Rare creatures from the black lagoon: Keratinising squamous cells in a malignant peritoneal effusion

Abstract

Malignant squamous cells in serous effusions are exceedingly rare, with most cases limited to case reports and small case series. Malignant urothelial carcinomas in serous effusions are also rare, and most reports are associated with widespread metastasis to other organs. We present a case of metastatic urothelial carcinoma with prominent squamous differentiation, which presented initially as malignant ascites. The 81-year-old female patient had a 7-year history of recurrent urinary tract infections and presented for investigation of abdominal distension. Abdominal CT reported ascites with omental caking and a heavily calcified bladder. On peritoneocentesis, 1800 mL of cloudy yellow fluid was aspirated. Cytology of the effusion revealed keratinising squamous cells and bizarre cell forms, which were CK5/6 and p40 positive on immunohistochemistry of cell blocks. Subsequently, a cystoscopy was performed which revealed a heavily necrotic and calcified bladder wall. A TURBT showed invasive tumour comprising of nests of markedly pleomorphic cells with squamous differentiation and keratin pearl formation. A diagnosis of high grade urothelial carcinoma with prominent squamous differentiation was made.