Abstract P5-12-01: Prognostic implications of tumor size and nodal involvement in invasive lobular carcinoma of breast

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Abstract Background: Invasive lobular carcinoma (ILC) represents the second most prevalent histological subtype of breast cancer, comprising 3-15% of all diagnosed cases. Despite its unique clinical and pathological profile when compared to invasive ductal carcinoma (IDC), there remains significant debate regarding the prognosis and optimal management strategies for ILC. This study aimed to investigate the differences in clinical and pathological features, survival between ILC and IDC and their impact on prognosis. Methods: We reviewed the data of the 15,489 IDC patients and 966 ILC patients who were treated between January 2005 and December 2022 at Seoul National University Hospital. The clinical and pathological characteristics of ILC and IDC were compared, and survival outcomes were analyzed using cox proportional hazard models. We conducted a parallel analysis using the Surveillance, Epidemiology, and End Results (SEER) database. We extracted data from 247,290 breast cancer patients diagnosed between 2010 and 2015, comprising 219,404 cases of IDC and 27,886 cases of ILC. Result: ILC had a larger tumor size, higher proportion of HR+/HER2- subtype, lower tumor grade, and lower Ki-67 index compared to IDC. Kaplan-Meier survival analysis with log-rank tests showed no statistically significant differences in breast cancer-specific survival (BCSS) or overall survival (OS) between patients with ILC and IDC (p = 0.52, 0.27, respectively). After adjusting for histologic grade and subtype, we found distinct patterns in survival determinants between ILC and IDC. In IDC, both tumor size (T stage) and nodal status (N stage) showed consistent prognostic significance. Compared to T1, T2 and T3 stages had 2.5-fold (95% CI: 2.00-3.00) and 4.4-fold (95% CI: 3.40-5.72) higher breast cancer-specific death risks, respectively. N1, N2, and N3 stages showed 2.5-fold (95% CI: 2.08-3.06), 3.8-fold (95% CI: 3.06-4.74), and 6.0-fold (95% CI: 4.66-7.67) increased risks compared to N0. Interestingly, in ILC, tumor size did not significantly impact survival (p=0.38 for T2, p=0.12 for T3 vs T1). However, advanced nodal involvement in ILC showed markedly higher risk, with N2 and N3 stages having 5.2-fold (95% CI: 1.66-16.57) and 26.1-fold (95% CI: 10.54-64.69) increased risks, respectively. SEER data analysis corroborated these findings. For IDC, T2 and T3 stages showed 2.2-fold (95% CI: 2.14-2.29) and 3.4-fold (95% CI: 3.20-3.51) increased risks, while N1, N2, and N3 stages had 1.7-fold, 2.8-fold, and 4.2-fold higher risks, respectively. In ILC, the impact of tumor size was less pronounced, with T2 and T3 stages showing 1.9-fold (95% CI: 1.69-2.04) and 2.43-fold (95% CI: 2.17-2.71) increased risks. However, nodal involvement in ILC again showed higher risk, with N2 and N3 stages having 3.5-fold (95% CI: 1.66-16.57) and 6.18-fold (95% CI: 5.53-6.90) increased risks, compared to 2.83-fold and 4.17-fold in IDC for N2 and N3, respectively. Conclusion: In conclusion, our study highlights that ILC exhibits distinct characteristics and prognostic patterns compared to IDC. Although ILC is often detected at a larger size, the impact of tumor size on survival differs from that of IDC. In ILC, tumor size alone may not be as strong a predictor of survival as it is in IDC. Conversely, nodal involvement in ILC, especially at advanced stages, carries a particularly high risk for breast cancer-specific mortality. These findings underscore the need for tailored prognostic assessments and potentially different treatment strategies for ILC and IDC. Citation Format: Eunhye Kang, Ik Beom Shin, Hyerim Kang, Changhoon Lee, Min Jung Lee, Jinyoung Byeon, Ji-Jung Jung, Hong-Kyu Kim, Han-Byoel Lee, Wonshik Han, Hyeong-Gon Moon. Prognostic implications of tumor size and nodal involvement in invasive lobular carcinoma of breast [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 P5-12-01.

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