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25 October 2017 - Case of the Week #441

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Case of the Week #441

Clinical history:
A 59 year old man presented with abdominal bloating and vague abdominal pain. CT and PET scan showed ascites and findings suggestive of peritoneal carcinomatosis. However, no primary could be identified. A diagnostic laparoscopy with omental and peritoneal biopsies was performed. The patient underwent a debulking procedure. He died 6 months later.

Cytology images:



Histopathology images:




What is your diagnosis?

Myxoid variant of peritoneal epithelioid mesothelioma

Special stains:



Test question (answer at the end):
Which of the following immunostains would be positive in peritoneal myxoid mesothelioma but not pseudomyxoma peritonei?

B. Claudin4
C. D2-40
D. Pankeratin


Up to one third of malignant mesotheliomas arise in the peritoneum, and epithelioid mesothelioma is the most frequent histologic type of malignant mesothelioma (sarcomatoid and biphasic mesotheliomas are less common). There are several subtypes of mesothelioma, including common and rare variants. The myxoid variant of epithelioid mesothelioma is a rare subtype that retains the secretory properties of normal mesothelium. About two dozen cases have been reported in the literature, with only 4 of these arising in the peritoneum (Virchows Arch 2005;447:828, Cesk Patol 2014;50:149). There is no sex predilection. Patients can present at any stage, and about half of the published cases report asbestos exposure. Overall, myxoid features appear to confer a slightly improved prognosis in epithelioid mesothelioma.

Histologically, the tumor consists of dyscohesive medium sized to large epithelioid cells with a moderate to abundant amount of eosinophilic cytoplasm dispersed in a myxoid background. The nuclei show coarse chromatin with prominent nucleoli. Mitotic figures are usually inconspicuous. Immunohistochemically, the tumor cells show diffuse positivity for calretinin, D2-40, EMA, CK5 / CK6 and vimentin. MOC31, BerEP4 and Claudin4 are negative in tumor cells, supporting mesothelial differentiation (Am J Clin Pathol 2013;139:611).

The main differential diagnoses are mucinous adenocarcinoma and pseudomyxoma peritonei. All of these lesions are positive for keratins, but only mesothelioma is positive for the above mesothelial markers.

Test Question Answer:

C. D2-40

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