Is the Imaging Radiation Oncology Core Head and Neck Credentialing Phantom an Effective Surrogate for Different Anatomic Sites?

Abstract

Purpose: The Imaging Radiation Oncology Core (IROC) head and neck (H&N) phantom is used to credential institutions for intensity modulated radiation therapy delivery for all anatomic sites where delivery of modulated therapy is a primary challenge. This study evaluated how appropriate the use of this phantom is for varied clinical anatomy by evaluating how closely the IROC H&N phantom described clinical dose errors from beam modeling compared with various anatomic sites.

Methods and materials: The multileaf collimator (MLC) offset, transmission, percent depth dose, and 7 additional beam modeling parameters for a Varian accelerator were modified in RayStation to match community data at the 2.5th, 25th, 50th, 75th, and 97.5th percentile levels. Modifications were evaluated on 25 H&N phantom cases and 25 clinical cases (H&N, prostate, lung, mesothelioma, and brain), generating 2000 plan perturbations. Differences in mean dose delivered to clinical target volumes and maximum dose to organs at risk were compared between phantom and clinical plans to assess the relationship between dose deviations in phantom versus clinical target volumes and as a function of 18 different complexity metrics.

Results: Perturbations to MLC offset and transmission parameters demonstrated the greatest impact on dose accuracy for phantom and clinical plans (for all anatomic sites). The phantom demonstrated equivalent or greater sensitivity to these parameter perturbations compared with clinical sites, largely aligning with treatment complexity. The mean MLC gap best described the impact of errors in treatment planning system beam modeling parameters in phantom plans and clinical plans from various anatomic sites.

Conclusions: When compared across various anatomic sites, the IROC H&N credentialing phantom exhibited similar or greater sensitivity to errors in the treatment planning system. As such, it is a suitable surrogate device for assessing institutional performance across various anatomic sites. If an institution successfully irradiates the phantom, that result confers confidence that intensity modulated radiation therapy to a wide range of anatomic sites can be successfully delivered by the institution.