Pseudomyxoma Peritonei as a First Manifestation of KRAS-Mutated Urachal Mucinous Cystadenocarcinoma of the Bladder: A Case Report

: Only 28 cases of pseudomyxoma peritonei (PMP) arising from urachal neoplasms have been reported. We report one example of this extremely rare disease with KRAS mutational status in its spectrum of pathology. A 45-year-old woman presented with urachal frankly invasive mucinous cystadenocarcinoma confined to the dome of the bladder, which clinically manifested as PMP and was not detected at the first surgery. The primary tumour was revealed 6 months later because of its recurrence as PMP. Microscopic investigation revealed tubular adenoma and cystadenocarcinoma communicating with the bladder lumen and transitioning from the urachal urothelium to the mucinous epithelium. A urachal remnant was identified near the neoplasm. On immunohistochemistry, the tumour proved positive for CK7, CK20, CEA, and CDX2. Staining for β-catenin revealed expression in both the cytoplasm and cell membrane. Mismatch repair protein expression was normal. Somatic KRAS-mutation (G12V) was revealed in tubular adenoma, cystadenocarcinoma, and mucinous carcinoma peritonei and may play an oncogenic role in the malignant transformation of urachal mucosa and the development of PMP.