What is small cell lung cancer?
Note: This document is specifically about the small cell type of lung cancer. Treatment for the two major types of lung cancer (small cell vs. non-small cell) is very different, so much of the information for one type will not apply to the other type. If you are not sure which type of lung cancer you have, it is very important to ask your doctor so you can be sure the information you receive is correct.
Lung cancer is a cancer that starts in the lungs. To understand lung cancer, it helps to know about the normal structure and function of the lungs.
Your lungs are 2 sponge-like organs in your chest. Your right lung is divided into 3 sections, called lobes. Your left lung has 2 lobes. The left lung is smaller because your heart takes up more room on that side of the body.
When you breathe in, air enters through your mouth and nose and goes into your lungs through the trachea (windpipe). The trachea divides into tubes called the bronchi (singular, bronchus), which enter the lungs and divide into smaller branches called the bronchioles. At the end of the bronchioles are tiny air sacs known as alveoli.
Many tiny blood vessels run through the alveoli. They absorb oxygen from the inhaled air into your bloodstream and pass carbon dioxide from the body into the alveoli. This is expelled from the body when you exhale. Taking in oxygen and getting rid of carbon dioxide are your lungs’ main functions.
A thin lining called the pleura surrounds the lungs. The pleura protects your lungs and helps them slide back and forth against the chest wall as they expand and contract during breathing.
Below the lungs, a thin, dome-shaped muscle called the diaphragm separates the chest from the abdomen. When you breathe, the diaphragm moves up and down, forcing air in and out of the lungs.
Start and spread of lung cancer
Lung cancers start in the cells lining the bronchi and in other parts of the lung such as the bronchioles or alveoli.
Lung cancers are thought to start as areas of pre-cancerous changes in the lung. The first changes in the genes (DNA) inside the lung cells may cause the cells to grow faster. These cells may look a bit abnormal if seen under a microscope, but at this point they do not form a mass or tumor. They cannot be seen on an x-ray and they do not cause symptoms.
Over time, the abnormal cells may acquire other gene changes, which cause them to progress to true cancer. As a cancer develops, the cancer cells may make chemicals that cause new blood vessels to form nearby. These blood vessels nourish the cancer cells, which can continue to grow and form a tumor large enough to be seen on imaging tests such as x-rays.
At some point, cells from the cancer may break away from the original tumor and spread (metastasize) to other parts of the body. Lung cancer is often a life-threatening disease because it tends to spread in this way even before it can be detected on an imaging test such as a chest x-ray.
The lymph (lymphatic) system
The lymph system is one of the ways in which lung cancers can spread. This system has several parts:
- Lymph nodes are small, bean-shaped collections of immune system cells (cells that fight infections) that are connected by lymphatic vessels.
- Lymphatic vessels are like small veins, except that they carry a clear fluid called lymph (instead of blood) away from the lungs.
- Lymph contains excess fluid and waste products from body tissues, as well as immune system cells.
Lung cancer cells can enter lymphatic vessels and begin to grow in lymph nodes around the bronchi and in the mediastinum (the area between the 2 lungs). Once lung cancer cells have reached the lymph nodes, they are more likely to have spread to other organs of the body as well. The stage (extent) of the cancer and decisions about treatment are based in part on whether or not the cancer has spread to the nearby lymph nodes in the mediastinum. These topics are discussed later in the section “How is small cell lung cancer staged?”
Types of lung cancer
There are 2 major types of lung cancer:
- Small cell lung cancer (SCLC)
- Non-small cell lung cancer (NSCLC)
(If a lung cancer has some cells with characteristics of SCLC and other cells with characteristics of NSCLC it is called a combined small cell/non-small cell cancer. This is uncommon.)
These 2 types of lung cancer are treated very differently. This document focuses on small cell lung cancer. Non-small cell lung cancer is discussed in the separate document, Lung Cancer (Non-Small Cell).
Small cell lung cancer
About 10% to 15% of all lung cancers are small cell lung cancer (SCLC), named for the size of the cancer cells when seen under a microscope. Other names for SCLC are oat cell cancer, oat cell carcinoma, and small cell undifferentiated carcinoma.
SCLC often starts in the bronchi near the center of the chest. It tends to grow and spread quickly, and it has almost always spread to distant parts of the body before it is found.
Non-small cell lung cancer
About 85% to 90% of lung cancers are non-small cell lung cancer (NSCLC). There are 3 main subtypes of NSCLC:
- Squamous cell carcinoma
- Large cell carcinoma
The cells in these subtypes differ in size, shape, and chemical make-up when looked at under a microscope. But they are grouped together because the approach to treatment and prognosis (outlook) are similar. They are discussed further in our document Lung Cancer (Non-Small Cell).
Other types of lung cancer
Along with the 2 main types of lung cancer, other tumors can occur in the lungs.
Lung carcinoid tumors: Carcinoid tumors of the lung account for less than 5% of lung tumors. Most are slow-growing tumors that are called typical carcinoid tumors. They are generally cured by surgery. Some typical carcinoid tumors can spread, but they usually have a better prognosis (outlook) than small cell or non-small cell lung cancer. Atypical carcinoid tumors are less common. The outlook for these tumors is somewhere in between typical carcinoids and small cell lung cancer. For more information about typical and atypical carcinoid tumors, see our document, Lung Carcinoid Tumor.
Other lung tumors: Other types of lung tumors such as adenoid cystic carcinomas, hamartomas, lymphomas, and sarcomas, are rare and are treated differently from the more common lung cancers. They are not discussed in this document.
Cancers that spread to the lungs: Cancers that start in other organs (such as the breast, pancreas, kidney, or skin) can sometimes spread (metastasize) to the lungs, but these are not lung cancers. For example, cancer that starts in the breast and spreads to the lungs is still breast cancer, not lung cancer. Treatment for metastatic cancer to the lungs is based on where it started (the primary cancer site). For information on these cancers, refer to our separate documents on each.
Last Medical Review: 09/12/2014
Last Revised: 03/09/2015