Answer: Atypical renal cyst

Histology: The lesion consists of a cyst lined by cuboidal epithelium (image 1) with bland round nuclei and clear cytoplasm (image 2). There is a range from small papillary projections to small but more complex branching papillae (image 2). The nuclei in areas lines up in the middle of the cytoplasm or is inverted at the apical surface (image 3).

Discussion: The cytology and architecture, other than the cystic nature of the lesion, is typical of clear cell papillary renal cell carcinoma. Clear cell papillary renal cell carcinoma usually consists of well-formed tubules and papillary structures lined by very bland cuboidal epithelium with clear cytoplasm. A distinctive feature is that the nuclei are often in the middle of the cell or towards the apical surface. This case is unusual in that the lesion is cystic whereas most clear cell papillary renal cell carcinomas are solid. To date, clear cell papillary renal cell carcinomas have not been shown to metastasize. If with a greater number of cases studied with longer follow-up, these findings persist it may be that these tumors will be reclassified as clear cell papillary adenomas. These lesions are not merely a hybrid of clear cell renal cell carcinoma and papillary renal cell carcinoma, not a variant of either of these entities. Rather, it is a distinct lesion with unique molecular and immunohistochemical findings. Clear cell papillary renal cell carcinomas express CK7 (present in papillary RCC) and CAIX (present in clear cell RCC) with negative stains for AMACR (present in papillary RCC and negative for CD10 and RCC (present in clear cell RCC). They also lack loss of chromosome 3p and lack trisomy 7, 17, and loss of y seen in clear cell RCC and papillary RCC, respectively.

Reference(s):
– Am J Surg Pathol 32: 1239-45, 2008