Breast Tangent Beam Energy, Surgical Bed-to-Skin Distance and Local Recurrence after Breast-Conserving Treatment.

Abstract

PURPOSE\nHigher energy (>6 MV) photons reduce dose inhomogeneity with breast tangent beams, thereby reducing late breast toxicity, but skin and superficial tissue sparing by higher energy beams raises concerns about local recurrence (LR) risk. This study aimed to determine whether beam energy and surgical bed-to-skin distance affect LR.\n\n\nMETHODS AND MATERIALS\nThis population-based study included newly diagnosed invasive breast cancers without skin involvement (pT1-4a, any-N, M0) treated with breast-conserving surgery and adjuvant whole breast radiotherapy without bolus or beam spoilers. The primary endpoint was the cumulative incidence of LR (CILR). A multivariable analysis (MVA) included mean beam energy, age, T-stage, nodal status, overall stage, lymphovascular invasion (LVI), grade, margin status, extensive intraductal component (EIC), breast cancer subtype, hormone therapy and chemotherapy. In a subgroup with contoured surgical beds, another MVA included surgical bed-to-skin distance.\n\n\nRESULTS\nThe cohort consisted of 10,083 women treated from 2002 to 2011, 327 with 4MV, 6,006 with 6 MV, 2,083 with >6-10 MV and 1,667 with >10 MV tangents. The median follow-up time was 11.1 years. The 10-year CILR was 3.1% [95% confidence interval 1.6,5.4] with 4 MV, 2.8% [2.4,3.3] with 6 MV, 4.2% [3.4,5.3] with >6-10 MV and 2.6% [1.9,3.5] with >10 MV. On MVA of the entire cohort, LR risk was increased with positive margins, LVI, EIC, and lack of hormone therapy, but was not associated with beam energy (HR\u202f=\u202f1.01 [0.96,1.05], p\u202f=\u202f0.8). On MVA of 3,359 patients with contoured surgical beds, LR risk was not associated with surgical bed-to-skin distance (HR\u202f=\u202f1.00 [0.99,1.02], p\u202f=\u202f0.8).\n\n\nCONCLUSIONS\nUse of higher breast tangent beam energies is not associated with increased risk of local recurrence, including in cases with surgical beds that are close to the skin.