Note: This document covers only the small-cell type of lung cancer. The treatment for small cell lung cancer is very different from the treatment for non-small cell lung cancer. Much of the information for one type will not apply to the other type. If you don't know which type of lung cancer you have, ask your doctor so you can be sure you are looking at the right information.

Lung cancer is a cancer starts in the lungs. To understand lung cancer, it helps to know something about the lungs and how they work.

The lungs

The lungs are 2 sponge-like organs found in the chest. The right lung has 3 sections, called lobes. The left lung has 2 lobes, as shown in the picture below. The left lung is smaller because the 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 windpipe (trachea). The trachea divides into tubes called the bronchi, which divide into smaller branches called the bronchioles. At the end of the bronchioles are tiny air sacs known as alveoli.

The lining around the lungs, called the pleura, helps to protect the lungs and allows them to move during breathing.

Start and spread of lung cancer

Most lung cancer starts in the lining of the bronchi, but it can also start in other parts of the lung.

Lung cancer often takes many years to develop. First, there may be pre-cancer changes in the lung. These changes are not a mass or tumor. They can't be seen on an x-ray and they don't cause symptoms.

Over time, these pre-cancer areas may go on to become true cancer. The cancer makes chemicals that cause new blood vessels to form nearby. These new blood vessels feed the cancer cells and allow a tumor to form. In time, the tumor becomes large enough to show up on an x-ray.

At some point, lung cancer cells can break away and spread to other parts of the body in a process called metastasis. Lung cancer is a life-threatening disease because it often spreads this way before it is found.

The lymph system

One of the ways lung cancer can spread is through the lymph (pronounced "limf") system. Lymph vessels are like veins, but they carry lymph instead of blood. Lymph is a clear fluid that contains tissue waste and cells that fight infection. Lung cancer cells can enter lymph vessels and begin to grow in lymph nodes around the bronchi and in the space between the lungs. When lung cancer cells have reached the lymph nodes, they are more likely to have spread to other organs of the body, too. The extent (stage) of the cancer and decisions about treatment are based on whether or not the cancer has spread to the nearby lymph nodes. We talk about this more in the section, "After the tests: Staging."

Types of lung cancer

There are 2 main types of lung cancer and they are treated very differently.

• Small cell lung cancer (SCLC)
• Non-small cell lung cancer (NSCLC)

If the cancer has features of both types, it is called mixed small cell/large cell cancer. The information here only covers small cell lung cancer. Non-small cell lung cancer is covered in our document, Lung Cancer (Non-Small Cell).

Small cell lung cancer (SCLC)

About 10% to15% of all lung cancers are the small cell type. Other names for SCLC are oat cell carcinoma and small cell undifferentiated carcinoma.

This cancer often starts in the bronchi near the center of the chest. Although the cancer cells are small, they can divide quickly, form large tumors, and spread to lymph nodes and other organs throughout the body. This is important because it means that surgery is rarely an option and never the only treatment given. Treatment must include drugs to kill the widespread disease.

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

Other types of lung cancer

Along with the 2 main types of lung cancer, other tumors can also be found in the lungs. Some of these are not cancer and others are. Carcinoid tumors, for instance, are slow-growing and usually cured by surgery. We have more information about lung carcinoid tumors in our document Lung Carcinoid Tumor.

Keep in mind that 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 these cancers that have spread to the lungs depends on where the cancer started.