Urachal adenocarcinoma that metastasized to breast was misinterpreted as primary breast mucinous carcinoma: A rare case report and literature review

BACKGROUND: The urachus is a vestigial tubular structure that connects the urinary bladder to the allantois during early embryonic development. Urachal carcinoma develops in the urachus, which is an embryological remnant of the urogenital sinus and allantois. The estimated annual incidence of urachal carcinoma in the general population is 0.01% of all cancers in adults. Moreover, urachal carcinoma accounts for 0.34% to 0.7% of all bladder carcinoma cases. And breast metastasis is extremely rarer. METHODS AND RESULTS: A 42-year-old woman was admitted to our hospital with a palpable mass in the outer upper quadrant of the right breast, which was misinterpreted as a carcinoma that originated from the breast. Subsequently, she underwent surgery without any further meticulous examination. Immunohistochemistry analysis revealed positivity for CK20, Villin, and CDX-2 and negativity for CK7. After further inspection, a mass was found in the bladder dome using 18F-fluorodeoxyglucose positron emission tomography and computed tomography. The mass was surgically removed. CONCLUSION: Pathologic and immunohistochemical examination confirmed that the mass was urachal mucinous adenocarcinoma and mucinous adenocarcinoma to the right breast. The patient has been followed up without recurrence for 8 months.