Atypia in inverted urothelial papillomas: pathology and prognostic significance

Inverted papillomas of the bladder are considered benign urothelial neoplasms, based on their histology and clinical course. There are scant data on inverted papillomas with atypical features. Whether to designate them as inverted papillomas with atypia or low-grade transitional cell carcinomas with inverted features is controversial. In the present study, 11 cases of inverted papillomas with atypia and 10 controls of classic inverted papillomas without atypia were collected from 2 institutions. The inverted papillomas with atypia had the typical architectural features of inverted papillomas consisting of thin anastomosing trabeculae of urothelium growing downward into the stroma without an exophytic papillary component. The atypical areas in the current series were focal, with other areas exhibiting the benign cytology of classic inverted papillomas. Cases with atypia were subdivided into the following groups: (1) 5 cases notable for areas containing prominent nucleoli, (2) 2 cases with foci with atypical squamous features, (3) 2 cases with areas of dysplasia, approaching the level of carcinoma in situ, (4) 1 case with degenerative-appearing multinucleated giant cells, and (5) 1 case notable for nests of atypical squamous cells associated with large, atypical squamoid cells with a pagetoid appearance in addition to degenerative-appearing multinucleated giant cells. Ki67 was slightly increased in 1 case, with focal dysplasia approaching carcinoma in situ and in 1 case with prominent nucleoli (increased Ki67 in both the atypical and non-atypical areas) and in the case with atypical squamous, pagetoid, and giant cells (no increased Ki67 in the atypical components). Two of the atypical inverted papilloma cases with prominent nucleoli demonstrated an increase in p53 staining throughout the lesions. Cytokeratin (CK) 20 staining was negative in all cases of inverted papillomas. No significant increase in Ki67 staining was found in any of the 10 control cases; increased p53 staining was seen in 1 control case. CK20 staining was negative in the 10 control cases. In the 11 cases with atypia, clinical follow-up revealed no history of prior or subsequent bladder neoplasms. In the cases reviewed, most inverted papillomas with atypia did not demonstrate significantly increased cellular proliferation in comparison with inverted papillomas without atypical features. To date, there has been no association with urothelial carcinoma in the individuals diagnosed with atypical inverted papillomas. These findings suggest that these lesions are currently best classified as inverted papillomas with atypia, not as low-grade transitional carcinomas, and that they merit continued evaluation as a distinct group.