Inflammatory pseudotumor of the urinary bladder with an aberrant expression of cytokeratin.

A case of inflammatory pseudotumor of the urinary bladder in a 47 year old Japanese male patient is presented. Inflammatory pseudotumor of the urinary bladder is a benign but rare proliferative lesion of the submucosal stroma, easily mistaken for a malignant neoplasm. Based on the clinical diagnosis of bladder cancer by cystoscopy and magnetic resonance imaging (MRI), urologists started chemotherapy before results of the histological report were available which described inflammatory pseudotumor on the biopsy. Biopsied materials showed marked proliferation of irregularly bundled spindle cells, varied in size and shape and separated in severe loose myxoid stroma with moderate infiltration of the inflammatory cells and capillary proliferations. At a glance, these findings resemble the sarcomatous pattern. However neither severe nuclear atypism nor atypical mitoses were present. Immunohistochemically, these spindle cells, which were positive for vimentin and alpha-smooth muscle actin, showed a diffuse aberrant expression of cytokeratin. Some of them were positive for phosphotungstic acid hematoxylin. Electron microscopy revealed only the fibroblasts. No recurrence has been observed for 10 months. These findings indicate that inflammatory pseudotumor is a benign mesenchymal lesion that must be discriminated from true sarcoma to avoid subjecting the patient to unnecessary therapy. Only careful histological examination can enable a successful diagnosis.