Mixed Invasive Ductal and Lobular Carcinoma of the Breast: Prognosis and the Importance of Histologic Grade

AbstractBackground.The diagnosis of mixed invasive ductal and lobular carcinoma (IDC‐L) in clinical practice is often associated with uncertainty related to its prognosis and response to systemic therapies. With the increasing recognition of invasive lobular carcinoma (ILC) as a distinct disease subtype, questions surrounding IDC‐L become even more relevant. In this study, we took advantage of a detailed clinical database to compare IDC‐L and ILC regarding clinicopathologic and treatment characteristics, prognostic power of histologic grade, and survival outcomes.Materials and Methods.In this retrospective cohort study, we identified 811 patients diagnosed with early‐stage breast cancer with IDC‐L or ILC. Descriptive statistics were performed to compare baseline clinicopathologic characteristics and treatments. Survival rates were subsequently analyzed using the Kaplan–Meier method and compared using the Cox proportional hazards model.Results.Patients with ILC had more commonly multifocal disease, low to intermediate histologic grade, and HER2‐negative disease. Histologic grade was prognostic for patients with IDC‐L but had no significant discriminatory power in patients with ILC. Among postmenopausal women, those with IDC‐L had significantly better outcomes when compared with those with ILC: disease‐free survival (DFS) and overall survival (OS; adjusted hazard ratio [HR], 0.54; 95% confidence interval [CI] 0.31–0.95). Finally, postmenopausal women treated with an aromatase inhibitor had more favorable DFS and OS than those treated with tamoxifen only (OS adjusted HR, 0.50; 95% CI, 0.29–0.87), which was similar for both histologic types (p = .212).Conclusion.IDC‐L tumors have a better prognosis than ILC tumors, particularly among postmenopausal women. Histologic grade is an important prognostic factor in IDC‐L but not in ILC.Implications for Practice.This study compared mixed invasive ductal and lobular carcinoma (IDC‐L) with invasive lobular carcinomas (ILCs) to assess the overall prognosis, the prognostic role of histologic grade, and response to systemic therapy. It was found that patients with IDC‐L tumors have a better prognosis than ILC, particularly among postmenopausal women, which may impact follow‐up strategies. Moreover, although histologic grade failed to stratify the risk of ILC, it showed an important prognostic power in IDC‐L, thus highlighting its clinical utility to guide treatment decisions of IDC‐L. Finally, the disease‐free survival advantage of adjuvant aromatase inhibitors over tamoxifen in ILC was consistent in IDC‐L.

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