Answer: B

Histological Description: There is a well circumscribed tumor that is not encapsulated. The tumor is composed of small tubules and nests with focal papillary formations separated by broad bands of fibromuscular stroma. Cytologically, the nuclei are bland with small nucleoli and there is voluminous clear cytoplasm.

Discussion: Recent studies have demonstrated that renal cell carcinoma (RCC) with fibromyomatous stroma (FMS) represents a distinct entity that can be separated from some other RCCs that exhibit clear cells, as well as tubulopapillary morphology and smooth muscle/fibromatous stroma such as clear cell papillary renal cell tumor (former clear cell papillary RCC). Clear cell papillary renal cell tumor would have blunt ended tubules with not as abundant cytoplasm and areas with subnuclear vacuoles and smaller round nuclei. I sign out these tumors as in the answer B above and then in a note say that the following entities can have this appearance, including clear cell RCC, RCC with TCEB1 mutations, RCC with TSC mutations (either sporadic or hereditary). Without molecular studies, the distinction cannot be made and from a clinical standpoint, the tumors are managed the same and if the clinicians want molecular studies they can request it. Diffuse CK7 positivity is typical and is generally required for the diagnosis; reactivity for CAIX and CD10 are also usually present, with overlap amongst the entities in the differential diagnosis. Based on the relatively limited follow-up, most RCC FMS have demonstrated an indolent biological behavior; however, lymph node metastases have been reported in rare cases associated with TSC.