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 0.001). We also analyzed 17,882 patients from the long-term cohort with available OncotypeDx results. Among patients with high OncotypeDx recurrence score, the percentage of patients with low, intermediate, and high CTS5 risk-group were 47.3%, 41.3%, 11.4% respectively, whereas the percentages were 50.9%, 38.6%, and 10.5% for those with low OncotypeDx recurrence score. We found that CTS5 provided significant prognostic information for long-term OS across all Oncotype Dx risks group. Patients with high CTS5 risk-group had worse long-term OS when compared to those with low CTS5 risk-group in patients with low (HR 4.6, 95% CI 2.90-7.30; p 0.001), intermediate (HR 4.33, 95% CI 3.43-5.46; p 0.001), and high (HR 3.99, 95% CI 2.19-7.28; p 0.001) OncotypeDx recurrence score. A multivariable model with CTS5 in addition to OncotypeDx recurrence score showed significant association with long-term OS (Likelihood ratio test, p 0.001). Conclusion: CTS5 is an independent predictor of long-term OS in patients with ILC and it can identify patients with substantially higher risk of late death beyond 5 years. CTS5 adds independent prognostic information that persists even after adjusting for genomic factors such as OncotypeDx recurrence score. Combining CTS5 score and OncotypeDx recurrence score into clinical practice may provide better information for treatment decisions such as extending endocrine therapy or intensifying surveillance in this population. Citation Format: J. Avila, X. Xue, A. Gyamfi, J. D. Anampa. Prognostic Effect of Clinical Treatment Score Post-5 years (CTS5) in patients with Invasive Lobular Carcinoma and its relationship with Oncotype Dx abstract. In: Proceedings of the San Antonio Breast Cancer Symposium 2025; 2025 Dec 9-12; San Antonio, TX. Philadelphia (PA): AACR; Clin Cancer Res 2026;32(4 Suppl):Abstract nr PS1-05-19.