Mucinous tubular and spindle cell carcinoma of the kidney with prominent papillary component, a non-classic morphologic variant. A histologic, immunohistochemical, electron microscopic and fluorescence in situ hybridization study.

Mucinous tubular and spindle cell carcinoma (MTSCC) is a rare type of kidney tumor with relatively indolent behavior. Non-classic morphological variants have not been well studied and rarely been reported. We report a challenging case MTSCC with a peculiar morphology in a 42-year-old man, arising in a background of end-stage renal disease (ESRD). Predominant areas with extensive papillary architecture, psammoma bodies and stromal macrophageal aggregates, reminiscent of a papillary renal cell carcinoma (papillary RCC), were intermixed with foci that transitioned into a MTSCC-like morphology exhibiting elongated tubules and a low grade spindle cell component in a background of mucinous stroma. Immunohistochemistry demonstrated diffuse positivity for P504s/AMACR and vimentin in tumor cells. Focal positivity for RCC, CD10 and CK7 was also noted. Kidney-specific cadherin, cytokeratin 34betaE12 and TFE3 stains were negative in the tumor. The major differential diagnostic considerations were papillary RCC, clear cell papillary RCC, and Xp11.2 translocation carcinoma. Negative FISH studies for trisomy 7 and 17 in both papillary and spindled components supported the diagnosis of MTSCC. The ultrastructural profile was not entirely indicative of the cellular origin of the tumor. Cytogenetic analysis should be performed in atypical cases of MTSCC for precise diagnosis.