Inverted papilloma and papillary transitional cell carcinoma of urinary bladder: report of four cases of inverted papilloma, one showing papillary malignant transformation and review of the literature.

Four new cases of inverted urothelial papillomas are described and added to the 47 cases previously reported in the literature. Three papillomas present benign gross and microscopic characteristics. The fourth case, however, demonstrates inverted papilloma and papillary transitional cell carcinoma in a single polypoid lesion. This papillary malignant transformation, not previously observed in inverted papillomas, cautions against overconfidence in benign nature of inverted papilloma.