Which of the following is not classified as a malignant renal mass?

Oncocytoma is a benign renal mass and is not classified as malignant. It is a type of tumor that arises from the intercalated cells of the renal tubules. Typically, oncocytomas are well-circumscribed, encapsulated, and do not invade surrounding tissues or metastasize, which are characteristics that differentiate them from malignant tumors.

In contrast, renal cell carcinoma, adenocarcinoma of the kidney, and transitional cell carcinoma are all considered malignant. Renal cell carcinoma, the most common type of kidney cancer, tends to invade locally and has the potential for metastasis. Adenocarcinoma of the kidney also refers to similar malignancies arising from the renal parenchyma. Transitional cell carcinoma arises from the urothelium, commonly affecting the renal pelvis and the ureters, and is likewise associated with aggressive behavior and metastasis.

Thus, the distinction lies in the behavior and characteristics of oncocytomas compared to the confirmed malignancies categorized under the other options.