Metastatic renal mucinous tubular and spindle cell carcinoma. Atypical behavior of a rare, morphologically bland tumor.

Mucinous tubular and spindle cell carcinoma (MTSCC) is a rare, distinctive renal neoplasm characterized by an admixture of cuboidal cells in tubules and sheets of spindle cells, typically with low-grade nuclei and a myxoid or mucinous background. It is characteristically of low malignant potential, and only rare metastatic cases have been reported. We describe a case in which the patient presented with extensive regional and distant metastases, but both primary and metastatic tumor showed the typical histomorphology of bland cuboidal or spindle cells lacking pleomorphism, mitoses, and necrosis. Almost all previous cases of metastatic MTSCCs have shown nuclear atypia or sarcomatoid morphology of the primary tumor; and metastatic renal MTSCC in which the primary neoplasm does not display atypical features is exceptional, serving to highlight that these rare tumors can behave aggressively even with "indolent" histological appearances.