LAUSR

Micro‐Abstract Using molecular and immunohistochemical‐based testing for gene and protein expression patterns, the most commonly studied breast cancer variants are the luminal A, luminal B, HER2, and basal subtypes. Previous reports on outcomes for the breast cancer subtypes have focused on patients treated with traditional breast‐conserving therapy with whole‐breast irradiation. In this analysis, we observed 5‐year local control rates in 278 women after treatment with accelerated partial breast irradiation, which is excellent for luminal, HER2, and basal phenotypes of early‐stage breast cancer. Background: The purpose of the study was to determine outcomes for patients treated with accelerated partial breast irradiation (APBI) on the basis of breast cancer subtype (BCST). Patients and Methods: Our single‐institution, institutional review board‐approved APBI database was queried for patients who had complete testing results for the estrogen (ER), progesterone (PR), and HER2/neu receptors to determine outcomes for each BCST. Women were assigned as luminal A (LA), luminal B (LB), HER2, and basal BCST using their ER, PR, and HER2/neu receptor status. Degree of ER expression supplemented the receptor‐based luminal BCST assignment. Two hundred seventy‐eight patients had results for all 3 receptors (LA = 164 [59%], LB = 81 [29%], HER2 = 5 [2%], basal = 28 [10%]), which were submitted for analysis (ipsilateral breast tumor recurrence [IBTR], regional nodal failure, distant metastasis [DM], disease‐free survival [DFS], cause‐specific survival [CSS], and overall survival [OS]). Results: Median follow‐up was 5.4 years (range, 0.1‐12.4 years). Basal and HER2 subtype patients had higher histologic grades (Grade 3 = 75% vs. 10% LA/LB; P < .001), larger tumors (13.0 mm basal vs. 10.7 mm LA/LB; P = .059), and were more likely to receive chemotherapy (68% vs. 15% LA/LB; P < .001). Margin and nodal status were similar among BCSTs. At 5 years, IBTR rates were similar (1.8%, 2.9%, 0%, and 4.8%) for LA, LB, HER2, and basal subtypes, respectively (P = .62). DM was only seen in LA (2.9%) and LB (1.3%) (P = .83). DFS (95%‐100%), CSS (97%‐100%), and OS (80%‐100%) were not statistically different (P = .97, .87, .46, respectively). Conclusion: Five‐year local control rates after breast‐conserving surgery, APBI, and appropriate systemic therapy are excellent for luminal, HER2, and basal phenotypes of early‐stage breast cancer; however, further study of receptor subtype effect on risk stratification in early‐stage breast cancer is needed.