Figure 910

A, contrast-enhanced computed tomography (CT) for evaluation of a left renal stone in a 67-year-old man. A cystic mass was found at the lower pole of the right kidney. Only careful examination revealed that the walls of the mass (arrows) were too thick for a simple cyst (see Fig. 9-5 for comparison). B, The echo pattern of the mass was very heterogeneous (arrows), clearly different from the echo-free appearance of a simple cyst (see Fig. 9-3 for comparison). C, Magnetic resonance imaging showed thick, irregular walls and a hyperintense central area (arrows). At surgery, the mass proved to be a largely necrotic renal cell carcinoma. Thus, although renal carcinoma is not a true cystic disease, it occasionally has a cystic appearance on CT and can mimic a simple cyst. (Courtesy of A. Dardenne.)

Medullary sponge kidney (MSK) diagnosed by intravenous urography in 53-year-old woman with a history of recurrent kidney stones. Pseudocystic collections of contrast medium in the papillary areas (arrows) are the typical feature of MSK. They result from congenital dilatation of collecting ducts (involving part or all of one or both kidneys), ranging from mild ectasia (appearing on urography as linear striations in the papillae, or papillary "blush") to frank cystic pools, as in this case (giving a spongelike appearance on section of the kidney). MSK has an estimated prevalence of 1 in 5000 [2]. It predisposes to stone formation in the dilated ducts: on plain films, clustering of calcifications in the papillary areas is very suggestive of the condition. MSK may be associated with a variety of other congenital and inherited disorders, including corporeal hemihypertrophy, Beckwith-Wiedemann syndrome (macroglossia, omphalocele, visceromegaly, microcephaly, and mental retardation), polycystic kidney disease (about 3% of patients with autosomal-dominant polycystic kidney disease have evidence of MSK), congenital hepatic fibrosis, and Caroli's disease [7].

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