Likelihood of late relapse assessed by the Breast Cancer Index (BCI) in LN-invasive lobular compared to invasive ductal carcinoma.

Abstract

e12025 Background: Women with HR+ breast cancer remain at risk for late distant recurrences (DR) despite optimal adjuvant therapy. Invasive lobular carcinoma (ILC) is the 2nd most common subtype of invasive breast cancer. When compared to invasive ductal carcinoma (IDC), ILC has distinguishing clinical and pathologic characteristics that may result in different response to therapy and long term prognosis. BCI is a validated gene expression-based assay for pts with early stage HR+ breast cancer that provides risk of late (5-10 y) DR and predicts likelihood of benefit of extended endocrine therapy (EET). Here, we compared BCI assay results in pts with HR+, LN- ILC vs IDC. Methods: The BCI Clinical Database for Correlative Studies is an IRB-approved de-identified database that contains >50 clinicopathologic and molecular variables from cases submitted for BCI in clinical practice (N=14,463). Molecular variables include BCI Prognostic score, HoxB13/IL17BR ratio (H/I), and molecular grade index (MGI). Clinicopathologic variables were abstracted from pathology reports (available for a subset of cases). LN- pts with available pathologic data were analyzed. Chi-squared tests were used to compare BCI results between IDC and ILC subgroups. Results: Analyses included 2554 LN- pts with available histologic subtype data (80.7% IDC; 13.7% ILC; 2.6% mixed; 2.9% other). Median age was 59.3y (range 27-89y; 74% ≥ 50y). BCI Prognostic had a broad distribution of individual risk assessment in both IDC and ILC. However, BCI Prognostic had a lower median BCI scores (P<0.0001) in ILC, and classified a smaller proportion of pts with ILC as high risk for late DR compared to IDC (36.5% vs 53.1%; P<0.0001). IDC had a higher median molecular proliferative status (MGI) compared to ILC (P<0.0001). BCI Predictive (H/I) identified a slightly decreased proportion of pts with ILC benefiting from extended endocrine therapy compared to IDC (38.2% vs 41.1%). There was no difference in quantitative ER or PR (by PCR) between groups. Conclusions: BCI identified a smaller proportion of pts with ILC at high risk of late DR compared to IDC. Further studies evaluating outcomes are warranted to further validate BCI in pts with ILC.