Pilot study tests single-fraction stereotactic radiation plus endocrine therapy without lumpectomy in early breast cancer

This prospective, single-arm pilot from the Renaissance Institute of Precision Oncology and Radiosurgery tests whether a single fraction of highly conformal stereotactic radiation can serve as definitive local treatment for carefully selected, small, intact HR-positive, HER2-negative breast cancers without lumpectomy. The prescribed treatment is 24 Gy to the planning target volume with a simultaneous integrated boost to 30 Gy to the clinical target volume, followed by standard endocrine therapy with an aromatase inhibitor or tamoxifen.

The main safety focus is grade 2 or higher acute skin toxicity within 90 days, with a stopping rule for excess toxicity. Activity assessments include biopsy-confirmed absence of residual invasive cancer at 12 months and imaging response. Surgery remains the usual local treatment, with radiation, nodal staging, and endocrine treatment adapted to age, risk, and clinical circumstances. This pilot asks whether a nonsurgical local approach can provide adequate control in carefully selected patients; surgery omission with this regimen remains investigational.