Abstract Objective: A subset of invasive lobular carcinoma (ILC) is managed with neoadjuvant therapy (NAT). The clinical and pathologic features of ILC following NAT are not well characterized. The aim of this study is to characterize the pathologic response in patients with ILC treated with NAT, including neoadjuvant endocrine therapy (NET), neoadjuvant chemotherapy (NCT), and Herceptin-targeted NCT (H-NCT). Methods: We conducted a retrospective cohort study of 65 patients with ILC and 290 patients with invasive ductal carcinoma (IDC) who received neoadjuvant therapy followed by resection between 2019 and 2024. Clinicopathologic parameters were recorded, including age, tumor grade, histologic type, staging, ER, PR, and HER2 status. Residual cancer burden (RCB) was evaluated on the post NAT resection specimens through an online calculator (www. mdanderson. org/breastcancerRCB). Cases were categorized into three subtypes: ER+/HER2-, HER2+, and triple negative. Statistical analyses were performed to compare ILCs and IDCs with the same immunohistochemical (IHC) profiles. Results: Patients with ER+/HER2- and HER2+ ILCs were older at diagnosis compared to those with ER+/HER2- and HER2+ IDCs (p0. 01). Among patients with ILCs, 41 (63%) had ER+/HER2- tumor and received NET or NCT, 6 (9%) had triple negative tumor and received NCT only, and 18 (28%) had HER2+ tumor and received H-NCT. The pathologic complete response (pCR) rate was 44% in HER2+ ILCs and 60% in HER2+ IDCs. No pCR was achieved in triple negative and ER+/HER2- ILCs and IDCs. The majority of ER+/HER2- ILCs (87%) and all triple negative ILCs (100%) were classified as RCB II or RCB III, comparable to IDC cases with the same immunohistochemical subtypes (Table 1). Conclusion: ILCs demonstrated a similar response to neoadjuvant therapy as IDCs when matched by IHC subtype. These findings suggest that the effectiveness of neoadjuvant therapy is primarily determined by IHC-based tumor subtype rather than histologic subtype. Citation Format: T. Jordan, D. L. Rimm, H. Zhan. Pathologic response in invasive lobular carcinoma versus invasive ductal carcinoma following neoadjuvant therapy: A single institution study abstract. In: Proceedings of the San Antonio Breast Cancer Symposium 2025; 2025 Dec 9-12; San Antonio, TX. Philadelphia (PA): AACR; Clin Cancer Res 2026;32 (4 Suppl): Abstract nr PS2-05-17.
Jordan et al. (Tue,) studied this question.