The stage of a cancer is a term used to describe its size and whether it has spread beyond its original site. Knowing the extent of the cancer helps the doctors to decide on the most appropriate treatment. A staging system has not yet been established for peritoneal mesothelioma. There are several staging systems for pleural mesothelioma. A commonly used system is described below: Localized malignant mesothelioma Stage I: The cancer cells are found in the pleura near the lung and heart or in the diaphragm or the lung. Advanced malignant mesothelioma Stage II: The cancer has spread beyond the pleura to lymph nodes in the chest. Stage III: The cancer has spread into the chest wall, the center of the chest, the heart, through the diaphragm, or abdominal lining, and in some cases into nearby lymph nodes. Stage IV: The cancer has spread to distant organs or tissues. Back to Mesothelioma Glossary.