Answer: Nephrogenic adenoma and carcinoma in situ

Histology: Focally the surface epithelium shows hyperchromatic, enlarged nuclei compared to the underlying von Brunn nest. This area is diagnostic of CIS. Elsewhere, the surface epithelium is papillary lined by cuboidal epithelium. Within the underlying lamina propria there are also atrophic tubules predominantly lined by cuboidal epithelium. Others are also atrophic yet lined by hobnail cell with eosinophilic cytoplasm. One of these tubules contains a nest of cells with hyperchromatic enlarged nuclei.

Discussion: Nephrogenic adenomas may have multiple patterns. Three of the classic patterns are seen in the current case. One of them is a papillary lesion lined by cuboidal epithelium. The second are atrophic tubules lined by flattened cuboidal epithelium resembling renal tubules. The third are vascular-like structures lined by hobnail cells. Nephrogenic adenomas may have cytologic atypia. However, the atypia tends to be in the hobnail cells. In the current case, the solid nest of cells with hyperchromatic enlarged nuclei is consistent with CIS extending down into nephrogenic adenoma. I have seen one other similar such case. If one has a question whether a lesion is nephrogenic adenoma or carcinoma or as in this case whether CIS is involving nephrogenic adenoma one can also utilize stains for Ki67 and p53. We have demonstrated that nephrogenic adenomas have extremely low Ki67 proliferation rates of approximately 1%. p53 staining tends to be negative. In contrast, CIS would show marked elevation of both Ki67 and p53 expression.