Abstract P3-05-38: Biology and clinical course of lobular cancer in breast cancer (BC)
Authors: Raghavendra Akshara Singareeka , Bassett Roland , Mouabbi Jason , Layman Rachel M. , Tripathy Debu 
Published: March 2023

Abstract
Background: Invasive lobular carcinoma (ILC) accounts for 10–15% of all breast cancers. Due to its distinctive growth pattern and biology, lobular carcinoma often fails to form distinct masses that can be easily detected by palpation or mammography. Because it is substantially less common than infiltrating ductal carcinoma (IDC), knowledge about clinical outcomes of ILC has been based on studies including relatively small patient (pt) numbers. Thus, we sought to assess the biologic and genomic features of ILC in the context of clinical outcomes.
Methods: Eligible pts were adults with metastatic lobular or ductal BC seen at MD Anderson between 1997 and 2020. Overall survival (OS), progression-free survival (PFS), and disease-free interval (DFI) from the initial diagnosis were estimated using the Kaplan-Meier method. Survival distributions were compared using the log-rank test. Multivariate Cox proportional hazards regression models were applied to assess effect of covariates of interest on OS and DFS. No statistical adjustment was made for multiple testing.
Results: A total of 7642 IDC and 1159 ILC female metastatic BC pts were included in the analysis. Clinical characteristics are presented in the table. Pts with ILC were on average older, had fewer metastases, and were more likely to have a family history of breast or ovarian cancer than pts with IDC. Lobular cases were less likely to be HER2+, had lower Ki-67, and lower nuclear grade than ductal cancer cases.
The median follow-up was 4.37 years. The median OS for all pts from initial diagnosis was 5.38 years; 5.21 years and 6.57 years for IDC and ILC, respectively. ER positivity was associated with longer OS in both IDC [Hazard ratio (HR) 0.47, 95% confidence interval (CI): 0.45 - 0.49, P&lt; 0.0001] and ILC (HR 0.63, 95% CI: 0.54 - 0.75, P&lt; 0.0001). In de novo metastatic disease, the median OS was 3.74 years in IDC and 4.15 years in ILC. In recurrent IDC and ILC, the median OS was 5.84 and 7.89 years, respectively. De novo presentation had better OS from time of metastatic disease diagnosis for pts with both ILC and IDC than those presenting with recurrent cancer; however, de novo presentation was associated with worse PFS on 1st line therapy. In ILC, grade III had a poorer prognosis than grade I/II; (HR 1.47, 95% CI: 1.30 - 1.68, P&lt; 0:0001). Among pts with IDC, grade I had the best outcomes, followed by grade II, then grade III. In IDC, significantly improved OS was observed in HER2+ BC (HR 0.85, 95% CI: 0.79 - 0.92), P&lt; 0:0001). The median PFS for all pts was 0.53 years and the median DFI was 2.61 years. In both ILC and IDC, PFS and DFI were better in cancers that were ER+ or grade I/II.
Conclusions: In metastatic BC pts, the biologic phenotypes and clinical behavior of ILC and IDC differ. More complete and reliable characterization of ILC may yield useful information regarding the biologic nature of ILC resulting in discovery of actionable findings leading to more personalized therapy. Further analyses of genomic features and patterns of metastatic sites between IDC and ILC will be presented.
Patient characteristics of ILC and IDC
Citation Format: Akshara Singareeka Raghavendra, Roland Bassett, Jason Mouabbi, Rachel M. Layman, Debu Tripathy. Biology and clinical course of lobular cancer in breast cancer (BC) [abstract]. In: Proceedings of the 2022 San Antonio Breast Cancer Symposium; 2022 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2023;83(5 Suppl):Abstract nr P3-05-38.