Peritoneal mesothelioma is a rare disease. The total number of cases per year in the United States is estimated between 100 and 500. There are no epidemiologic clues to its causation or association except that a small percentage of patients have a history of asbestos exposure. Because of the frequent dissemination of pleural mesothelioma to the peritoneal cavity, one must rule out spread from a primary pleural malignancy as the cause of disease. No sex, age or ethnic predominance exists. No genetic, dietary, employment or geographic associations have been reported. The primary peritoneal surface malignancies are currently regarded as sporadic malignancies in need of careful epidemiologic investigation.

Peritoneal mesothelioma is unusual in that it may show a wide spectrum of biological aggressiveness. The cystic variant of mesothelioma may cause recurrent episodes of severe lower abdominal pain but usually do not result in the death of the patient. In contrast, the most aggressive mesothelioma variants may show metastases from the peritoneal surface to mesenteric lymph nodes at the time of initial surgery. Our patient was diagnosed as having a malignant mesothelioma by histologic study. The frequent mitoses and marked cellularity along with the departure of cellular morphology from that of normal mesothelium indicated a malignant process. Because of the small size of the nodules, an invasive potential of the malignancy into surrounding tissues could not be determined. All the nodules demonstrated a uniform histology. No tumor foci that showed a transition pattern from benign to malignant were identified. There was one slightly larger tumor mass that may have been a single primary focus for the disease. Other tumor nodules were interpreted as spread via implantation to the peritoneal space. A continuation of this process of dissemination by cancer seeding and peritoneal fluid production would result in disease progression. As the peritoneal fluid produced by mesothelial nodules increased, dissemination out of the pelvis and to sites of peritoneal fluid resorption would be expected. Patients who are diagnosed with peritoneal mesothelioma invariably present to their physician with a large volume of ascites.

The widespread progression of malignant cells on peritoneal surfaces results in copious fluid production. The fluid production is assumed to be the retention of a functional property of normal mesothelial cells. In these patients the peritoneal space becomes a free conduit for mesothelioma cells to migrate from place to place. In the production of ascites fluid, the cancer cells provide themselves with a carrier solution to disseminate throughout the abdominal and pelvic spaces. Large accumulations of widely disseminated tumor would obscure a discrete primary tumor nodule. The study of patients at a very early stage of the disease would help elucidate the natural history of this malignant process.

A hypothesis can be made from the observations recorded in this patient. It may be that there was an early dissemination process from a single primary focus of disease. The small primary mesothelioma would be expected to exfoliate mesothelioma cells from its surface. This hypothesis would account for a primary cancer focus and multiple smaller but relatively uniform sized tumor nodules randomly distributed on peritoneal surfaces proximal to the primary site. No great differences in size of the initial malignant focus and its cancer seedlings should be expected. In this model the disease would progress as primary tumor and implants continue to exfoliate tumor cells. Distribution would be rather uniform throughout the abdomen and pelvis with large tumor deposits observed in dependent sites and at sites of peritoneal fluid resorption. No predominant primary tumor mass would exist when the disease is studied, as the patient became symptomatic as a result of gross disease and copious fluid production.

Alternatively, the mesothelial surface could undergo multiple sites of malignant degeneration. In this model one would expect tumor nodules of variable sizes, depending on when the benign to malignant transition occurred. Also, with multiple primary sites, heterogeneity of histologic types would be expected. The large tumor mass that results late in the disease might resemble many different subtypes of mesothelioma pressed together in the peritoneal space of a single patient. However, some patients with primary peritoneal surface malignancy do show a wide divergence of histologic types of peritoneal surface cancer.