Renal collision tumor composed of oncocytoma and mucinous tubular and spindle cell carcinoma: case report of an unprecedented entity.

Primary renal collision tumors composed of 2 histologically distinct components are rare with only isolated case reports in the medical literature. Oncocytoma is a benign renal neoplasm, which is thought to originate from distal tubular epithelial cells. Mucinous tubular and spindle cell carcinoma (MTSCC) of kidney is a rare, relatively recently described renal neoplasm that was first included in the 2004 World Health Organization renal cell carcinoma (RCC) classification as a distinct entity. Current studies suggest that the tumor originates from the proximal nephron, although it is still controversial. The presence of concurrent oncocytoma with conventional (clear cell) RCC, chromophobe RCC, and papillary RCC has been previously described. However, the association of oncocytoma with MTSCC has not yet been reported to our knowledge. Herein, we report the first case of a renal collision tumor composed of oncocytoma and MTSCC.