Lung Cancer1

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.

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 precancerous changes in the lung. The first changes happen in 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 precancerous changes may progress to 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.

There are two 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 cancer. This is uncommon.

The two types of lung cancer are treated very differently. This discussion focuses on non-small cell lung cancer.

About 85%–90% of lung cancers are non-small cell lung cancer. There are three subtypes of NSCLC. The cells in these subtypes differ in size, shape, and chemical make-up when looked at under a microscope.

Squamous cell carcinoma: About 25%–30% of all lung cancers are squamous cell carcinomas. They are often linked to a history of smoking and tend to be found in the middle of the lungs, near a bronchus.

Adenocarcinoma: About 40% of lung cancers are adenocarcinomas. These are usually found in the outer region of lung. People with one type of adenocarcinoma, sometimes called bronchioalveolar carcinoma, tend to have a better outlook (prognosis) than those with other types of lung cancer.

Large-cell (undifferentiated) carcinoma: This type of cancer accounts for about 10%–15% of lung cancers. It may appear in any part of the lung. It tends to grow and spread quickly, which can make it harder to treat.

Stage and Grade of NSCLC2 Staging is the process of finding out how far a cancer has spread. Your treatment and prognosis (outlook) depend, to a large extent, on the cancer's stage. There are actually two types of stages.

  • The clinical stage of the cancer is based on the results of the physical exam, biopsies, and imaging tests (e.g., CT scan, chest x-ray, PET scan).
  • If you have surgery, your doctor can also determine a pathologic stage, which is based on the same factors as the clinical stage, plus what is found as a result of the surgery.

The clinical and pathologic stages may be different in some cases. For example, during surgery the doctor may find cancer in an area that did not show up on imaging tests, which might give the cancer a more advanced pathologic stage.

Because most patients with lung cancer do not have exploratory surgery, the clinical stage is most often used when describing the extent of this cancer. However, when it is available, the pathologic stage is likely to be more accurate than the clinical stage, as it uses the additional information obtained at surgery.

The TNM staging system

The system used to describe the growth and spread of non-small cell lung cancer (NSCLC) is the American Joint Committee on Cancer (AJCC) TNM staging system. The TNM system describes three key pieces of information:

  • T indicates the size of the main (primary) tumor and whether it has grown into nearby areas.
  • N describes how much the cancer has spread to nearby (regional) lymph nodes. Lymph nodes are small, bean-shaped collections of immune system cells that help fight infections. Cancers often spread to the lymph nodes before going to other parts of the body.
  • M indicates whether the cancer has spread (metastasized) to other organs of the body. (The most common sites are the liver, bones, and brain.)

Numbers or letters appear after T, N, and M to provide more details about each of these factors. The numbers 0 through 4 indicate increasing severity. The letter X means "cannot be assessed because the information is not available."

Another factor that can affect the outlook for survival is the grade of the cancer. Grade is a description of how closely the cancer looks like normal lung tissue under a microscope. Low-grade means the tissue looks more normal; high-grade means the tissue looks less normal. Most of the time, the outlook is not as good for high-grade cancers as it is for low-grade cancers.

In addition, a tumor may be tested for molecular markers. This information is combined with TNM stage and grade to help improve prognostic information.

Key Statistics About Lung Cancer3 Most statistics concerning lung cancer include both small cell and non-small cell lung cancers. Lung cancer (both small cell and non-small cell) is the second most common cancer in both men (after prostate cancer) and women (after breast cancer). It accounts for about 15% of all new cancers.

The American Cancer Society's most recent estimates for lung cancer in the United States are for 2009:

  • About 219,440 new cases of lung cancer will be diagnosed (116,090 among men and 103,350 among women).
  • There will be an estimated 159,390 deaths from lung cancer (88,900 among men and 70,490 among women), accounting for about 28% of all cancer deaths.

Lung cancer is by far the leading cause of cancer death among both men and women. More people die of lung cancer than of colon, breast, and prostate cancers combined.

If you have been diagnosed with lung cancer, talk to your doctor about treatments that may be right for you. You can also find a list of support groups in the patient resources section of this Web site.

  1. American Cancer Society. What Is Non-Small Cell Lung Cancer? Accessed 3-1-10.
  2. American Cancer Society. How Is Non-Small Cell Lung Cancer Staged? Accessed 3-1-10.
  3. American Cancer Society. What Are the Key Statistics About Lung Cancer? Accessed 3-1-10.