Cardiopulmonary Toxicity from Intensity Modulated Proton Therapy for Thymic Malignancies.

PURPOSE/OBJECTIVE(S): Use of radiation therapy for thymic malignancies is limited by excess dose to organs at risk (OARs) including heart, lung, and esophagus. Intensity Modulated Proton Therapy with Pencil Beam Scanning (IMPT/PBS) allows the conformality benefits of volumetric modulated arc therapy (VMAT) combined with dosimetry benefits of protons making it an exciting tool to treat thymic tumors. Very limited clinical data are reported with the use of IMPT/PBS to treat thymic malignancies. This study evaluates the incidence of acute and delayed toxicities among patients who underwent IMPT/PBS for thymic tumors.

MATERIALS/METHODS: Our single center retrospective study identified 27 patients with diagnosis of either thymic carcinoma or thymoma who received IMPT/PBS between 2015 and 2022. Patient demographics, IMPT treatment details and clinical outcomes (toxicity, recurrence, and survival) were recorded. Frequency distributions are described for primary endpoints of acute (≤ 90 days) and late (>90 days) toxicity graded using CTCAE version 5.0. Specific toxicities assessed were dermatitis, esophagitis, pneumonitis, pulmonary fibrosis, and cardiac toxicity. Recurrence and survival data were analyzed as secondary endpoint using Kaplan-Meier method.

RESULTS: Median follow-up was 22 months. Median age of the patients was 59 years (range, 30-87), predominantly female (55%), and white (66%), and stage ¾ (72%). Histologically showed mainly thymoma (59%) and Masaoka stage ¾ (70%). Surgery prior to IMPT was performed in 19 (70%) patients; of whom 5 patients had positive margins. Chemotherapy was used in 12 (44%) patients. Median IMPT dose was 50.4 GyE. Patients were primarily planned with 2 or 3 fields (81%), coplanar distribution (74%), using SFO technique (70%). Robust planning was performed accounting for 5 mm margin and 3.5-5% range uncertainty. All patients required use of range shifter ranging from 2-5 cm. Median of Heart-mean (10.3 GyE), Heart-max (54 GyE), Lung-mean (8.1 GyE), Lung V20 Gy (16.1%), Lung-V30 Gy (11.4%), Esophagus-mean (10.3 GyE) and Cord-Max (1.4 GyE). QACT was performed in 21 (77%) patients with replan needed in 5 of them. Only 1 (3.7%) patient had a grade 3+ acute toxicity (dermatitis) and only 2 (7.4%) patients had a grade 3+ late toxicity (both pulmonary). No patients had any acute or delayed cardiac-related adverse effects following PBT treatment. One (3.7%) patient had an infield recurrence of malignancy and 6 had out-of-field metastatic failure. Local control and overall survival were 74.1% and 85.2%, respectively.

CONCLUSION: In this largest single-institution analysis of IMPT/PBS experience, we note extremely low incidence of grade 3+ acute or late toxicity with excellent local control and overall survival. No marginal failures were noted. In a patient population at high risk of cardiopulmonary radiation toxicities, IMPT/PBS should be strongly explored as a possible treatment option.