Abstract PS1-05-19: Prognostic Effect of Clinical Treatment Score Post-5 years (CTS5) in patients with Invasive Lobular Carcinoma and its relationship with Oncotype Dx

Abstract Background: While patients with early-stage invasive lobular carcinoma (ILC) of the breast often present better survival rates initially, the risk of recurrence and mortality increases later in the disease course. The Clinical Treatment Score Post-5 years (CTS5) is a validated tool used to estimate risk of late distant recurrence in patients with Hormone Receptor positive breast cancer after 5 years of adjuvant treatment; however, its application in patients with ILC has not been fully analyzed. OncotypeDx provides prognostic information in breast cancer, but there is limited data about OncotypeDx to predict long-term survival outcomes in ILC. We aimed to assess whether CTS can predict long-term OS for patients with ILC, and to evaluate its prognostic effect beyond OncotypeDx recurrence score. Methods: We conducted a retrospective analysis using the National Cancer Database (NCDB) from 2006-2020. Inclusion criteria were female gender, age ≥18 years, diagnosis of ILC, breast cancer stage I-III, removal of breast tumor and axillary lymph nodes, and alive status at least 5 years after diagnosis. Patients were classified into CTS5 low, intermediate and high-risk categories. Our primary endpoint, long-term overall survival (OS) included survival analysis after 5 years from diagnosis. We used the Pearson X2 test to evaluate the relationship between categorical variables and the t test for continuous variables. Cox proportional models were conducted to identify factors associated with long-term OS. Results: 67,927 patients were analyzed for our long-term survival analysis. Median age for our group was 63 years. Most of our patients were Non-Hispanic White (82%), with breast cancer stage I (50%), HR positive (ER+ [100%], PR+ [87%])/HER2 negative (73%) tumors. Patients were divided into 3 groups based on their CTS5 risk: low (n = 32,625 [48%]), intermediate (n = 21,007 [31%]) and high (n = 14,295 [21%]). 10-year OS rates differed significantly among these groups with OS rates of 91%, 84% and 64% respectively (p < 0.001 each). In multivariate Cox regression models adjusted for patient, tumor, and treatment characteristics, CTS5 remained an independent predictor of long-term OS. Patients with high CTS5 score had worse long-term OS when compared to those with low CTS5 (HR 3.63, 95% CI 3.42-3.86; p