Clinical outcomes of patients with HER2-positive microinvasive breast cancer.

Abstract

544 Background: Although previous studies have reported that adjuvant trastuzumab with paclitaxel resulted in excellent survival outcomes in small, node-negative HER2-positive breast cancer, the necessity of adjuvant chemotherapy in HER2-positive microinvasive breast cancer (MIBC) remains controversial. Methods: Between January 2000 and January 2020, women with curatively resected pT1mi N0 and/or N1mi in Asan Medical Center were retrospectively identified and their survival outcomes including invasive disease-free survival (IDFS, excluding second primary cancer), distant recurrence-free survival (DRFS), and overall survival (OS) and the effect of adjuvant chemotherapy were analyzed. Results: A total of 833 patients were included. The median age was 51 years, and 51% (n = 426) were premenopausal. Pathologic nodal stage was pN0 in 96% (n = 799) and pN1mi in 0.7% (n = 6), and estrogen receptor (ER) was positive in 30% (n = 247). Adjuvant chemotherapy was administered to 18% (n = 147) of patients, of which 95% were fluorouracil-based. Six patients (0.7%) received adjuvant trastuzumab. With the median follow-up of 118.7 months, the 8-year IDFS and DRFS rates were 90.6% (95% CI, 88.4-92.7), and 97.3% (95% CI, 96.1-98.4), respectively, and the 8-year OS were 98.6% (95% CI, 97.8-99.5). IDFS did not differ by adjuvant chemotherapy, as the 8-year IDFS rates for patients who did and did not receive adjuvant chemotherapy were 91.1% (95% CI, 86.6-95.8) and 90.3% (95% CI, 87.9-92.8), respectively ( p = 0.930). The lack of differences in IDFS by chemotherapy was consistent across subgroups including pre-/postmenopausal patients, grade 1-2/3 tumors, and ER-negative diseases. Conclusions: In HER2-positive MIBC, a clinically meaningful proportion of patients experience IDFS events with long-term follow-up. The role of modern adjuvant chemotherapy regimens, as well as the role of adjuvant trastuzumab, should be further explored in this early-stage patient population.