A case of a malignant peritoneal mesothelioma mimicking an autoinflammatory syndrome in a 12-year-old boy is reported. The patient initially presented with lymphadenopathy and weight loss but without abdominal pain. Three things confounded the initial diagnosis: a positive test result for a gene related to cryopyrin-associated periodic syndrome, a positive response to the autoinflammatory syndrome treatment, and a lymph node biopsy which showed "hyperplastic mesothelial cells in the lymph sinuses." His symptoms relapsed several years later, and a peritoneal biopsy confirmed the final diagnosis. Complete morphological, immunohistochemical, and molecular diagnoses are described. A translocation in the TERT gene involving the truncation of the promoter was found in the mesothelioma. The translocation has never been described in mesotheliomas and is of an unknown significance.
Keywords: TERT promoter translocation; autoinflammatory syndrome; epithelioid; malignant mesothelioma; pediatric; peritoneum.