Patterns of Adaptive Replanning in Proton Therapy for Thoracic Malignancies

Abstract

Theability to deliver high doses of localized radiation therapy (RT) while preferentially sparing surrounding normal tissue makes the use of proton beam therapy (PBT) advantageous for thoracic malignancies. However, these traits also make PBT more susceptible to treatment related changes to the tumor or target volume, patient anatomy or surrounding tissue. This may result in increased need for adaptive re-planning during PBT. We present a single institution experience examining patterns of PBT adaptive re-planning in patients treated for thoracic malignancies. Consecutive patients treated with PBT for thoracic malignancies from 2017-2022 at an academic proton center were retrospectively analyzed. Patients were treated with intensity modulated proton therapy (IMPT) either once or twice daily (BID), with or without concurrent systemic therapy. Patients with lung and thymic malignancies were included. Quality assurance CT (QACT) scans were assessed for dosimetric changes. Number and frequency of QACTs was determined by departmental policy and clinical factors such as histology, cancer stage and concurrent systemic therapy. Statistical comparisons of the frequency of replans and pathological and patient characteristics were performed with two-tailed Fisher's exact tests and Chi-Square tests. Of the 340 patients who were retrospectively analyzed, 80 (24%) required replanning during treatment. Median age was 67 years (range 20-89) and 49 (61%) patients were men. Median radiation dose was 59.5 Gy (range 16.2 -70 Gy). Pathology requiring replan included small cell lung cancer (n = 6, 8%), lung adenocarcinoma (n = 23, 29%), lung squamous cell carcinoma (n = 25, 31%), mesothelioma (n = 7, 9%), thymic malignancy (n = 5, 6%) or other histology (n = 10, 13%). 46 (58%) patients received concurrent systemic or chemotherapy. Six (8%) patients were treated in a BID approach. Histology, frequency of treatment, and concurrent systemic treatment were not significant predictors of patients requiring replan. Of the 80 total patients that required replanning, a total of 122 replans were completed. A total of 360 quality assurance CT (QACT) scans were used for all replans. Sixty (75%) patients required only one re-plan, 20 (25%) required two or more re-plans. Median number of treatments prior to the initial replan was 13 treatments (range 2 -41). Reason for requiring replanning included change in tumor size (n = 21), change in anatomy (n = 41, 51%), concern for toxicity (n = 4, 5%), change to radiation plan (n = 2, 3%), or technical/technique related changes (n = 12, 15%). Our single institution experience highlights the need for adaptive replanning in patients with thoracic malignancies treated with PBT to ensure adequate target coverage and to help spare normal tissue.