Impact of the number of cautionary/unsuitable risk factors on outcomes following accelerated partial breast irradiation.

146 Background: Current guidelines for APBI stratify appropriateness of treatment outside of a clinical trial. Limited data are available, however, regarding the impact of the number of risk factors (RFs) per patient on clinical outcomes. METHODS 692 patients were treated with APBI at a single institution between 10/1992 and 10/2011. Patients were stratified by the ASTRO guidelines as suitable, cautionary, and unsuitable. Outcomes including ipsilateral/contralateral breast tumor recurrence (IBTR/CBTR), regional recurrence (RR), distant metastases (DM), disease-free survival (DFS), cause specific survival (CSS), and overall survival (OS) were evaluated by risk group and number of RFs. RESULTS Median follow-up was 5.2 years (range: 0-18.3).Distribution within CP risk-groups was suitable: 240, cautionary: 343, and unsuitable: 109 patients. Increased IBTR (2.0% v. 0.6%, p=0.03), DM (6.5% v. 1.5%, p=0.02), and decreased DFS (92% v. 98%, p=0.01) were noted for patients with 2+ cautionary RFs (n=115) vs. 1 RF (n=228). Those with 2+ unsuitable RFs had higher RR (7.7% v. 1.7%, p=0.05). Pooled analysis revealed increased IBTR/RR for patients with 3+ combined cautionary/unsuitable RFs vs. 2 or fewer combined RFs (Table). Univariate analysis showed increased DM with increasing tumor size/T-stage (p<0.01), ER negativity (p=0.04), LVSI (p=0.01), + LN (p<0.01), and increasing number of RFs (p<0.01). No single RF was associated with an increased risk of local recurrence on UVA. CONCLUSIONS Three or more cautionary or unsuitable APBI RFs is associated with higher local, regional, and distant recurrence. Patients with fewer than 3 total RFs have a 98% locoregional control at 5 years and likely remain good candidates for APBI. Future attempts to risk stratify patients may need to account for the number of RF present in order to appropriately classify patients. [Table: see text].