Most mesothelioma occurs in the pleura, which is a thin membrane lining the lung. However a substantial minority of mesothelioma cases (probably slightly less than 20%) occurs in the mesothelial tissue lining the peritonum (i.e., in the abdomen).
While it is generally recognized that peritoneal mesothelioma is caused by exposure to asbestos, some authors have advanced the hypothesis that for asbestos to cause peritoneal mesothelioma, exposures must be of a higher duration and intensity than those required to cause pleural mesothelioma. A study by Laura Welch et al., published in the International Journal of Occupational Health in 2005 challenges that hypothesis.
Welch conducted a case-control study of patients treated for peritoneal mesothelioma at the Washington Cancer Institute between 1989 and 2001. One of the co-authors of the study was Dr. Paul Sugarbaker, an oncologist at the Washington Cancer Institute who specializes in the treatment of peritoneal mesothelioma.
Fifty-three cases of peritoneal mesothelioma were included in the study, and of those 40 patients or their next of kin were interviewed to determine whether asbestos exposure was present, and what the intensity of the exposure was. This was the largest series of peritoneal mesotheliomas ever reported in the medical literature for which in depth interviews were conducted.
The average age of the male patients studied was 53 years. Seventy five percent of them were college educated. The study found a significant association between asbestos exposure and peritoneal mesothelioma, but it also found that many of the asbestos exposures leading to peritoneal mesothelioma were lower than had previously been described in the literature. The authors of the study noted that prior studies often do not separate out peritoneal mesotheliomas from pleural mesotheliomas, and those that do have not had good access to asbestos exposure histories.
The authors acknowledged that peritoneal mesotheliomas are more common in more highly exposed groups of people, but concluded that that does not mean that peritoneal mesotheliomas cannot appear in groups with less exposure. Their study indicated that relatively low exposures can result in peritoneal mesotheliomas, and it supported the hypothesis that there is no threshold of exposure below which peritoneal mesotheliomas will not occur.