Factors Associated With Radiation Pneumonitis in Patients Receiving Electron Boost Radiation for Breast-Conserving Therapy: A Retrospective Review

Purpose This study aimed to investigate risk factors for radiation pneumonitis (RP) caused by electron beam (EB) boost irradiation during breast-conserving therapy. Methods and Materials This single-institution retrospective study included patients with breast cancer treated with breast-conserving therapy from 2013 to 2019. Radiation therapy comprised whole-breast irradiation with a dose of 50 Gy and 10 Gy EB boost dose to the tumor bed. EB energies were 4, 6, 9, 12, and 15 MeV. The lung volume receiving ≥1.25 Gy (V1.25) was calculated and considered because the EB energies have a short range. All patients underwent computed tomography and positron emission tomography/computed tomography within 1 year of irradiation. Imaging evaluation was based on the Common Terminology Criteria for Adverse Events, version 5.0. Results Overall, 105 patients (median age, 62 years; range, 33-85) were included for analysis with a median follow-up period of 5 months. Average area of EB boost irradiation was 72 cm2 (range, 36-196). Grade 1 RP developed in the EB irradiation field in 22 (20.6%) patients; grade 2 RP developed in 1 (0.93%) patient. Even in patients with central lung distance (CLD) ≥1.8 cm, a positive correlation was found between RP and both energy (r = 0.36; P = .005) and V1.25 (r = 0.26; P = .04). No correlation was found between RP and irradiation field size (P = .47). The EB energy and V1.25 cutoff values were 12 MeV and 24 cm3, respectively. Conclusions CLD of ≥1.8 cm, EB energy of ≥12 MeV, and V1.25 of ≥24 cm3 were risk factors associated with RP. Although the frequency of severe RP was not high, patients receiving high-energy electron treatment and those with a large CLD should be closely monitored.