In a single-center, prospective phase II study, carbon-ion radiotherapy provided durable tumor control with minimal side effects while preserving breast appearance in carefully selected patients who were unable or unwilling to undergo surgery. The study was published online on October 24, 2025, in the International Journal of Radiation Oncology*Biology*Physics.

The societal impact is clear: expanding access to a nonsurgical, organ-sparing treatment option has the potential to improve quality of life, particularly for older adults, patients with comorbidities, or those who wish to avoid mastectomy or lumpectomy. At five years, local control and disease-free survival rates were both 92%, overall survival was 100%, and no grade 2 or higher toxicities were observed. Most patients experienced only mild, transient skin reactions. Cosmetic outcomes were rated as “excellent” in all but one case, which later required mastectomy because of local recurrence.