Malignant mesothelioma of the peritoneum presenting as an inflammatory lesion: a report of four cases.

Most cases of malignant mesothelioma present with obvious diffuse tumor, and the presence of grossly visible diffuse tumor is usually cited as an important criterion for making the diagnosis. We report four cases of unsuspected malignant mesothelioma of the peritoneum presenting as localized acute inflammatory lesions. The clinical diagnoses were acute appendicitis in two cases, acute cholecystitis in the third case, and incarcerated umbilical hernia in the fourth case. In all cases tumor was not evident at initial surgical exploration or on gross pathologic examination, and the diagnosis was only made on microscopic examination of the resected specimens. All cases showed a tubulopapillary form of epithelial mesothelioma with obvious tissue invasion, but the foci of tumor were too small to be seen grossly or were present deep in fibrous tissue. On follow-up all patients developed grossly evident tumor, and one of these patients is alive without evidence of disease 5 years after presentation. We conclude that peritoneal mesotheliomas may occasionally present as inflammatory processes without grossly evident tumor and can be diagnosed by microscopic findings alone.