Answer: Xp11 translocation renal cell carcinoma

Histology: This is a high grade malignant neoplasm with clear cell morphology involving the kidney. In a patient of this age, the default diagnosis is that of a clear cell renal cell carcinoma. One clue regarding the true nature of this lesion is the focal area of biphasic cytology featuring larger and smaller cells, a typical finding in Xp11 translocation renal cell carcinomas. Unlike the expected results for a clear cell carcinoma, this case was negative for carbonic anhydrase-IX and labeled diffusely for cathepsin k. FISH demonstrated a TFE3 gene rearrangement, supporting the diagnosis of Xp11 translocation renal cell carcinoma.

Discussion: Clear cell carcinomas should label diffusely for carbonic anhydrase-IX, particularly when they are low grade. The do not label for cathepsin k. Chromophobe cell carcinomas have a more plant like cytologic appearance, with raisinoid nuclei and discrete cell borders. They do not label for cathepsin k. Adrenal cortical neoplasms may present as renal/perirenal masses, but typically have more bubbly cytoplasm and do not label for cathepsin k. One pitfall is that adrenal cortical neoplasms and Xp11 translocation renal cell carcinomas both may label for the melanocytic marker Melan A. Adrenal cortical neoplasms typically do not label for renal tubular markers such as PAX8, and do not demonstrate TFE3 gene rearrangements.