Pericardial mesothelioma is a rare form of mesothelioma that accounts for roughly 5% of all mesothelioma cases. Pericardial mesothelioma is a cancer that grows on the lining of the heart. The lining of the heart is extremely important because it plays an important role in protecting the heart from damage and so pericardial mesothelioma is an extremely serious condition. Pericardial mesothelioma becomes a terminal illness if not treated aggressively in its early stages.