Most mesotheliomas develop on the thin membrane that lines the outside of the lungs. This membrane is called the pleura and so these are known as pleural mesotheliomas.

A similar thin membrane surrounds the contents of the abdomen, including organs like the stomach, the liver and the pancreas. This membrane is called the peritoneum. It is possible to develop mesotheliomas of the peritoneum. But, this is much rarer than pleural mesothelioma.

Usually peritoneal mesotheliomas begin on that part of the peritoneum that lies immediately under the diaphragm (the muscle which separates the top of the abdomen from the bottom of the chest). From there they can spread to other parts of the peritoneum. Sometimes, as the tumours get larger, they grow into organs nearby, including the stomach.

So although mesotheliomas don't start in the stomach, they may sometimes affect the stomach as they grow.