Abstract ED02-01: Biological peculiarities of ILC

Abstract One in 8 women will be diagnosed with breast cancer in her lifetime and approximately 15% of them have invasive lobular breast cancer (ILC). ILC differs in many ways from the most common BC subtype, called non-special type (NST) (Van Baelen et al. Ann Oncol 2022). The microscopic hallmark of ILC is the presence of single files of relatively small discohesive tumor cells, which stems from a dysfunctional E-cadherin-catenins complex at the intercellular junctions, a feature that is uncommon in NST. More than 90% of ILC express the estrogen (ER) and/or progesterone receptor (PR) and lack HER2 amplification, while this is only the case in ~65% of NST. Patients with ILC also show a distinct clinical behavior as compared to NST: generally, a poor response to chemotherapy, more late recurrences, and a peculiar metastatic pattern. The first part of the presentation will focus on presenting the biological knowledge we have accumulated over the years about primary ILC. We will review the studies which compared ILC and NST at the genomic, transcriptomic and immune level. The second part of the presentation will be focused on metastatic breast cancer. Here we will review the published evidence regarding the metastatic dissemination of ILC, its genomic landscape and the contribution of the local tumor microenvironment. We will also dedicate attention to the important biological and clinical knowledge which can be gained by studying samples collected from patients with metastatic ILC who consented to post-mortem tissue donation. Finally, the last part of the presentation will be dedicated to mixed NST/ILC tumors. These tumors account for up to 18% of all invasive breast cancers and represent an even more under investigated area of research as pure ILC. Citation Format: C. Desmedt. Biological peculiarities of ILC abstract. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr ED02-01.