Unusual imaging features of urothelial cancer of the renal pelvicaliceal system can be challenging for any radiologist. These manifestations include noncalcified and calcified focal infiltrative parenchymal masses, incidentally detected tumors in a hydronephrotic kidney due to ureteropelvic junction obstruction, transpelvic infiltrating solid masses extending through the retroperitoneum, and tumors primarily invading the perirenal fat. Less common manifestations include massive papillary frond-like projections within a large hydronephrotic sac, large multiloculated cystic masses with thick and irregular septa, tumors invading the renal vein, and paraaortic lymph node metastases as the only sign of an undetectable primary tumor of the renal collecting system. Radiologists should be aware that an eccentric, focal, infiltrative pelvicaliceal carcinoma that causes distortion of the renal contour may simulate renal cell carcinoma; when calcified, such a pelvicaliceal carcinoma may simulate chronic inflammatory renal disease. The radiologist should also be aware of the importance of a meticulous sonographic evaluation of a hydronephrotic kidney, particularly in older patients, to avoid missing a urothelial cancer. When a nonfunctioning and enlarged kidney is present, radiologists should always consider infiltrative hydronephrotic urothelial carcinoma as a diagnostic possibility. Familiarity with unusual imaging features of urothelial cancer of the renal pelvicaliceal system may facilitate making the correct diagnosis, thus enabling adequate therapeutic management.

Unusual radiologic manifestations of a renal cell carcinoma (RCC) can present a diagnostic challenge. These manifestations include bilateral synchronous multifocal tumors, a small RCC with synchronous adrenal metastasis, and RCC associated with bulky abdominal lymphoma. Less common manifestations include multiseptated cystic carcinoma simulating a moderately complex renal cyst at ultrasonography (US), paraaortic metastatic adenopathy as the only sign of an undetectable primary renal neoplasm, RCC causing a large arteriovenous fistula, RCC simulating angiomyolipoma, and a nonfunctioning kidney due to transparenchymal renal propagation of cancer associated with a tumor thrombus occluding the renal vein. Radiologists should be aware of the possibility of tumor multifocality or of adrenal metastases from a high-grade small renal tumor, as well as of the association of RCC with lymphoma. They should also be aware of the importance of following up a multiseptated cystic mass found at US or a Bosniak category IIF renal cyst, since these lesions can serve as early indicators of cystic carcinoma. Because the clinical implications of and therapeutic strategies for RCC vary depending on imaging characterization of the nature and extent of the disease, familiarity with its more unusual radiologic manifestations facilitates accurate diagnosis and management.

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