Answer: C

Histological Description: The lesion is fairly well-circumscribed. There is predominantly an admixture of compressed elongated tubules and cords of cells in either a myxoid or a scant collagenous stroma. The cells have light eosinophilic cytoplasm. Cytologically, the epithelial cells are bland with small ovoid-round nuclei and inconspicuous nucleoli. Mitotic activity is low. There is a variable amount of stromal and intratubular mucin present.

Discussion: Mucinous tubular spindle cell carcinoma (MTSC) is a well-established relatively newer described renal cell carcinoma variant. It predominantly occurs in females and has an excellent prognosis with only rare cases having metastases. Cases of MTSC can uncommonly be high grade with either nucleolar grade 3 morphology or with extensive necrosis, where the risk of metastatic behavior is increased. Although the classic case of MTSC, has all three components (mucinous, tubular, spindled), the presence of the elements may vary from case to case, and in some cases only two of the three features are present. In this case, the key to the correct diagnosis is recognizing the lightly eosinophilic secretions that are either between or in the tubules, as this is not seen in other renal cell carcinomas. The spindle cell component in this case is not well-developed. In cases with a more prominent spindle cell component, it is important not to misdiagnose MTSC as sarcomatoid renal cell carcinoma. The low grade cytology and other features of MTSC are key in this regard. Immunohistochemical stains are not helpful in establishing the diagnosis. Tubulocystic carcinoma contains much more dilated cysts with a “Swiss cheese” appearance at low power and have grade 3 nucleoli. Medullary renal cell carcinoma and collecting duct carcinoma share with MTSC the presence of tubules, but have more cytological atypia, a prominent desmoplastic reaction, and a very infiltrative interface with the normal kidney.