Adaptive planning and toxicities of uniform scanning proton therapy for lung cancer patients

Purpose: Adaptive planning is often needed in lung cancer proton therapy to account for geometrical variations, such as tumor shrinkage and other anatomical changes. The purpose of this study is to present our findings in adaptive radiotherapy for lung cancer using uniform scanning proton beams, including clinical workflow, adaptation strategies and considerations, and toxicities. Methods: We analyzed 165 lung patients treated using uniform scanning proton beams at our center. Quality assurance (QA) plans were generated after repeated computerized tomography (CT) scan to evaluate anatomic and dosimetric change during the course of treatment. Plan adaptation was determined mutually by physicists and physicians after QA plan evaluation, based on several clinical and practical considerations including potential clinical benefit and associated cost in plan adaption. Detailed analysis was performed for all patients with a plan adaptation, including the type of anatomy change, at which fraction the adaption was made, and the strategy for adaptation. Toxicities were compared between patients with and without plan adaptation. Results: In total, 32 adaptive plans were made for 31 patients out of 165 patients, with one patient undergoing adaptive planning twice. Anatomy changes leading to plan adaptation included tumor shrinkage (17), pleural effusion (3), patient weight loss (2), and tumor growth or other anatomy change (9). The plan adaptation occurred at the 15th fraction on average and ranged from the 1st to 31st fraction. Strategies of plan adaptation included range change only (18), re-planning with new patient-specific hardware (9), and others (5). Most toxicities were Grade 1 or 2, with dermatitis the highest toxicity rate. Conclusions: Adaptive planning is necessary in proton therapy to account for anatomy change and its effect on proton penetration depth during the course of treatment. It is important to take practical considerations into account and fully understand the limitations of plan adaptation process and tools to make wise decision on adaptive planning. USPT is a safe treatment for lung cancer patients with no Grade 4 toxicity