Squamous differentiation in surgically resected malignant pleural mesothelioma with neoadjuvant chemotherapy

We describe a rare case of malignant pleural mesothelioma (MPM) that developed squamous differentiation. MPM can present various patterns of histology, but squamous differentiation has not been reported in any surgically resected cases to date. The patient was a 50-year-old female without smoking habit who had right MPM and underwent pleurectomy/decortication after chemotherapy. Pathological examination of the surgical specimen found that the MPM contained squamous cancer cells with apparent keratinization close to the tubulopapillary epithelioid tumor cells. Squamous differentiation was recognized close to the mesothelial proliferation, and the topographical origin of the tumor could not be recognized in the lung. The tubulopapillary tumor cells were positive for cytokeratin 5/6, Wilms tumor-1, and calretinin, and negative for thyroid transcription factor-1 (TTF-1), claudin-4, and p40. Squamous cells were positive for cytokeratin 5/6 and p40, and negative for Wilms tumor-1, calretinin, and TTF-1. Loss of BRCA1 associated protein-1 (BAP1) was observed in both the tubulopapillary and squamous tumor cells. Based on the loss of BAP1 and no history of smoking, we diagnosed this case as a rare differentiation of biphasic-type MPM into squamous cell carcinoma.