Answer: Mucinous tubular spindle cell carcinoma

Histology: The tumor consists of compact tubules with focal more solid areas. The tubules are lined by relatively bland cells with clear vacuolated cytoplasm and uniform nuclei with small nucleoli. Focally, there is subtle spindling of the cells. There is bubbly lightly basophilic secretions both within and in between the tubules.

Discussion: The lesion has the typical appearance of a mucinous tubular spindle cell carcinoma (MTSC). The key finding is the presence of elongated tubules, a feature not seen in other RCCs. In some cases, tubular, spindled, or mucinous features may be minor. The mucin can be highlighted by alcian blue stains. Occasionally, a pseudopapillary appearance may result from separation of epithelial cells from surrounding myxoid stroma. Bona fide papillary structures with fibrovascular cores are not seen. Variable presence of stromal foamy histiocytes can be seen, a nonspecific feature typically seen with papillary RCC. Other features seen in papillary RCC, such as hemosiderin deposition in epithelium and psammomatous calcifications are absent. Immunohistochemistry is not helpful. The prognosis is excellent with only rare cases with metastases.