Mucinous Cystadenocarcinoma of the Breast: A Clinicopathologic and Molecular Case Report With Literature Review.

Mucinous cystadenocarcinoma of the breast is rare and has been recently classified as a distinct entity in the fifth edition of the WHO Classification of Tumors. Here, we present a 47-year-old woman with a history of NST carcinoma in her right breast who subsequently developed a lump in the right chest wall close to the breast implant. Immunohistochemical analysis showed negativity for keratin 20 and CDX2, and positivity for mammaglobin, GCDFP15 and TRPS1 with a triple-negative phenotype. The diagnosis of primary mucinous cystadenocarcinoma of the breast was finally established. Next-generation sequencing identified likely pathogenetic mutations specifically in AKT1, EGFR, FANCA, BCL10, CTCF and RUNX1 and RP1-34H18.1::NAV3 gene fusion, making this as the first reported example in the literature harboring an RP1-34H18.1::NAV3 gene fusion identified within a comprehensive RNA-based molecular analysis, representing the 49th reported patient worldwide.