[Inverted urothelial papilloma and resembling tumors].

Two cases of inverted papilloma, developed on the bladder neck in 36- and 23-year old males, are presented. From the Japanese literature, 40 cases of inverted papillomas have been collected and are reviewed. In addition, a review of the consecutive series of 173 pathological specimens, obtained by bladder biopsy and at the time of bladder surgery over the past 5 years disclosed a resembling lesion in two cases of bladder tumors. An inverted growth of transitional-cell carcinoma was observed in one case. Transitional-cell carcinoma with abundant microcysts was covered by the non-neoplastic transitional-cell epithelium in the other case. In conclusion, the inverted urothelial papilloma is more likely an inverted growth of the neoplastic epithelium with epithelial metaplasia and microcyst formation, much as is observed in so-called chronic proliferative cystitis, than a neoplastic transformation from chronic proliferative cystitis.