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.

Inclusion Criteria: * Invasive ductal carcinoma or favorable special histology confirmed by core biopsy. * Unifocal tumor ≤2 cm; estrogen receptor-positive and HER2-negative; grade 1-2; clinically node-negative (cN0). * Age ≥65, or age 50-64 when surgery is unsuitable or declined (capped at 10 of 30 participants). * Tumor ≥5 mm from skin and pectoralis; willing to receive adjuvant endocrine therapy. * Zubrod performance status 0-2; able to undergo MRI and FDG PET/CT; informed consent. Exclusion Criteria: * Multifocal or multicentric disease; pure DCIS; invasive lobular carcinoma \>10%; extensive intraductal component or diffuse calcifications. * Grade 3, triple-negative, or HER2-positive disease; nodal or distant metastases. * Prior ipsilateral breast radiation or excisional biopsy/lumpectomy; neoadjuvant chemotherapy. * Tumor \60 cm³. * Connective tissue disease; prohibited other malignancy; pregnancy or lactation. * Pathogenic BRCA1, BRCA2, PALB2, or TP53 variant; investigator judgment that participation is inappropriate.