Answer: Malignant mesothelioma

Histology: The lesion is characterized by anastomosing cords of cuboidal cells associated with psammoma bodies. Fibrous stroma is present but it appears to be reactive rather than to be a component of neoplasm. Intracytoplasmic vacuoles are present in many of the tumor cells. In many areas, the tumor assumes a papillary configuration with broad branching fronds of tumor.

Discussion: Malignant peritoneal mesothelioma is rare in adults but is particularly unusual in children. Individual series usually report two to four cases. One review (Pediatric Radiology 30:251-255, 2000) indicates that pediatric peritoneal mesothelioma is more common in girls. Unlike malignant mesothelioma in adults, the lesion in children is not clearly associated with exposure to asbestos or irradiation. The microscopic features of this lesion are those of a primarily epithelial mesothelioma. The presence of psammoma bodies will cause confusion with primary ovarian carcinoma or with metastatic thyroid tumor, perhaps even from struma ovarii in an ovarian teratoma. The presence of intracytoplasmic vacuoles might raise the question of mucinous carcinoma although histochemical stains would show that these vacuoles contain hyaluronic acid and not neutral mucin. Monophasic epithelial synovial sarcoma would be another, although extremely rare, possibility.

Immunocytochemical stains in this case resolved the issue. The tumor cells were positive for cytokeratin and for calretinin but were negative for CEA and CD15. The differential diagnosis would also include reactive mesothelium, however, the huge mass and invasive quality of this lesion militates against that diagnosis. The separation of this lesion from granulosa cell tumor, which may have a trabecular growth pattern, would rest on the lack of nuclear grooves in the tumor cells and on immunocytochemical studies. Well differentiated papillary mesothelioma (WDPM) also occurs in the peritoneum. In this entity there are multiple peritoneal nodules ranging from a few millimeters to several sonometers in greatest dimention. WDPM usually has a well developed tubulopapillary pattern in at least some areas but may be associated with extensive fibrosis. This lesion may be difficult to separate from malignant mesothelioma, although that lesion, as in the present case is more clinically aggressive, has larger tumor masses and has atypical cellular histology with numerous mitotic figures.

Benign multicystic mesothelioma (BMM) is a rare lesion that often forms a large multicystic mass. This lesion has a characteristic gross and microscopic appearance with multiple, translucent membranous cysts often arranged in grapelike clusters and filled with fluid. The cysts are lined by one to several layers of flat cuboidal cells with occasional hobnail shapes and focus foci of squamous metaplasia may rarely be seen. The differential diagnosis of BMM could include lymphangioma as well. Primary serous papillary carcinoma of the peritoneum is a well-known entity. It usually occurs in women whose ovaries are free of tumor or only have small nodules of tumor on the ovarian surface. The separation of this lesion from mesothelioma will involve immunocytochemical and histochemical staining.