Typically the most important variable determining the prognosis and life expectancy of a mesothelioma patient is the stage of the cancer when it is diagnosed. Unfortunately, mesothelioma is more difficult to "stage" than other cancers. This is true for a couple of reasons: Since mesothelioma is usually diagnosed at an advanced stage, the statistics for five year survival for early stage mesothelioma are usually unreliable. It also can not be said with certainty which of the two types is a worse diagnosis; peritoneal mesothelioma or pleural mesothelioma. Many studies show that peritoneal is a more deadly, and rapidly spreading mesothelioma than pleural mesothelioma, but these studies are often contradicted by specialists who argue pleural mesothelioma is the more dangerous and more difficult to treat of the two. Usually patients diagnosed with either peritoneal or pleural mesothelioma are told that they may have less than a year to live. However, according to researchers in leading research centers around the world this is not necessarily the case. More recent studies indicate that patients with mesothelioma might, in some instances, have a better out look than originally believed.
1) because its somewhat rare, and
2) because its initial symptoms are subtle it is usually advanced when diagnosed, making it difficult to stage.
Peritoneal mesothelioma in particular can be difficult to stage because, while pleural mesothelioma has multiple staging systems, pathologists still haven't developed a staging system for peritoneal mesothelioma. Both pleural and peritoneal types of mesothelioma are very serious conditions and neither have a good outlook.