Answer: D

Histological Description: There is an admixture of compressed elongated tubular and focal spindle cells with light eosinophilic cytoplasm. The epithelial cells are bland with small ovoid-round nuclei and inconspicuous nucleoli. Mitotic activity is low. A variable amount of stromal bubbly eosinophilic mucin is present.

Discussion: The histological findings in this case are typical of a mucinous tubular spindle cell carcinoma. The proportions of the three elements can vary, such that some of these tumors may have only focal spindle cell or mucinous features. By definition, these are histologically low grade tumors. There are some tumors that resemble mucinous tubular spindle cell carcinoma yet have overt cytological atypia. There is some data that suggests that these higher grade lesions share molecular features with mucinous tubular spindle cell carcinoma. However, in order to keep “mucinous tubular spindle cell carcinoma” for an indolent neoplasm, higher grade similar lesions are designated as renal cell carcinoma with features of mucinous tubular spindle cell carcinoma yet with too much cytological atypia for that diagnosis. The prognosis of mucinous tubular spindle cell carcinoma is excellent with only rare cases with metastases.