Background  Benign mucinous metaplasia of the genitalia (BMM) is a rare condition typified by cells with foamy mucinous cytoplasm. Differential diagnoses include extramammary Paget disease (PD) and human papillomavirus (HPV)-induced vulval intraepithelial neoplasia (VIN) with mucinous differentiation.

Objectives  To characterize histopathological and immunohistochemical features of BMM and to forge criteria for differentiation from PD and VIN with mucinous differentiation.

Methods  Eight biopsy specimens of BMM were stained with haematoxylin and eosin, periodic acid–Schiff and alcian blue, and for cytokeratin (CK) 7, CK10, CK14, CK20, carcinoembryonic antigen (CEA), epithelial membrane antigen (EMA), S100, gross cystic disease fluid protein-15 (GCDFP-15), lysozyme and Ki67 and compared with PD. Polymerase chain reaction was performed in order to identify HPV-specific DNA.

Results  BMM showed mucin deposition in superficial epithelial layers ranging from numerous large goblet cells to subtle deposits. The epithelium often showed polygonal (squamoid) or cuboidal differentiation while columnar differentiation was an inconsistent feature. A band-like inflammatory infiltrate was consistently present. Metaplastic epithelium consistently expressed CK7, CEA and EMA either in the entire epithelium or in a superficial band, while CK14, CK10, GCDFP-15 and lysozyme were largely not expressed, and staining for CK20 and S100 was negative. Comparison with PD demonstrated similar staining characteristics, but in a scattered pattern of mucinous cells within preserved squamous epithelium and not in a band-like pattern as in BMM. Nuclear pleomorphism and Ki67-positive mucinous cells in superficial epithelial layers were seen only in PD; GCDFP-15 and/or lysozyme were expressed in the majority of cases of PD. No evidence of HPV-specific DNA was found in BMM.

Conclusions  The spectrum of changes in BMM is distinctive, and BMM can be differentiated with surety from both PD and VIN with mucinous differentiation.