Over the past decades, interest in second breast-conserving therapy (BCT) has increased due to, among others, advances in radiotherapy techniques. Preoperative partial breast irradiation (PBI) is an experimental treatment for patients with low-risk primary breast cancer. This approach can downstage the tumour and may possibly reduce toxicity and improve cosmetic outcomes compared with postoperative radiotherapy. This study aims to evaluate the feasibility of single-dose preoperative PBI and second breast-conserving surgery (BCS) for patients with an ipsilateral recurrent breast event (IRBE) after previous BCT.

The REPEAT trial is a multicentre, prospective, single-arm trial investigating ablative single-dose preoperative PBI in patients with an IRBE. Eligible patients are ≥50 years, have a unifocal non-lobular invasive breast cancer ≤2 cm, Bloom-Richardson grade 1 or 2, oestrogen receptor-positive, human epidermal growth factor receptor 2-negative and clinically negative axillary lymph nodes. The study plans to enrol 25 patients. Radiotherapy planning will involve the use of CT and MRI in the treatment position. Single-dose PBI of 20 Gy to the tumour and 15 Gy to the surrounding 2 cm of breast tissue will be delivered using a conventional or MR-guided linear accelerator. Tumour response will be monitored preoperatively using MRI and liquid biopsies to identify biomarkers for evaluating radiosensitivity. BCS will be performed 3 (±one) weeks post PBI. The primary endpoint is the incidence of grade 2 or higher treatment-associated acute toxicity within 90 days. Secondary endpoints include the evaluation of acute (grade 1) and late toxicity, radiologic and pathologic response, mastectomy rate, patient-reported outcomes, cosmetic outcome, local, regional and distant recurrence rates, survival outcome and biomarkers in liquid biopsies and tumour tissue. Patients will be followed up to 5 years after PBI.

Ethical approval from the Medical Research Ethics Committee of the Amsterdam UMC has been obtained (NL85983.018.24). The results will be disseminated via peer-reviewed academic journal and presentation at conferences. In addition, summaries will be shared with the participating patients.

The trial was registered prospectively on October 11^th 2024 at clinicaltrials.gov (NCT06640881).