Short-term and long-term survival outcomes in patients with invasive lobular carcinoma vs. invasive ductal carcinoma of the breast.

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e12564 Background: Invasive lobular carcinoma (ILC) accounts for 10% of all invasive breast cancer and has distinct histological and biological features compared to the most common breast cancer histological subtype, invasive ductal carcinoma (IDC). ILC is more endocrine sensitive and chemotherapy resistant than IDC and has a different metastatic pattern. There is lack of conclusive data on the differences in survival outcomes between ILC and IDC. We aimed to assess the short-term and long-term survival outcomes in patients with ILC and IDC in a large population database. Methods: This is a retrospective, population-based study using data from the Surveillance, Epidemiology, and End Results database. We included women > 18 years with stage I-III ILC or IDC diagnosed from 1992-2020. Multivariable logistic regression was used to assess factors associated with different treatment modalities in patients with IDC vs. ILC. We estimated breast cancer-specific survival (BCSS) using Kaplan-Meier methods and their survival rates were compared using the log-rank test. We analyzed BCSS at different time points using a cox regression model with time-varying coefficients. Results: 343,397 patients with IDC and 39,859 patients with ILC were included in this study. Patients with ILC were older (63.0 ± 12.5 years vs. 59.2 ± 13.5 years, p<0.001). There were more non-Hispanic white patients with ILC vs. IDC (76% vs. 67%, p<0.001). Patients with ILC had more advanced disease: more stage III disease (16% vs. 11%, p<0.001), more T3-T4 disease (15% vs. 6%, p<0.001), and more N2-N3 disease (12% vs. 9%, p<0.001). Patients with ILC also had a higher frequency of hormone receptor positive disease (97% vs. 81%, p<0.001). Fewer patients with ILC had grade 3-4 tumors (9% vs. 36%, p<0.001). Our multivariate analysis showed that compared to patients with IDC, patients with ILC were more likely to undergo mastectomy (OR=1.85, p < 0.001) and radiation (OR=1.20, p<0.001), but less likely to receive chemotherapy (OR= 0.92, p<0.001). After adjusting for clinicopathological features, treatment modalities, and socioeconomic variables; compared to patients with IDC, patients with ILC had better BCSS in the first five years (HR=0.71, p <0.001), but worse BCSS in later years (HR=1.30, p<0.001 in year 6-10; HR=1.75, p<0.001 in year 11-15; HR=2.17, p<0.001 in year 16-20). Annual hazard models show that IDC peaked at recurrence in the first 5 years after diagnoses, and the hazard rate declines gradually after that. ILC peaked at recurrence in the first 5 years after diagnosis with minimal decrease in hazard rates after that. Conclusions: BCSS of patients with ILC is better in early years but worse in later years compared to patients with IDC. Future studies are needed to identify predictors of late relapses in ILC, and novel therapeutic strategies are required for these patients.