SABR (definitive breast RT) + endocrine therapy hits 53% pCR, enabling surgery omission in low-risk HR+ breast cancer

Definitive stereotactic ablative radiotherapy (SABR; 5 fractions) plus endocrine therapy (ET) was tested as a non-operative strategy in a prospective Phase II trial (NCT02945579) for select patients aged ≥40 with cT1N0M0, unicentric HR+/HER2− breast cancer; patients received 3 months of ET → SABR, then vacuum-assisted biopsy of the tumor bed at 6–12 months to decide on surgery omission. Among 19 biopsied patients (median age 70.5), pCR was 53% (10/19; 95% CI 30–73%) and near-complete response was 37% (7/19); 12 patients ultimately omitted surgery and, at median follow-up 44.9 months, 3-year PFS was 92% (95% CI 54–99%) with no breast cancer recurrences (one non–breast cancer death). As a benchmark, current de-escalation standards still generally start with lumpectomy: in older, low-risk HR+ disease treated with lumpectomy + ET, adding whole-breast RT improves long-term local control (e.g., 10-year freedom from local/regional recurrence 98% with RT vs 90% without RT in CALGB 9343), while PRIME II similarly reported higher 10-year ipsilateral recurrence when RT is omitted after surgery (≈9–10% vs ≈1%), underscoring that this study is “RT instead of surgery,” not “less RT after surgery.”