Answer: Nephrogenic adenoma

Histology: There are two pieces of tissue removed. One piece consists of tubules lined by cuboidal flattened epithelium with dense pink secretions resembling thyroid follicles. Many of the follicles have associated intraluminal acute inflammation. The other piece of tissue shows a mixture of small tubules and structures resembling signet ring cells containing blue tinged mucinous secretions. Some of the small tubules and signet ring-like structures have a hyaline rim of collagen surrounding them. Both pieces of tissue on the surface are lined by flat cuboidal epithelium.

Discussion: This is a classic case of nephrogenic adenoma. Nephrogenic adenoma may have multiple histological patterns. Some of them, like the signet ring-like cell pattern, may be confused with adenocarcinoma, yet other patterns will typically be present, providing a clue to the correct diagnosis. In this case the thyroid-like tubules are particularly striking and is a characteristic feature of nephrogenic adenoma not seen with any of its mimickers. The signet-ring-like structures represent small tubules lined by flattened epithelium which depending on the plane of section may show only one nucleus resembling a signet ring cell carcinoma. Distinguishing it from signet ring cell carcinoma requires finding of slightly larger similar tubules with more than one nucleus, the hyaline rim of connective tissue surrounding some of these structures, and the other more characteristic features of nephrogenic adenoma seen elsewhere in the lesion. Features that should not be seen in nephrogenic adenoma include mitotic figures or necrosis. While I have seen rare cases with very focal clear cells and small solid nest of cells these are distinctly uncommon and more typically seen in malignant tumors resembling nephrogenic adenoma such as clear cell mesonephric