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

Our plain-English summary of A Rare Case of Pleural Deciduoid Mesothelioma With Histological, Immunohistochemical, Fluorescent In Situ Hybridization, and Ultrastructural Findings, by H. Sonobe, R. Omote, E. Niman, et al (Curēus, 2026).

The study

• Authors: H. Sonobe, R. Omote, E. Niman, K. Takahashi, K. Nabeshima
• Journal: Curēus
• Published: 2026;18(7):e113332
• Type of article: Case report
• Read it: PubMed · Publisher · Free full text

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.

Copyright © 2026, Sonobe et al.

In plain English

This case report described one patient: a 75-year-old man who had worked in the painting industry and had a history of asbestos exposure. The article did not name an asbestos fiber type. A CT scan showed fluid and multiple nodules on the left pleura, and pleural plaques were seen inside the chest. A removed nodule showed sheets of large, highly abnormal epithelioid cells. These cells stained positive for mesothelial markers including calretinin, podoplanin and WT-1, and had lost the proteins BAP1, MTAP and Merlin/NF2. A genetic test (FISH) found homozygous deletion of CDKN2A in 75 of 114 cells counted (65.8%). Electron microscopy showed bundles of intermediate filaments but no microvilli. The authors concluded that these findings supported a diagnosis of pleural deciduoid mesothelioma, a rare type. The patient started nivolumab plus ipilimumab, which was stopped after two courses because of immune-related enteritis and hepatitis. The authors reported no worsening seven months after therapy began and said long-term follow-up is needed.

We wrote this plain-English summary from the full article with the help of AI, and a second AI model checked each sentence against the article. It is not the authors' wording; the abstract and the article are their own account.

What the study did and found

• Design: Case report (single patient)
• Who was studied: One 75-year-old man with hypertension, emphysema and chronic heart failure who had smoked 20 cigarettes a day for 45 years. He was referred to the authors' hospital (affiliation: NHO Fukuyama Medical Center, Fukuyama, Japan). No dates of care are given.
• Exposure: Work in the painting industry with a history of asbestos exposure. No fiber type, product or dose is given.
• Main results:
  • CT showed left pleural effusion and multiple pleural nodules.
  • Hyaluronic acid in the pleural fluid was elevated above the upper limit of detection.
  • Pleural effusion and plaques were observed in the pleural cavity.
  • In the biopsy tissue, tumor cells grew in sheets, with multinucleated tumor cells present.
  • Tumor cells were positive for calretinin, podoplanin (D2-40), WT-1, cytokeratin AE1/AE3 and vimentin. Table 1 lists CEA and TTF1 as negative.
  • Tumor cells showed loss of BAP1, MTAP and Merlin/NF2 expression.
  • CDKN2A FISH found homozygous deletion in 75 of 114 cells (65.8%) and heterozygous deletion in 0 of 114 cells (0%).
  • Electron microscopy showed bundles of intracytoplasmic intermediate filaments but no microvilli on the cell surface.
  • Nivolumab plus ipilimumab was started.
  • After two courses, the patient developed immune-related enteritis and hepatitis, and therapy was stopped.
  • Therapy was not resumed, and the patient's condition remained stable.
  • The discussion reports no worsening of symptoms seven months after the start of combination therapy.
• The authors' conclusion: The authors concluded that the histological, immunohistochemical and FISH findings supported a diagnosis of deciduoid mesothelioma. They reported the disease was controlled at seven months but said further long-term follow-up is required.
• Limitations the authors state:
  • The deciduoid subtype could not be identified from the pleural fluid cytology.
  • The biopsy came from only one of several pleural lesions; other lesions might contain non-deciduoid mesothelioma, and this needs further investigation.
  • Long-term follow-up is needed to judge whether the treatment worked.
  • The meaning of the missing microvilli, compared with earlier cases, is unknown.

Topics

Medical terms in this study, explained in our Lexicon: Mesothelioma, Fluorescence In Situ Hybridization (FISH), Asbestos Exposure, Pleura, Pleural Effusion, Cytology, Epithelioid, Nuclear Atypia.

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• Adaptation of a commercial fluorescent in situ hybridization test to the diagnosis of malignant cells in effusions — C. Flores-Staino, E. Darai-Ramqvist, et al. (2010)
• MET Gene Status in Malignant Mesothelioma Using Fluorescent In Situ Hybridization — S. Varesano, S. Salvi, et al. (2016)
• MTAP immunohistochemistry is an accurate and reproducible surrogate for CDKN2A fluorescence in situ hybridization in diagnosis of malignant pleural mesothelioma — D.B. Chapel, J.J. Schulte, et al. (2020)
• Usefulness of p16/CDKN2A fluorescence in situ hybridization and BAP1 immunohistochemistry for the diagnosis of biphasic mesothelioma — D. Wu, K. Hiroshima, et al. (2017)
• Hemizygous loss of NF2 detected by fluorescence in situ hybridization is useful for the diagnosis of malignant pleural mesothelioma — Y. Kinoshita, M. Hamasaki, et al. (2020)

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