Characteristics of Radiation Therapy for Medullary Carcinoma of the Breast in the National Cancer Database

Abstract

Medullary breast carcinoma (MedBC) is an uncommon histology of invasive breast cancer that has a more favorable prognosis compared to invasive ductal carcinoma (IDC). There are no specific consensus guidelines for radiotherapy (RT) for MedBC. We aim to determine whether there are differences in the utilization of RT using the National Cancer Data Base (NCDB), a national oncology outcomes database. Patients with MedBC and IDC reported to the NCDB from 2004 to 2014 who had undergone either lumpectomy or mastectomy followed by adjuvant radiotherapy were analyzed for patient and treatment characteristics. Characteristics analyzed between the two histologies included age, race, primary insurance payer, Charles-Deyo score, grade, NCDB analytic stage, regional radiation dose, boost radiation dose, and types of surgery performed. Regional and boost dose for grades of MedBC were evaluated. A total of 1862 patients with MedBC and 589,358 patients with IDC were included in the analysis. Median follow-up was 59.6 months. MedBC patients were more likely to be black (26.4% vs. 11.5%, p< 0.001). There was a higher proportion of Stage II patients with MedBC (47% vs. 29.4% p < 0.001) and less Stage III (6.0% vs. 11.8% p< 0.001). MedBC patients were more likely to have triple negative receptor status compared to IDC (60.7% vs. 13.9% p< 0.001). MedBC had lower grade disease, 52.4%, 34.4% and 13.1% (well, moderately, poorly differentiated) compared to IDC - 20.6%, 41.8%, and 37.6 respectively (p<0.001). There was no significant difference in mean regional dose - 4865.6 and 4893 cGy for MedBC and IDC respectively and a small difference in boost dose, 1061.5 cGy for MedBC and 956.5 cGy for IDC (p<0.001). A greater proportion of patients with IDC received no boost dose compared to MedBC – 20.1% vs. 14.2% (p<0.001). There was no significant difference in the mean regional dose for well, moderate and poorly differentiated grade MedBC (4842.3 Gy, 4913.4 cGy, and 4894.9 cGy, p = 0.701) or boost dose (1165.9 cGy, 1036.0 cGy, and 1062.5 cGy, p = 0.592). There was a significant difference in 5 and 10 year survival, 83.7% and 77.1% for IDC compared to 89.1% and 84.4% for MedBC, respectively, (log-rank p = 0.012). On univariate and multivariate analysis, neither regional nor boost dose was associated with increased HR. On UVA, poorly differentiated grade had an associated HR=2.78 (p=0.035), with a trend on MVA (HR=2.62, p=0.071). The use of radiotherapy for MedBC does not appear to differ greatly for IDC with no difference in regional doses. A small difference in boost dose may be explained by a lower proportion of IDC patients receiving boost dose as compared to MedBC. The grade of disease does not appear to effect radiation dosing, and dose of therapy was not associated with survival. The improved outcomes of MedBC compared to IDC is likely related to disease biology and other factors.