Room: Exhibit Hall
Purpose: To examine the preliminary institutional irradiations of the proton Head & Neck (H&N) phantom used for clinical trial credentialing.
Methods: An anthropomorphic H&N phantom was created with proton-equivalent plastics and an embedded human skull. The phantom contains a horseshoe shaped target and cord and parotid organs at risk, as well as TLD and radiochromic film for dosimetric measurements. The phantom is simulated, planned, and irradiated using the institutionsâ€™ clinical procedures. Ninty-five percent of the target is to receive 6.6 Gy(RBE). The phantom was irradiated 12 times by 11 different proton therapy centers; 15 analyses were performed, as several institutions submitted multiple calculations using different algorithms. The criteria for point dose agreement between the TLD and the treatment plan was Â±7%, and the criteria for percent of film pixels passing a 7%/4 mm gamma analysis was 85%. The institutionsâ€™ treatment plans were also assessed for how well they met the target and OAR dose constraints.
Results: The phantom pass rate, based on the target dose results and gamma comparison, was 87%. The mean ratio of TLD/TPS was 0.97 (Ïƒ 0.03), 1.03 (Ïƒ 0.11), and 0.97 (Ïƒ 0.09) for the target, parotids, and cord, respectively. The large parotid dose Ïƒ is a result of the steep dose gradient at the edge of the OAR.The average percent of pixels passing the gamma analysis was 94% (Ïƒ 5.7%). Despite this fairly high phantom pass rate, an analysis of clinical dose constraints for the target and OARs showed only 33% of institutions were able to meet these criteria.
Conclusion: Many proton centers are able to deliver what they plan to this phantom, however many struggle to meet typical clinical dose constraints for H&N disease. Clinical trial groups should proceed with caution when adding proton therapy as a delivery modality for H&N trials.
Funding Support, Disclosures, and Conflict of Interest: This work is supported by the NCI grant # CA180803.