Five-year outcomes of partial breast proton beam therapy without lumpectomy for early-stage breast cancer: an interventional prospective study.

BACKGROUND: We report on our initial clinical experience with partial breast proton beam therapy (PB-PBT) without tumor resection as an alternative to partial mastectomy for low-risk early-stage breast cancer. METHODS: This trial commenced in 2015, and eligibility criteria included women with cT1N0M0 invasive ductal carcinoma, estrogen receptor positivity, human epidermal growth factor receptor type-2 negativity, age of 40-70 years, and a performance status of 0 or 1. In Phase I, PB-PBT was administered at 62.4 Gy (RBE) in 26 fractions using the field-in-field technique. After confirming that no serious adverse events occurred, Phase II was conducted using the same dose. Follow-up evaluations were conducted every 3 months for the first year after irradiation and every 6 months thereafter. RESULTS: As of December 2025, 11 patients had completed a follow-up of ≥ 5 years. The median follow-up time was 82 months. Tumors exhibited immediate shrinkage post-PB-PBT, with morphological changes on ultrasonography and magnetic resonance imaging observed within 3-6 months. No recurrence was observed within the irradiated field in patients with > 5 years of follow-up. Dermatitis was Grade 1 in 55% of cases and Grade 2 in 45%; there were no cases of Grade ≥ 3. Grade 1 telangiectasia was observed in 36% of patients after 2 years, although most patients tolerated the treatment well. CONCLUSION: Preliminary clinical observations suggest that PB-PBT without lumpectomy achieves results within specific criteria and that it could be an option for breast cancer treatment using radiation therapy alone.