Clinical and Demographic Factors, Treatment Patterns, and Overall Survival Associated With Rare Triple-Negative Breast Carcinomas in the US

IMPORTANCE: Triple-negative breast cancers are known collectively to demonstrate a more aggressive clinical course and earlier recurrence than cancers of other histological subtypes. However, the literature on rare triple-negative breast cancers and the association of histological type with survival and risk of metastasis is sparse. OBJECTIVE: To present the clinical and demographic characteristics, treatment patterns, and overall survival (OS) for histologically rare (<10% of breast cancers) triple-negative breast cancer types: medullary carcinoma, adenoid cystic carcinoma, and metaplastic breast carcinoma. DESIGN, SETTING, AND PARTICIPANTS: This cohort study was performed in the US using data reported by the National Cancer Database between 2010 and 2016. Confirmed cases of medullary carcinoma, adenoid cystic carcinoma, and metaplastic breast cancer were analyzed. Univariable analyses and multivariable Cox regression models were performed. Data analysis was performed from April to May 2020. MAIN OUTCOMES AND MEASURES: The primary outcome was 5-year OS. Secondary outcomes included site of metastasis, effect of immunohistochemistry, management, and 2-year mortality. RESULTS: A total of 8479 patients with breast cancer (mean [SD] age; 62.6 [14.3] years; 8435 women [99.48%]) were analyzed. Metaplastic carcinoma was the most commonly diagnosed histological type in this cohort, with 6867 patients (81%), followed by 1357 (16%) with adenoid cystic carcinoma and only 255 (3%) with medullary carcinoma. Medullary carcinoma presented earlier in life, at a median (interquartile range) age of 53 (45-62) years, compared with 62 (53-72) years for patients with adenoid cystic carcinoma and 63 (52-74) years for patients with metaplastic carcinoma. The proportion of tumors with triple-negative immunohistochemistry varied by histological type for medullary carcinoma (57 patients [22.4%]), adenoid cystic carcinoma (653 patients [48.1%]), and metaplastic carcinoma (3637 patients [53.0%]). Patients with adenoid cystic carcinoma were less likely to receive radiotherapy (711 patients [52.4%]) and chemotherapy (175 patients [12.9%]) compared with patients with medullary carcinoma (radiotherapy, 156 patients [61.2%]; chemotherapy, 190 patients [74.5%]) and metaplastic carcinoma (radiotherapy, 3416 patients [49.7%]; chemotherapy, 4709 patients [68.6%]). The 5-year OS rate was superior for patients with medullary (91.7%) and adenoid cystic carcinoma (88.4%) compared with patients with metaplastic carcinoma (63.1%). The 5-year mortality rate for adenoid cystic carcinoma was 8.33% vs 36.91% for metaplastic carcinoma. CONCLUSIONS AND RELEVANCE: Nationally, over the course of 7 years, medullary carcinoma was most common and metaplastic carcinoma had the worst 5-year OS among the rare histological breast cancer subtypes analyzed. Factors associated with a poor prognosis for metaplastic carcinoma included advanced stage, lung metastasis, older age, and not receiving chemotherapy or radiation therapy. Future research focusing on rare subtypes of breast cancer is desirable and could inform the optimal management of these relatively understudied carcinomas.