiple moving targets are present.