Evaluation of current consensus panel guidelines for APBI: A pooled analysis of William Beaumont Hospital and American Society of Breast Surgeons MammoSite registry trial data.

Abstract

145 Background: To determine whether the ASTRO Consensus Panel (CP) guidelines for accelerated partial breast irradiation (APBI) are associated with significantly different outcomes in a pooled analysis from William Beaumont Hospital (WBH) and the American Society of Breast Surgeons (ASBrS) MammoSite Registry Trial. Methods: 2,127 cases of early-stage breast cancer were treated using APBI (WBH: n=678; ASBrS: n=1,449). Three forms of APBI were used at WBH (interstitial, n=221; balloon-based, n=255; or 3D-CRT, n=206) while all Registry Trial patients received balloon-based brachytherapy. Patients with complete coding necessary for ASTRO CP assignment (n=1,813) were divided into suitable (n=661, 36.5%), cautionary (n=850, 46.9%), and unsuitable (n=302, 16.7%) categories. Tumor characteristics and clinical outcomes were analyzed according to CP group. Results: Median age was 65 years (32-94 years) and median tumor size was 10.0mm (0-45mm). The WBH cohort had more node-positive disease (6.9% vs. 2.6%, p<0.01) and cautionary patients (49.5% vs. 41.8%, p=0.06). Five-year rates of ipsilateral breast tumor recurrence (IBTR), regional nodal failure (RNF), and distant metastasis (DM) for the whole cohort were 2.8%, 0.6%, 1.6%, respectively. When analyzed by CP group, the rate of IBTR was not statistically higher for suitable (2.5%), cautionary (3.3%), or unsuitable (4.6%) patients (p=0.20). The non-significant increase in IBTR for grouped cautionary/unsuitable categories was due to increased elsewhere failures/new primaries (p=0.04), not true recurrences (1.1-1.2%, all groups, p=0.94). A higher rate of DM was observed within the cautionary and unsuitable groups (3.3-3.6% vs. 0.7% for suitable, p=0.01), although cause-specific survival was the same for all patient categories (98.0-98.6%, all groups, p=0.47). Conclusions: Excellent outcomes following breast conserving surgery and APBI were seen in our pooled analysis. The current ASTRO CP guidelines did not adequately differentiate patients at an increased risk of IBTR or tumor bed failure within the largest patient population of cases treated with APBI to date.