Cystic renal cell carcinoma: arose as a simple cyst, recurred repeatedly as a multicystic mass and finally presented as a solid mass.

An unusual case of cystic renal cell carcinoma in a 70-year-old Japanese male is reported. He had had a simple renal cyst removed 1 year ago. On presentation the right kidney was surrounded by multiple translucent cysts, which varied in size from 1 to 50 mm. The cyst walls were lined by single-layered cuboidal epithelium. The differential diagnosis included cystic mesothelioma, cystic lymphangioma and multicystic dysplastic kidney. An immunohistochemical and ultrastructural study and flow cytometric analysis of DNA ploidy were performed. The tumor was differentiated to such an extent that it was difficult to diagnose as carcinoma; however, it recurred repeatedly. Three and a half years after initial presentation the tumor had invaded the ileum with the pathological change to be almost solid when viewed grossly and, microscopically, showed tubulo-papillary structures in addition to a cystic pattern. The DNA ploidy pattern revealed a near-diploid aneuploid in the early specimen and a polyploid aneuploid in the last specimen.