Key Points
- Single-institution retrospective study of 70 consecutive patients treated from 2019–2025 with pencil-beam scanning proton reirradiation for isolated locoregional breast cancer recurrence after overlapping prior breast radiotherapy.
- Median first-course, reirradiation, and cumulative doses were 53, 50, and 107 Gy(RBE), respectively; 82.9% received regional nodal irradiation with or without whole-breast or chest-wall treatment.
- At 30 months’ median follow-up, cumulative incidence of any recurrence was 14% at two years and 17% at three years; three-year overall survival was 95%. Local and regional recurrence occurred in 7.1% and 1.4%, respectively.
- Maximum acute grade 3 toxicity occurred in 7.1% and maximum late grade 3 toxicity in 10%; no grade 4 or higher events and no cardiac or pulmonary toxicities were reported.
- Rib fractures occurred in 21.1% without grade 3 events. Higher chest-wall D200cc during both radiation courses and higher first-course rib D10cc were associated with fracture risk; greater breast, chest-wall, and nodal overlap was associated with late grade ≥2 toxicity.
CLINICAL TAKEAWAY
Pencil-beam scanning proton therapy may enable comprehensive salvage reirradiation for carefully selected patients with high-risk locoregional breast cancer recurrence while maintaining relatively low rates of severe toxicity. The findings are clinically relevant but remain hypothesis-generating because of the retrospective, single-institution design, incomplete availability of prior treatment plans, and limited follow-up for very late effects.