Abstract PO5-02-13: Distinct differences in the prognostic implication of tumor size for invasive lobular carcinoma of breast when compared to invasive ductal carcinoma

Abstract Purpose Invasive lobular carcinoma (ILC) is the second common types of breast cancer and its clinicopathological feature and outcomes differ from invasive ductal carcinoma (IDC). However, in terms of survival outcomes, there are conflicting data on the prognostic implication of the ILC when compared to that of IDC. This study aims to evaluate the survival outcomes of patients diagnosed with ILC at single center and compared them with those of IDC. Methods We reviewed the data of the 13,034 IDC patients and 935 ILC patients who were treated between January 2005 and December 2022 at Seoul National University Hospital. We investigated the overall treatment outcomes of two histologic types with a special focus on the tumor size and lymph nodes. Result ILC exhibited distinct characteristics compared to IDC, including a larger tumor size (2.6 ± 1.8 vs. 1.9 ± 1.1 cm, p < 0.001) and a higher hormone receptor positivity (97.4% vs. 79.3%, p < 0.001). Additionally, the presence of lymphovascular emboli was lower in ILC (8.0% vs. 24.6%, p < 0.001). While IDC patients showed slightly better overall survival (OS) than ILC (10-year OS, ILC 89.9% vs. IDC 91.1%, the difference was not statistically significant (p = 0.85). However, distant metastasis-free survival (DMFS) was significantly higher in ILC (10-year DMFS, ILC 93.2% vs. IDC 89.9%, p = 0.001). Age at diagnosis, tumor size, presence of lymphovascular emboli, high expression of Ki-67, and higher pathologic N stage were poor prognostic markers for ILC. Multivariate analysis for DMFS showed that age, high expression of Ki-67 and higher pathologic N stage were poor prognostic markers for ILC. Tumor size was not an independent prognostic factor for DMFS (HR: 0.92, 95% CI: 0.72-1.17, p = 0.491) and for OS (HR: 1.18, 95% CI: 0.99-1.41, p = 0.060) in ILC. Conclusion Our data suggest that the tumor size, which is a well-known prognostic factor in IDC, carries distinctly different prognostic implication in ILC. These observation indicate that adjuvant treatment decisions based on tumor size can be tailored to different histologic subtypes of breast cancer. Citation Format: Ik Beom Shin, Eunhye Kang, Ji-Jung Jung, Hawjeong Lee, Jin Young Byeon, Yunhee Choi, Changjin Lim, Jong-Ho Cheun, Hong-Kyu Kim, Han-Byoel Lee, Wonshik Han, Hyeong-Gon Moon. Distinct differences in the prognostic implication of tumor size for invasive lobular carcinoma of breast when compared to invasive ductal carcinoma [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 PO5-02-13.