Invasive stratified mucin-producing carcinoma of the uterine cervix masquerading as squamous cell carcinoma on biopsy: a diagnostic pitfall

Invasive stratified mucin-producing carcinoma (ISMC) of the uterine cervix can masquerade as non-keratinizing squamous cell carcinoma (SCC) on small biopsies because it often grows as solid nests with minimal gland formation and little or no keratinization, and may show patchy cytokeratin 5/6 (CK5/6) positivity. We report a 44-year-old woman whose outside cervical biopsy was interpreted as SCC. Repeat biopsy showed SCC-like solid nests without keratinization and patchy CK5/6 positivity, but p40 and p63 were negative and periodic acid-Schiff with diastase (D-PAS) demonstrated diastase-resistant intracytoplasmic mucin within tumor nests. Radical hysterectomy revealed a human papillomavirus (HPV)-associated endocervical adenocarcinoma with a predominant ISMC component, lymphovascular invasion, and nodal metastases. This case highlights a practical diagnostic pitfall and supports a minimal, reproducible workup for p16 block-positive, SCC-like biopsies: p40/p63 plus a mucin stain (D-PAS and/or Alcian blue), with PAX8 as needed. Funding: Japan Society for the Promotion of Science (JSPS) KAKENHI [25 K10294].