Vulval intraepithelial neoplasia with mucinous differentiation: report of 2 cases of a hitherto undescribed phenomenon.

We report 2 cases of classic vulval intraepithelial neoplasia (VIN) with collections of cells throughout the full epithelial thickness and within epithelial downgrowths containing intracytoplasmic mucin. The mucinous cells were bland with eccentrically placed small regular nuclei and were positive with CAM5.2, carcinoembryonic antigen, epithelial membrane antigen, and cytokeratin 7, markers that are typically positive in the neoplastic cells of Paget disease. Both cases were strongly positive with p16 and linear array genotyping demonstrated the presence of high-risk human papillomavirus type 16. As far as we are aware, mucinous differentiation has not been described in VIN, although mucinous metaplasia has rarely been reported in non-neoplastic vulval skin and in cutaneous squamous cell carcinoma in situ at extravulval sites. The main differential diagnosis of the cases we describe is Paget disease or coexistent VIN and Paget disease and the immunophenotype of the mucinous cells may further highlight the potential for misdiagnosis. Other possible differential diagnoses include Pagetoid squamous cell carcinoma in situ (Pagetoid VIN), basal cell carcinoma with sebaceous differentiation, and VIN involving skin appendage structures. The mucinous cells are likely to be metaplastic and a result of aberrant differentiation in a premalignant squamous lesion.