Abstract: 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.
Introduction: Mesotheliomas arising from the serous membranes are most commonly found in the pleura (accounting for more than 80% of cases), followed by the peritoneum (approximately 15-20%), with extremely rare occurrence in the pericardium and tunica vaginalis testis (below 1%) [1]. Asbestos is known to be associated with several diseases, including lung cancer, mesothelioma, and plaque formation. The majority of patients with mesotheliomas (approximately 80%) have a history of asbestos exposure; in which asbestos fibers cause abnormal changes in cyclin-dependent kinase inhibitor 2A (CDKN2A) [2]; however, mesotheliomas may actually occur in patients with an unknown history of asbestos exposure [3]. According to the World Health Organization (WHO) classification (2021) [1], mesotheliomas are classified as epithelioid (approximately 60-80%), sarcomatoid (approximately 10-20%), or biphasic mesotheliomas (approximately 10-20%). Biphasic mesotheliomas are defined as comprising at least 10% each of epithelioid and sarcomatoid components. Epithelioid mesotheliomas show diverse histo-architectural and cytomorphological characteristics. The histo-architectural characteristics of tumor cells include adenomatoid tumor-like, solid, and micropapillary patterns, and their cytomorphological distinctions include rhabdoid, small cell, clear cell, signet ring cell, deciduoid, and lymphohistiocytoid mesotheliomas. In addition, mesothelioma with extensive myxoid stroma exists. Among them, epithelioid mesothelioma with deciduoid features (deciduoid mesothelioma) is exceedingly rare and exhibits a morphological pattern similar to decidua-like features of the uterine stroma occurring in a progesterone-dominant state, [1,4,5]. Therefore, deciduoid mesothelioma may be mistaken for ectopic decidua. This tumor was first reported approximately 40 years ago by Talerman et al. [5] In their report, a peritoneal lesion in a 13-year-old girl was initially misdiagnosed as ectopic decidua, and this case was published together with a review of similar cases. Likely owing to the rarity of this malignancy, there remains no consensus regarding its clinical or prognostic evaluation. In addition, few detailed studies have been conducted on the morphological characteristics of this rare deciduoid mesotheliomas. In this paper, we report the results of cytological, histological, immunohistochemical, and CDKN2A-fluorescent in situ hybridization (FISH), and ultrastructural studies in a case of deciduoid mesothelioma arising in the pleura of an elderly man.
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