Answer: Clear cell hidradenoma

Histology: This is a nested and papillary neoplasm which is composed predominantly of bland epithelioid cells with clear cytoplasm. Focally, one can appreciate a second cell layer which resembles umbrella cells draped on the top of the clear cells. The lesion is associated with extensive cystic change, and essentially no mitoses or cytologic atypia is appreciated. This patient had no prior history of malignancy. The lesion was diffusely immunoreactive for p63 and p40. These results support the diagnosis clear cell hidradenoma.

Differential Diagnosis: Clear cell hidradenoma cytologically resembles benign urothelium. Metastatic urothelial carcinoma can be excluded by both history and the absence of cytologic atypia or mitotic activity in the current lesion. The areas of clear cells and papillary architecture raised the differential diagnosis of Xp11 translocation renal cell carcinoma, and the supraclavicular area would be a good site for metastasis from an abdominal primary. However, the urothelial like morphology and diffuse labeling for p40 and p63, along with the absence of a renal mass, argues against Xp11 translocation renal cell carcinoma. Thymoma may occasionally metastasize, and are characterized by bland neoplastic epithelial cells with often squamoid morphology, similar to what is seen in the current case. Thymomas also may be extensive cystic. However, thymoma would be associated immature lymphocytes which would label for TdT, and clinically would be associated with mediastinal mass.