21-gene expression assay in postpartum vs non-postpartum breast cancer and correlation with outcomes in young women with breast cancer.

Abstract

e12538 Background: The incidence of breast cancer (BC) in young women is rising and noted for more aggressive features and worse prognosis. Concurrently, increasing maternal age in the United States has led to a higher incidence of pregnancy-associated BC, known for worse survival outcomes. Specifically, post-partum BC (PPBC), characterized by mammary gland involution and remodeling after lactation, exhibits unique molecular characteristics thought to contribute to its poor prognosis. A higher prevalence of PPBC in YWBC may partly explain their worse outcomes, however genomic profiles of PPBC and non-PPBC are not well understood. We hypothesized that YWBC with PPBC may have higher 21-gene expression assay scores—Oncotype DX Recurrence Scores (RS)—and worse outcomes compared to YWBC with non-PPBC. Methods: A retrospective chart review was conducted, pooling data from three cohorts of YWBC with ER+HER2- BC over different time periods between 2011-2024 including patients (pts) with PPBC, pts with node positive BC, and a contemporary cohort of pts. YWBC were defined as women ≤45 years old. PPBC was defined as BC diagnosed ≤5 years postpartum and excluded cases diagnosed during pregnancy. Clinical outcomes, including 5-year distant recurrence-free survival (DRFS), overall survival (OS), and RS, were analyzed and compared between PPBC and non-PPBC using descriptive statistics, two proportion z-test, student’s t-test, Kaplan-Meier curves and log-rank test. Results: From the pooled cohorts, there were 43 YWBC with ER+HER2- BC. 44.2% of pts had stage I BC, 39.5% of pts had lymph node metastasis, and 58.1% were treated with systemic chemotherapy. 24 YWBC had non-PPBC while 19 had PPBC. Of these pts, 54.2% of non-PPBC had stage I disease compared to 31.6% of PPBC (p=0.128). 45.8% of non-PPBC had lymph node involvement compared to 73.7% of PPBC (p=0.066) and 45.8% of non-PPBC were treated with systemic chemotherapy compared to 68.4% of PPBC (p=0.128). 24 (55.8%) pts had RS available for review including 16 non-PPBC and 8 with PPBC. Average RS was 18.9 in the non-PPBC and 19.3 in the PPBC group (p=0.919). A higher proportion of PPBC pts had RS ≥16—75% in PPBC vs. 50% in non-PPBC (p=0.242). With a median follow-up of 66.1 months, distant recurrence rates were 8.33% and 21.1% in the non-PPBC and PPBC groups, respectively. The 5-year DRFS was 88.8% in non-PPBC and 80.3% in PPBC pts (p=0.26), with corresponding OS rates of 100% and 94.1% (p=0.55). Conclusions: From this pooled cohort, YWBC with PPBC have more lymph node involvement, higher chemotherapy use, and worse RFS than those with non-PPBC. However, differences in RS between these groups were less pronounced. Ongoing work will expand our cohorts to confirm these findings, while further studies are needed to better elucidate the biology lending to worse outcomes for pts with ER+ PPBC.