(Sunday, 7/29/2018) 3:00 PM - 6:00 PM
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 institutions’ 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 ±7%, and the criteria for percent of film pixels passing a 7%/4 mm gamma analysis was 85%. The institutions’ 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 (σ 0.03), 1.03 (σ 0.11), and 0.97 (σ 0.09) for the target, parotids, and cord, respectively. The large parotid dose σ 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% (σ 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.
Keywords
Protons, Phantoms, Clinical Trials
Taxonomy
TH- External Beam- Particle therapy: Proton therapy - quality assurance
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