Clinicopathological and molecular characteristics of mammary adenoid cystic carcinoma with adipocytic differentiation with emphasis on the identification of a novel BRAF mutation

Background/Aim: Adenoid cystic carcinoma accounts for fewer than 1% of all breast malignancies. Herein, we describe the clinicopathological characteristics and immunophenotype of mammary adenoid cystic carcinoma that exhibited adipocytic differentiation. Furthermore, we aimed to identify mutations potentially associated with this rare tumor. Case Report: A 52-year-old woman was referred to our Institution after detection of an incidental breast mass. The resected breast specimen contained a well-circumscribed, firm mass surrounded by ill-defined, soft, yellow bulging nodules and measured 2.5 cm in the greatest dimension. The surgically-resected breast lesion was subjected to pathological examination and immunohistochemical staining. Next-generation sequencing was also carried out on an Ion Torrent Personal Genome Machine, using Ion AmpliSeq Cancer Hotspot Panel v2. Histologically, the tumor had two components: a central area of complex glandular proliferation with solid and cribriform architectures and a peripheral area showing adipocytic differentiation with spindle cell proliferation. Immunohistochemical staining revealed positive immunoreactivity for cytokeratin 5/6, epidermal growth factor receptor, and smooth muscle actin in both the central and peripheral areas. Next-generation sequencing-based genetic analysis identified a homozygous missense mutation (NM_004333.4:c.1742A>G; p.N581S) in the v-Raf murine sarcoma viral oncogene homolog B (BRAF) gene. Conclusion: We report the first case, as far as we are aware, of primary mammary adenoid cystic carcinoma with adipocytic differentiation. A novel missense mutation in BRAF was detected. The potential response to BRAF inhibitors in the presence of this mutation remains a matter for further studies.