Update on selected renal cell tumors with clear cell features. With emphasis on multilocular cystic clear cell renal cell carcinoma

Clear cell renal cell carcinoma (CCRCC) is the most common malignant tumor of renal epithelial origin and, with the exception of some rare tumors, the most deadly. The exception is represented by the multilocular cystic CCRCC, whose prognosis is excellent with survival rates of 100% when diagnosis is made according to the WHO definition. For this reason a proposal has been made to rename this tumor as multilocular cystic renal cell neoplasm of low malignant potential. Another exemption could be the clear cell (tubulo) papillary renal cell carcinoma/clear cell papillary renal cell carcinoma (CCPRCC), a tumor with tubulopapillary architecture and clear cytoplasm. Published data indicates that these are neoplasms with indolent clinical behavior. No cases with metastasis have been reported. Neoplasms meeting criteria for CCPRCC will subsequently be reclassified as of "low malignant potential" rather than carcinoma. The stroma of CCPRCC not infrequently demonstrates smooth muscle metaplasia. It should be remembered, however, that smooth muscle stromal metaplasia and proliferation are not entirely specific to this entity. Hence, it is suggested that smooth muscle metaplasia in the kidney may be a nonspecific common reaction to a variety of stimuli. Xp11 translocation renal cell carcinomas are a group of neoplasms distinguished by chromosomal translocations with breakpoints involving the TFE3 transcription factor gene, which maps to the Xp11.2 locus. The most distinctive histologic pattern of the Xp11 translocation renal cell carcinoma is that of a neoplasm with both clear cells and papillary architecture, and abundant psammoma bodies. TFE3 immunohistochemical staining is reported to be sensitive and specific for a diagnosis of translocation-associated carcinoma as long as the labeling is strong, diffuse, and nuclear. This immunostaining is particularly useful if the differential diagnosis includes CCRCC and CCPRCC. In conclusion, recognition of CCRCC and differentiation from other renal cell neoplasms with clear cytoplasm is important not only for prognostication but also for treatment-related reasons.