SU-E-T-437: Interfractional Dosimetric Verification of Lung Patients Treated by Passive Double Scattering Proton Radiotherapy.

PURPOSE Proton radiotherapy, with the ability to confine the dose at desired depth, can potentially benefit lung tumor patients by significantly sparing the healthy lung tissue. However, the superior proton dose distribution could be affected by tumor shrinkage due to the quick response and by motion especially related to the respiration. Thus the treatment should be frequently verified and be adjusted accordingly if necessary to achieve the initial treatment goal. MATERIAL AND METHODS A cohort of 20 patients were selected from lung patients treated with passive proton radiotherapy. All those patients were evaluated via 4D-CT scans and found to have tumor motion less than 1 cm. The internal target volumes (ITV) were derived based on the full inspiration and expiration phases. The average of the 4D-CT scan, full inspiration and expiration phases were used for the initial treatment planning. The planning objective was 95% of the prescription dose to at least 95% volume of the ITV. Bi-weekly verification 4D-CT scans were performed to assess the robustness of the initial treatment plan and no replanning was required for target dose variations less then 3%. RESULTS Compared with the initial treatment plan, the standard deviations of target coverage on inspiration, expiration, and average verification CT scans are within 3% for all the patients, with the maximum difference up to 7%. No statistically significant differences were found among the initial and verification plans (p>0.1). The percentage deviations of OAR sparing were highly variable, e.g., up to 40% for mean lung dose, 100% for mean heart dose, 50% for max cord dose, particularly for OARs receiving small amount of doses. However, the absolute dose deviations are all with OAR's tolerance. CONCLUSION Overall, the passive double scattering proton modality allows for robust proton treatment planning and delivery to treat the lung tumors with limited motion.