Post-Mastectomy Radiation Therapy for Invasive Lobular Carcinoma: A Comparative Utilization and Outcomes Study

We used the Surveillance, Epidemiology, and End Results database to compare histology-specific utilization of post-mastectomy radiation therapy (PMRT) by tumor and patient characteristics. Among patients with a definite indication for PMRT, radiation improved 5-year breast cancer-specific survival from 71.4% to 77.0% for invasive ductal carcinoma (P < .0001) and from 80.9% to 84.7% for invasive lobular carcinoma (P = .0003). Moreover, PMRT appears to be significantly underutilized for both subtypes. Background: To date, there have been no analyses to assess factors that influence post-mastectomy radiation therapy (PMRT) utilization in invasive lobular carcinoma (ILC) or to quantify the benefit of PMRT in ILC as compared with invasive ductal carcinoma (IDC). We compared histology-specific utilization of PMRT by tumor and patient characteristics and estimated the effect of PMRT on overall and breast cancer-specific survival in ILC and IDC patients meeting American College of Radiology (ACR) criteria for PMRT. Patients and Methods: We used the Surveillance, Epidemiology, and End Results database to identify women diagnosed with ILC or IDC from 2004 to 2009 who underwent mastectomy. We assessed utilization of PMRT by T and N stage, analyzed factors associated with PMRT use, and quantified the histology-specific survival benefit of PMRT using log-rank tests and multivariate Cox regression analysis. Results: We identified 86,098 IDC and 12,703 ILC patients. Within this cohort, 18.7% of IDC patients and 26.1% of ILC met ACR criteria for PMRT. Among patients with a definite indication, PMRT was more commonly employed in ILC than in IDC (59.6% vs. 56.3%; P = .0004). Among patients with a definite indication for PMRT, radiation improved 5-year breast cancer-specific survival from 71.4% to 77.0% for IDC (P < .0001) and from 80.9% to 84.7% for ILC (P = .0003). Conclusions: PMRT was used more commonly in ILC than in IDC. PMRT significantly improves 5-year overall survival and breast cancer-specific survival for ILC patients to a degree comparable with that seen in IDC. Moreover, among ILC and IDC patients who meet ACR criteria, PMRT appears to be significantly underutilized.