Proton reirradiation for recurrent or new primary breast cancer in the setting of prior breast irradiation

Background and purpose: Late local recurrences and second primary breast cancers are increasingly common. Proton beam therapy (PBT) reirradiation (reRT) may allow safer delivery of a second definitive radiotherapy (RT) course. We analyzed outcomes of patients with recurrent or new primary breast cancer who underwent reRT. Materials and methods: In an IRB-approved retrospective study, patient/tumor characteristics, treatment parameters, outcomes, and toxicities were collected for all consecutive patients with recurrent or new primary non-metastatic breast cancer previously treated with breast or chest wall RT who underwent PBT reRT. Results: Forty-six patients received reRT using uniform (70%) or pencil beam (30%) scanning PBT. Median first RT, reRT, and cumulative doses were 60 Gy (range 45–66 Gy), 50.4 Gy(RBE) (40–66.6 Gy(RBE)), and 110 Gy(RBE) (96.6–169.4 Gy(RBE)), respectively. Median follow-up was 21 months. There were no local or regional recurrences; 17% developed distant recurrence. Two-year DMFS and OS were 92.0% and 93.6%, respectively. Nine of 13 (69.2%) patients who underwent implant or flap reconstruction developed capsular contracture, 3 (23.1%) requiring surgical intervention. One (7.7%) patient developed grade 3 breast pain requiring mastectomy after breast conserving surgery. No acute or late grade 4–5 toxicities were seen. Increased body mass index (BMI) was protective of grade ≥ 2 acute toxicity (OR = 0.84, 95%CI = 0.70–1.00). Conclusion: In the largest series to date of PBT reRT for breast cancer recurrence or new primary after prior definitive breast or chest wall RT, excellent locoregional control and few high-grade toxicities were encountered. PBT reRT may provide a relatively safe and highly effective salvage option. Additional patients and follow-up are needed to correlate composite normal tissue doses with toxicities and assess long-term outcomes.