It's not clear what causes carcinoid tumors. In general, cancer occurs when a cell develops mutations in its DNA. The mutations allow the cell to continue growing and dividing when healthy cells would normally die.

The accumulating cells form a tumor. Cancer cells can invade nearby healthy tissue and spread to other parts of the body.

Doctors don't know what causes the mutations that can lead to carcinoid tumors. But they know that carcinoid tumors develop in neuroendocrine cells.

Neuroendocrine cells are found in various organs throughout the body. They perform some nerve cell functions and some hormone-producing endocrine cell functions. Some hormones that are produced by neuroendocrine cells are cortisol, histamine, insulin and serotonin.

Dec. 20, 2012