Five-Year Outcomes of a Phase 1/2 Trial of Accelerated Partial Breast Irradiation Using Proton Therapy for Women With Stage 0-IIA Breast Cancer.

Purpose: We report the results of a phase 1/2 trial of external beam partial breast radiation using proton therapy.

Methods And Materials: Eligible patients included stage 0-IIA breast cancer pTis-T2, N0, and size ≤3 cm. Proton beam radiation was used to deliver 3.