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What are lung carcinoid tumors?

Carcinoid tumors start from cells from the neuroendocrine system; they are like nerve cells in some ways and like hormone-making endocrine cells in other ways. Neuroendocrine cells do not form an actual gland but are scattered throughout the chest and abdomen.

Lung neuroendocrine cells sometimes grow out of control and form tumors. These are known as neuroendocrine tumors (NETs) or neuroendocrine cancers. NETs can develop anywhere in the body.

There are two other types of lung carcinoid tumors, and their differences can be seen under a microscope:
  • Typical carcinoids grow slowly and only rarely spread beyond the lungs. Nearly 90% of lung carcinoids are typical carcinoids. They are usually without symptoms but can sometimes block a bronchial tube and trigger asthma symptoms.
  • Atypical carcinoids grow a little faster and are somewhat more likely to spread to other organs. Seen under a microscope, they have more cells in the process of dividing and look more like a fast-growing tumor. They are much less common than typical carcinoids.
Carcinoid tumors are sometimes also classified by where they form in the lung:
  • Central carcinoids form in the walls of large airways near the center of the lungs. Most lung carcinoid tumors are central carcinoids, and nearly all of these are also typical carcinoids.
  • Peripheral carcinoids develop in the narrower airways toward the edges of the lungs. Most peripheral carcinoids are also typical carcinoids.
Very little is known about what causes lung carcinoid tumors. They probably develop from tiny clusters of neuroendocrine cells in the lung airways called carcinoid tumorlets. Researchers still do not understand how carcinoid tumorlets develop from lung neuroendocrine cells or why tumorlets sometimes grow to become carcinoid tumors, and why others materialize but never grow.

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