Cross-modality Dosimetry Audit in Head-and-neck Radiotherapy: A Phantom-based Approach for Photon and Proton Beams.

Purpose: The purpose of this study was to implement and evaluate an end-to-end (E2E) test using an anthropomorphic head-and-neck (H and N) phantom across different radiotherapy modality platforms for photon and proton. Materials and Methods: The CIRS SHANE anthropomorphic phantom was used for the E2E test across multiple modalities, including Varian Clinac iX, four Varian TrueBeam systems, two Varian Halcyon systems, one Varian ETHOS system, and one ProBeam system. The volumetric modulated arc therapy plans were independently optimized for each photon machine while the intensity modulated proton therapy was planned for proton. Dosimetric measurements were taken using a 0.125 cc semiflex ionization chamber at three planning target volumes (PTVs) and one spinal cord point. Results: The Kruskal-Wallis test revealed no significant differences in dose/volume parameters across the different modalities (P = 0.976). However, the Varian TrueBeam system showed a higher percentage dose difference compared to the other modalities, with the largest deviation of -3.7%. The O-ring gantry systems, Varian Halcyon and Varian ETHOS, demonstrated smaller dose differences and deviations compared to the C-based gantry systems. The Varian ETHOS delivered the highest number of monitor units at 641.6, while the Clinac iX delivered the lowest at 407.0. The ProBeam system showed good results for PTV and spinal cord. Conclusions: The implementation of the E2E test using an anthropomorphic H and N phantom has proven feasible across multiple radiotherapy modalities in both photon and proton machines. This approach offers a valuable tool for center with advanced radiotherapy technologies, particularly those without a national dosimetry audit program.