A Rare Case of Pleural Deciduoid Mesothelioma With Histological, Immunohistochemical, Fluorescent In Situ Hybridization, and Ultrastructural Findings.

Owing to the rarity of deciduoid mesotheliomas, few studies have reported detailed morphological analyses. Here, we report a case of deciduoid mesothelioma along with cytological, histological, immunohistochemical, fluorescent in situ hybridization (FISH), and electron microscopic findings. The patient was a 75-year-old man with a history of asbestos exposure. Computed tomography revealed multiple nodular shadows in the left pleura, and pleural effusion. Pleural effusion cytology revealed large epithelioid tumor cells with high nuclear atypia, suggesting epithelioid mesothelioma. A biopsy revealed that the tumor tissue consisted of sheets of highly atypical, large epithelioid cells of various sizes. Immunostaining revealed that the tumor cells were positive for mesothelial cell markers, including calretinin, Wilms tumor 1, and podoplanin, and showed loss of expression of BRCA1-associated protein 1, methylthioadenosine phosphorylase, and merlin/neurofibromatosis 2. Furthermore, cyclin-dependent kinase inhibitor 2A-FISH revealed a high frequency of homozygous deletions. Based on these findings, the tumor was diagnosed as a deciduoid mesothelioma. Reflecting the histological features, electron microscopy revealed that the tumor exhibited sheets of large epithelioid cells containing round nuclei with single or multiple distinct nucleoli, and intracytoplasmic intermediate filaments; however, microvillous structures were not evident. After the diagnosis was confirmed, the patient was treated with a combination of immune checkpoint inhibitors and chemotherapy. Long-term follow-up is required, although no disease progression was observed seven months after the initiation of therapy.