Answer: B. Tubulocystic carcinoma

Histology: The lesion is well-circumscribed and unencapsulated without necrosis or hemorrhage. Microscopically, the tumor is composed of small to medium-sized cysts. A focal papillary component was present. Cytologically, there are large, round to polygonal cells with abundant eosinophilic cytoplasm. Cysts are lined by cells with hobnail appearance with vesicular nuclei and prominent nucleoli. Nuclei vary in shape, and some have irregular shapes.

Discussion: A variant of oncocytoma that can be confused with tubulocystic carcinoma is composed of tubules some with cystic dilatation. However, in addition, oncocytoma will typically also have some nests, which are lacking in tubulocystic carcinoma. In oncocytoma, one never see significant papillary component. Although both entities have dense Oncocytic cytoplasm, the nuclei in oncocytomas
are uniformly round with variably sized nucleoli. Cystic nephromas can have a similar architectural pattern although the cysts tend to be larger and lack the small cysts in tubulocystic carcinoma. Cystic nephromas are lined by cuboidal or hob nail epithelium and lack abundant oncocytic cytoplasm. ESC has more solid components and not just cysts lined by a single cell layer in tubulocystic carcinoma. Tubulocystic carcinomas are low grade with only uncommon metastases, despite having more prominent nucleoli, and should not be graded. In some tubulocystic RCCs there are poorly differentiated foci with solid area, areas resembling collecting duct carcinoma or prominent papillary carcinoma. In cases with poorly differentiated foci, the risk of metastases is significantly higher and a subset of these cases show low of fumarate hydratase (FH), typically sporadic unassociated with hereditary leiomyomatosis associated RCC (HLRCC).