Reirradiation of thoracic malignancies is a treatment challenge, with concerns for toxicity and the inability to deliver definitive doses. Intensity modulated proton therapy (IMPT) may allow safe delivery of a higher dose of radiation to the tumor while minimizing toxicities.

Between 2011 and 2016, 27 patients who received IMPT for reirradiation of thoracic malignancies with definitive intent were retrospectively analyzed. Patients were included if they received a prior thoracic radiation course. All doses were recalculated to an equivalent dose in 2-Gy fractions (EQD2). Patients received IMPT to a median dose of 66 EQD2 Gy (range, 43.2-84 Gy) for recurrence of thoracic cancer (93%) or sequentially after a course of thoracic stereotactic ablative radiation therapy (7%).

Twenty-two patients (81%) were treated for non-small cell lung cancer. The median time to reirradiation was 29.5 months. At a median follow-up for all patients of 11.2 months (25.9 surviving patients), the median overall survival was 18.0 months, with a 1-year overall survival of 54%. Four patients (15%) experienced an in-field local failure (LF), with a 1-year freedom from LF rate of 78%. The 1-year freedom from locoregional failure and 1-year progression-free survival rates were 61% and 51%, respectively. Patients who received 66 EQD2 Gy or higher had improved 1-year freedom from LF (100% vs 49%; P = .013), 1-year freedom from locoregional failure (84% vs 23%; P = .035), and 1-year progression-free survival (76% vs 14%; P = .050). Reirradiation was well tolerated, with only 2 patients (7%) experiencing late grade 3 pulmonary toxicity, and none with grade 3 or higher esophagitis. There were no grade 4-5 toxicities.

These data represent the largest series of patients treated with IMPT for definitive reirradiation of thoracic cancers. They demonstrate that IMPT provided durable local control with minimal toxicity and suggest that higher doses may improve outcomes.