Resection Margin Status and Radiation Boost to Surgical Cavity after Breast Conserving Surgery, a Pattern-of-Practice Study in British Columbia, Canada

Abstract

The 2014 Society of Surgical Oncology-American Society for Radiation Oncology (SSO-ASTRO) guideline defines ‘no tumor on ink’ as the adequate margin after breast conserving surgery (BCS). This study evaluates resection margin status and the use of surgical cavity boost following whole-breast radiation (WBRT) in British Columbia before and after the guideline publication. Using Breast Cancer Outcome Unit (BCOU) database, we identified 8,186 patients referred to BC Cancer with pT1-T3, any N, M0 invasive carcinoma diagnosed between 2010 and 2016. Patients were treated with BCS followed by WBRT with or without boost. Logistic regression was used to model the determinants of boost delivery. Fine and Gray competing risk analysis, with death and distant relapse as competing events, was used to assess the impact of boost on local control. The median follow-up was 5.6 years. The proportion of close (<2mm) or positive margin status was 25.5% in 2010-2014 and 25.9% in 2015-2106 (p = 0.66). Boost was used in 24.8% in margin-negative and 74.8% in close- or positive-margin patients. Boost utilization was 37.9% in 2010-2014 and 36.9% in 2015-2016 (p = 0.37). On multivariable analysis, younger age (OR 1.08, 95% CI = 1.07-1.08, p<0.0001), grade 3 (OR 1.34, 95%CI = 1.12-1.60, p = 0.0009), and positive or close margins (OR 12.6, CI = 11.1-14.4, p<0.0001) were significantly associated with boost delivery. Diagnosis in 2010-2014 compared to 2015-2016 was not associated with boost receipt (OR 1.05, 95%CI = 0.93-1.19, p = 0.46). On competing risk analysis, after adjusting for margin status, age, stage, grade, endocrine and HER2 status, boost was not associated with local control (HR 0.89, 95%CI = 0.56-1.43, p = 0.64). This study did not demonstrate a change in the likelihood of a close- or positive-margin status or boost utilization after the publication of the SSO-ASTRO consensus. For patients treated from 2010-2016, boost receipt was not associated with improved local control.