BACKGROUND: Breast cancer is the leading cause of global cancer incidence. In early-stage disease, standard treatment with breast-conserving surgery followed by whole breast irradiation (WBI) is associated with excellent outcomes. Multiple studies with extensive follow-up periods have demonstrated the comparative efficacy and toxicity outcomes of partial breast irradiation (PBI) in contrast to WBI. Various dose fractionation schedules for PBI have been recommended in clinical practice guidelines. In British Columbia (BC), a dose of 26 Gy in 5 fractions has been adopted. Early studies on single-fraction (SF) radiation for PBI have investigated its safety and effect on cosmetic outcomes, with promising initial results. In the context of the ongoing health care crisis, single-fraction PBI has the potential to reduce wait times and improve access to radiation. This study will therefore investigate a single-fraction PBI dose of 13 Gy. METHODS: This is a phase II randomized controlled trial, with a primary objective of testing the feasibility of randomizing participants to 1 vs. 5 fractions of PBI for early stage, node negative, breast cancer. The primary endpoint is the ability to accrue 60 participants at 4 of the 6 BC Cancer centres over a 2- year period and to randomize them to 1 vs. 5 fractions of radiotherapy for PBI. Its secondary endpoints are time from CT simulation to partial breast radiotherapy, local control rates, quality of life as measured by Prospective Outcomes and Support Initiative (POSI)-Breast, rates of provider-rated toxicities as measured by Common Terminology Criteria for Adverse Events (CTCAE), rates of participant-reported toxicities as measured by participant reported outcome version of CTCAE (PRO-CTCAE), overall survival, and progression-free survival. DISCUSSION: One of the trial's objectives is testing the feasibility of randomizing participants to single vs. multiple fractions for PBI. If successful, it will lead to a phase III non-inferiority trial with the potential to inform breast cancer treatment guidelines. Ultimately, if found to be non-inferior, a single-fraction PBI can reduce wait times, facilitate access to radiation, and improve patient convenience, particularly for those in rural and remote communities who must travel long distances to receive high-quality cancer care. TRIAL REGISTRATION: Clinicaltrials.gov identifier: NCT06885671. Date of Registration: 14 March 2025.