Cellular Mesoblastic Nephroma

Comments:
Cellular Mesoblastic Nephroma: Cellular variant accounts for about 65% of all mesoblastic nephromas. It is highly cellular and consists of plump spindle or ovoid cells arranged in densely packed sheets (as shown here) or poorly-formed fascicles. It resembles infantile fibrosarcoma and both carry ETV6-NTRK fusion, suggesting that they are the same entity.

The tumor cells have eosinophilic cytoplasm and ovoid or round vesicular nuclei with punctate nucleoli. Mitotic activity is brisk. Necrosis and hemorrhage are frequently seen. The blood vessels have staghorn configuration (see next image). Entrapped glomeruli and tubules are present within the tumor. Less well-differentiated, primitive-appearing foci may be present. The tumor has a tendency to infiltrate renal pelvis or perirenal fat.