Abstract P4-11-10: Long-term Outcome of Slowly Proliferating Luminal Type ILC versus IDC

Abstract Introduction: Invasive lobular carcinoma (ILC) ranks as the second most common histopathological subtype of breast cancer, following invasive ductal carcinoma (IDC). Prior studies have demonstrated that while ILC patients initially exhibit better survival rates within the first 5 years, their long-term outcomes are comparable to, or worse than, those of IDC patients. However, the lower prevalence of ILC relative to IDC and their distinct clinicopathological characteristics complicate direct comparisons between the two groups. This study aims to evaluate the long-term outcomes of luminal type A IDC versus ILC. Methods: This retrospective cohort study analyzed data from January 2008 to December 2015 at Samsung Medical Center. The study included patients with patients with primary breast cancer who underwent surgery and were histologically diagnosed with either IDC or ILC, were hormone receptor-positive, HER2-negative, and had a Ki-67 index ≤20. Patients with mixed histopathology, those undergoing palliative surgery, and those treated with neoadjuvant chemotherapy were excluded. Cohort analyses were performed in various subsets: real-world setting, patients receiving more than 2 years of endocrine therapy, and propensity score-matched cohorts. Results: In the real-world setting, of the 3439 patients, 3156 had IDC and 283 had ILC. With a median follow-up of 110 months (range 0-195 and 1-190 months, respectively), ILC patients exhibited worse long-term outcomes. Although breast cancer-specific survival (BCSS) did not show a significant difference (p=0.081), analysis by 5-year periods indicated significantly poorer survival for ILC patients beyond 5 years (5 to 10 years, p=0.004; 10 to 15 years, p=0.014). Disease-free survival (DFS) and distant metastasis-free survival (DMFS) were significantly worse in the ILC group (p<0.001 and p=0.005, respectively). In the second analysis, excluding patients with less than 2 years of endocrine therapy, 2919 IDC and 269 ILC patients remained. With median follow-ups of 112 months (range 17-195 months) and 110 months (range 19-190 months) respectively, the ILC group showed poorer long-term DFS and DMFS (p<0.001 and p=0.007, respectively). However, the ILC group exhibited a significantly higher incidence of synchronous contralateral breast cancer, pathologic T and N stages, and a lower nuclear grade compared to the IDC group (p<0.001, p<0.001, p=0.040, p<0.001, respectively). The third analysis used propensity score matching to control for confounding variables, including age, BMI, menopausal status, BRCA status, synchronous contralateral breast cancer, pathologic T and N stages, TNM stage, nuclear grade, histologic grade, adjuvant chemotherapy, and duration of endocrine therapy. In this analysis, long-term survival outcomes did not significantly differ between groups (BCSS p=0.248, DFS p=0.265, DMFS p=0.429). Conclusion: The study reveals that in the real-world setting, the long-term outcomes of ILC are worse compared to IDC. However, when adjusted for clinicopathological parameters, the long-term outcomes of ILC do not significantly differ from those of IDC. Citation Format: Ji Won Yoo, Jai Min Ryu, Se Kyung Lee, Byung Joo Chae, Jonghan Yu, Seok Won Kim, Seok Jin Nam, Jeong Eon Lee. Long-term Outcome of Slowly Proliferating Luminal Type ILC versus IDC [abstract]. In: Proceedings of the San Antonio Breast Cancer Symposium 2024; 2024 Dec 10-13; San Antonio, TX. Philadelphia (PA): AACR; Clin Cancer Res 2025;31(12 Suppl):Abstract nr P4-11-10.