Abstract PS1-07-19: Comparing Utilization and Survival Outcomes of Post-mastectomy Radiation Therapy in Invasive Lobular versus Ductal Carcinoma: A National Cancer Database Analysis

Abstract Background: Post-mastectomy radiation therapy (PMRT) can improve locoregional control and overall survival in patients with invasive breast cancer, particularly those with high-risk features such as ≥4 positive lymph nodes or T3-T4 tumors. However, data on the utilization and effectiveness of PMRT across different histological subtypes remain limited. Invasive lobular carcinoma (ILC), the second most common histologic subtype of breast cancer, differs from invasive ductal carcinoma (IDC) in its features and clinical outcomes, with more hormone receptor positive status and lower grade, but typically higher stage at presentation, higher risk of positive surgical margins, and propensity for late recurrences. Given the unique features of ILC, including a characteristic diffuse growth pattern, we hypothesized that more patients with ILC would receive PMRT, and, that the use of PMRT might have relatively greater survival benefit in patients with ILC than those with IDC. Methods: We used the National Cancer Database (NCDB) to identify patients diagnosed with pathologic stage I-III breast cancer between 2010 and 2020 who underwent mastectomy, with or without adjuvant radiation therapy. Patients who received neoadjuvant radiotherapy or intraoperative radiotherapy were excluded. We considered the following criteria to be an indication for PMRT: T3 tumor with age