Purpose: To evaluate the phantom’s effectiveness at predicting the impact of atypical TPS beam modeling on clinical dose delivery across various anatomical sites, as it is often used for general IMRT credentialing.
Methods: The MLC offset, MLC transmission, and eight additional beam modeling parameters for a Varian accelerator were modified in RayStation to match data obtained from a community survey at the 2.5, 25, 75 and 97.5 percentile levels (Glenn-MedPhys:2020). Modifications were evaluated on fifty H&N phantom and clinical cases (H&N, prostate, lung, mesothelioma, and brain), generating two thousand plan perturbations. Differences in mean dose delivered to phantom, clinical target volumes (CTV), and organs at risk (OAR) were evaluated with respect to dose delivered using reference (50th-percentile) parameter values. Linear regression was used to assess the relationship between dose deviations in phantom and CTVs and 18 complexity metrics. The best complexity metric was selected based on average R-squared values.
Results: Perturbations to MLC offset and transmission parameters demonstrated the greatest impact on dose differences from 50th-percentile values in all target structures and OAR. The phantom demonstrated equivalent or greater sensitivity to these parameter perturbations compared to clinical sites. Maximum dose deviations were: 6.8% and 22.1% to phantom target structures and OAR, and 5.7% and 16.1% to CTVs and clinical OARs, respectively. Phantom and clinical plans exhibiting higher complexity showed greater dose perturbation with extreme parameter values. The mean MLC Gap best described the impact of varying TPS beam modeling parameters in phantoms and across all anatomical sites.
Conclusion: In general, when compared across various anatomical sites, the IROC H&N credentialing phantom exhibits similar or greater sensitivity to perturbations to the MLC offset and transmission beam modeling parameters. As such, it can be considered an effective surrogate device for predicting clinically significant perturbations in TPS modeling parameters, across various anatomical sites.
Funding Support, Disclosures, and Conflict of Interest: This work was supported by Public Health Service Grants CA180803 and CA214526, awarded by the National Cancer Institute, United States Department of Health and Human Services.
Phantoms, Dose, Dosimetry Protocols