Grossly, WT tends to present as large, solitary, well-demarcated mass, which bulges from the kidney and compresses the residual parenchyma to form a surrounding thin rim. The yellow color of renal cell carcinoma is not seen. The tumor is well circumscribed, compresses the surrounding tissue and nearly replaces the whole kidney. The classic microscopic picture of WT is triphasic with epithelial elements in the form of abortive tubules or glomeruli, spindle-shaped stromal cells and small, round blue blastema in various proportions. This represents attempts to recapitulate the stages of development of the kidney.
Occasional tumors contain only two or one of the elements.
Classic triphasic pattern of Wilm's tumor with tubules, spindle-shaped stromal cells and small, round, blue blastema cells. Another example of classic triphasic Wilm's tumor. Wilm's tumor showing a monophasic pattern with small, round, blue blastema. Tumor is to the right and renal parenchyma is on the left. The stage and the presence of anaplasia, which tends to occur in older, nonwhite patients, affect the prognosis of the disease.