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 can cause changes in lung cells that lead to carcinomas, the more common type of lung cancer. But these factors are not thought to play a large role in the development of lung carcinoid tumors.

Carcinoid tumors probably develop from tiny clusters of neuroendocrine cells in the lung airways called carcinoid tumorlets. Tumorlets are sometimes found unexpectedly in lung biopsies done to treat or diagnose other conditions. Under the microscope, tumorlets resemble carcinoid tumors, except that they are much smaller -- less than 5 mm (¼ inch) across.

Most tumorlets never grow any bigger, but some may eventually become carcinoid tumors. Researchers still do not understand how carcinoid tumorlets develop from lung neuroendocrine cells or why tumorlets sometimes grow to become carcinoid tumors.

Last Medical Review: 03/09/2009
Last Revised: 03/09/2009