Inflammatory pseudotumor of the urinary bladder--possible relationship to nodular fasciitis. Two case reports, cytologic observations, and ultrastructural observations.

An inflammatory pseudotumor of the urinary bladder might cause a large, pedunculated intraluminal mass that clinically and radiologically simulates a malignant tumor. This benign lesion can easily be mistaken for a sarcoma because by light microscopy it consists of clon-gated strap cells that resemble rhabdomyoblasts. The loose texture, the myxoid background, the relative paucity of mitoses, and the absence of nuclear hyperchromasia are important histologic determinants of benignity. Smears and imprints yield voluminous tapering cells in an inflammatory background. There are no cytoplasmic cross-striations or Z-bands and the tumor cells have the ultrastructural characteristics of myofibroblasts. It is important to recognize this lesion as benign to avoid the potentially serious consequences of misdiagnosis.