SOLIS: Single-Dose Radiotherapy With Omission of Lumpectomy for Intact Small Breast Cancers

SOLIS is a prospective, single-arm, single-institution pilot trial evaluating single-fraction stereotactic radiotherapy (24 Gy to the planning target volume with a simultaneous integrated boost of 30 Gy to the clinical target volume) delivered with a cleared linear accelerator as definitive non-operative local treatment for early-stage, hormone receptor-positive, HER2-negative breast cancer with planned omission of lumpectomy. Adjuvant endocrine therapy is prescribed on-label. The primary endpoint is the incidence of CTCAE v5.0 Grade ≥2 skin adverse events within 90 days of treatment, monitored with a pre-specified Bayesian safety stopping rule. Secondary endpoints include biopsy-confirmed absence of residual invasive carcinoma at 12 months, imaging response on multiparametric MRI and FDG PET/CT, late toxicity, cosmesis, patient-reported outcomes, salvage surgery, and disease control through 36 months.