Renal pseudotumor due to a retained perirenal sponge: CT features

A case of a surgically retained, encapsulated sponge presenting as an unusual cause of renal pseudotumor is described in a man who had a partial nephrectomy for renal cell carcinoma 1 year before. The nonspecific sonographic appearance of a hyperechoic renal mass and the atypical CT features of a thick-walled indeterminate mass suggested a hematoma, a hemorrhagic cyst, or a hemorrhagic neoplasm. A retained surgical sponge within a thick fibrotic capsule adhesive to the renal capsule was found at surgery.