NRG/NSABP B-35 (tamoxifen vs anastrozole) suggests “close” DCIS margins don’t meaningfully worsen IBTR with WBI

In the Phase 3, double-blind NSABP B-35 trial (NCT00053898), 3104 postmenopausal women with HR-positive DCIS underwent lumpectomy + whole-breast irradiation (WBI) and were randomized to 5 years of tamoxifen (SERM) vs anastrozole (aromatase inhibitor); this ancillary analysis tested whether narrower negative margins predict ipsilateral breast tumor recurrence (IBTR). IBTR was infrequent overall (90/2707, 3.3% first events) but was numerically higher with margins <1mm vs ≥1mm (10-year cumulative incidence 5.6% vs 4.0%; P=.04) and with margins ≤2mm vs >2mm (5.3% vs 3.8%; P=.05), yet margin width was not significant after multivariable adjustment (2-mm threshold HR 1.33, 95% CI 0.86–2.06). Clinically, this lands close to (and arguably within) the “acceptable recurrence” territory that underpins current DCIS margin guidance—2mm is typically considered adequate with WBI, and routinely chasing wider margins has not shown clear incremental IBTR benefit.