Neuroendocrine cells contain little sacs called secretory granules that release substances such as hormones and peptides. Neuroendocrine cells assist in healing a thin layer of tissue — known as the epithelium — that lines internal organs and other surfaces inside the body following injury, such as might occur following an infection. Occasionally, however, neuroendocrine cells become overactive and, eventually, may become cancerous.
Neuroendocrine tumors are a highly diverse group of tumors formed by neuroendocrine cells. Because these cells are found throughout the body, neuroendocrine tumors can arise in a variety of organs, not just in the lungs. They also may occur in the gastrointestinal system, thymus, and skin. Another type of tumor that arises in the adrenal glands — called pheochromocytoma — is sometimes classified as a neuroendocrine tumor.
(For more information about neuroendocrine tumors that arise in the gastrointestinal system, please visit the Gastrointestinal Neuroendocrine Tumors section of our Web site.)
Doctors may diagnose and treat neuroendocrine tumors according to the types of cells involved, the appearance of the tumor cells under the microscope, and their location in the body.
Types of Pulmonary Neuroendocrine Tumors
A small percentage of all lung cancers are neuroendocrine tumors. These tumors represent a wide spectrum of disease based on the appearance of the cells under the microscope and how they progress in patients. There are several distinct types of pulmonary neuroendocrine tumors.
Up to 25 percent of all carcinoid tumors are found in the lungs. The majority of these tumors develop in the major bronchi (large tubes that lead air from the trachea into the lungs). There is no direct evidence that smoking causes pulmonary carcinoid tumors, yet studies suggest that a higher percentage of patients with atypical pulmonary carcinoid tumors smoke.
Pulmonary carcinoid tumors are further classified as follows:
- Typical Carcinoid Tumors
The majority of all pulmonary carcinoid tumors are typical (low-grade), which means that they are slow-growing cancers.
- Atypical Carcinoid Tumors
Atypical (intermediate-grade) pulmonary carcinoid tumors are more rare. These tumors grow more rapidly than typical carcinoid tumors, but are much less aggressive than non-carcinoid tumors found in the lungs, such as adenocarcinoma of the lungs.
This fast-growing type of cancer represents a small percentage of all lung cancers. Less commonly, large cell neuroendocrine tumors may be found in the gastrointestinal system. (For more information about neuroendocrine tumors that arise in the gastrointestinal system, please visit the Gastrointestinal Neuroendocrine Tumors section of our Web site.) Large cell neuroendocrine carcinoma of the lungs is four times more common in men than in women. Smoking is a significant risk factor for developing this type of lung cancer.
Another aggressive type of neuroendocrine tumor, small cell lung carcinoma accounts for approximately 10 to 13 percent of all lung cancers. Like large cell neuroendocrine carcinoma, this type of cancer also occasionally arises in the gastrointestinal tract. Smoking is the single biggest risk factor for developing small cell lung carcinoma. Additional risk factors include exposure to radon and asbestos. Small cell lung cancer also can occur in people with certain electrolyte and neurologic disorders.
For more information about small cell lung carcinoma, visit the Lung Cancer section on our Web site.
Because pulmonary carcinoid tumors frequently develop in the bronchial tubes, they can cause symptoms such as:
- coughing up blood
- recurrent pneumonia
Some people also may develop pain in the chest wall. However, up to one-third of patients with carcinoid tumors in the lungs have no symptoms.
Pulmonary carcinoid tumors can cause a variety of symptoms known as carcinoid syndrome. These symptoms, including diarrhea, flushing, and wheezing, may occur when the tumors produce excess amounts of serotonin, a hormone found in blood platelets, the digestive tract, and the brain. (Carcinoid syndrome is more common in patients with gastrointestinal carcinoid tumors. For more information about gastrointestinal carcinoid tumors, visit the Gastrointestinal Neuroendocrine Tumors section for more information.)
In rare cases, carcinoid lung tumors may produce too much ACTH, a substance that causes the adrenal glands to make too much cortisol and other hormones. This can lead to weight gain, weakness, diabetes, increased body and facial hair, and thin or darkened skin (Cushing’s syndrome). Less common symptoms of lung carcinoid tumors may include sudden growth of the hands and feet (acromegaly) and increased blood calcium (hypercalcemia).
Large cell neuroendocrine carcinoma of the lungs and small cell lung cancer can cause symptoms such as coughing, wheezing, recurrent pneumonia, or chest wall pain. However, many people with these types of tumors experience no symptoms. For more information about small cell lung carcinoma, visit the Lung Cancer overview on our Web site.