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.

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 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 as they expand and contract during breathing.

Below the lungs, a 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

Most lung cancers start in the bronchi, but they can also begin in other areas such as the trachea, bronchioles, or alveoli.

Lung cancers are thought to develop over many years. They may start as areas of pre-cancerous changes in the lung. The first changes happen within the cells themselves, but at this point the cells do not form a mass or tumor. They cannot be seen on an x-ray and they do not cause symptoms. Over time, these pre-cancerous changes may progress to true cancer. As a cancer develops, the cancer cells may make chemicals that cause new blood vessels to form nearby. These new 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 a life-threatening disease because it often spreads 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 important to understand because it 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). When 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 to some extent on whether or not the cancer has spread to certain groups of lymph nodes. 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 characteristics of both types it is called a mixed small cell/large cell carcinoma. This is uncommon.)

These 2 main types of lung cancer are discussed separately because their treatments are very different. 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 small cells that make up these cancers. 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, and it tends to spread widely through the body fairly early in the course of the disease (usually before it starts to cause symptoms). The cancer cells can multiply quickly, form large tumors, and spread to lymph nodes and other organs, such as the bones, brain, adrenal glands, and liver. This is important because it means that surgery is rarely an option (and never the only treatment given). Treatment must include drugs to try to kill the widespread disease.

Small cell lung cancer is almost always caused by smoking. It is very rare for someone who has never smoked to have small cell lung cancer.

Non-small cell lung cancer

About 85% to 90% of lung cancers are non-small cell lung cancer (NSCLC). There are 3 subtypes of NSCLC:

• adenocarcinoma
• 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. They are discussed further in the separate 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.

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 the document, Lung Carcinoid Tumor.

There are other, even more rare, lung tumors such as adenoid cystic carcinomas, hamartomas, lymphomas, and sarcomas. These tumors are treated differently from the more common lung cancers. They are not discussed in this document.

Cancer that starts in other organs (such as the breast, pancreas, kidney, or skin) and spreads (metastasizes) to the lungs is not the same as lung cancer. 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 depends on where it started (the primary cancer site). For information on these cancers, refer to our specific documents on each.

Last Medical Review: 10/13/2009
Last Revised: 10/13/2009