Very little is known about the causes of lung carcinoid tumors. Researchers have learned a lot about how certain risk factors like cancer-causing chemicals or radiation cause lung cells to become carcinomas, the usual type of lung cancer. But similar studies of lung carcinoid tumors have not found any risk factors.

Sometimes, tiny clusters of neuroendocrine cells that are similar to those seen in lung carcinoid tumors are seen under the microscope as an unexpected finding in lung biopsies done to treat or diagnose other conditions. These lesions, called carcinoid tumorlets, develop in small airways. Under the microscope, tumorlets have a striking resemblance to peripheral carcinoid tumors, except that they are usually much smaller (¼ inch).

Central carcinoid tumors are believed to develop from the glands under the surface of the large air passages. Researchers still do not understand how carcinoid tumorlets develop from lung neuroendocrine cells or why some tumorlets may eventually grow to become carcinoid tumors.

Revised: 08/07/2006