Diagnosis: Mucinous Carcinoma
• Mucinous carcinomas of the breast are (1) distinct from grade and estrogen receptor–matched invasive ductal carcinomas of no special type (NST) at the genomic and transcriptomic levels; (2) display a relatively low level of genetic instability; and (iii) lack the concurrent loss of 16q and gain of 1q, a hallmark feature of grade I invasive ductal carcinoma NST and classic lobular carcinomas. These data suggest that mucinous carcinomas may evolve through genetic pathways distinct from pathways altered in tumors from the low-grade breast neoplasia family.
• Pure mucinous carcinomas of the breast are defined as tumors with greater than 90% of mucinous component that display bland cytologic features, with uniform neoplastic cells showing mild nuclear atypia and low mitotic activity, arranged in clusters floating in a large amount of mucin. By definition, most mucinous breast carcinomas are classified as low histologic grade.
• Mucinous carcinomas harbor a low level of genetic instability and rare recurrent amplifications and often display a simplex genomic pattern. A comprehensive loss of heterozygosity analysis of pure mucinous carcinomas revealed that these tumors harbor loss of heterozygosity significantly less frequently than invasive ductal carcinoma NST.
• Unsupervised clustering analysis revealed that mixed mucinous carcinomas preferentially cluster together with pure mucinous carcinomas and separately from invasive ductal carcinoma NST, providing strong circumstantial evidence to suggest that mixed mucinous carcinomas have genomic profiles more similar to profiles of pure mucinous carcinomas than invasive ductal carcinoma NST.
• Mucinous carcinomas are subdivided into two subtypes, mucinous A (hypocellular variant) and mucinous B (hypercellular variant). Mucinous B tumors often display features of neuroendocrine differentiation and transcriptomic profiles remarkably similar to neuroendocrine breast cancers.
