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Medically reviewed by Last updated on Jan 12, 2022.

What is Cancer?

Harvard Health Publishing

Cancer is not a single disease. It's a group of diseases characterized by their ability to cause cells to change abnormally and grow out of control.

Most types of cancer form a tumor, a lump or mass of cancerous cells. Cells from a tumor may break away and travel to other parts of the body, where they can settle and multiply. This spreading process is called metastasis. New cancers that have broken off and spread from the original tumor are called metastases.

Not all tumors are cancerous or malignant; some are benign (non-malignant), don't spread, and aren't life-threatening. And a few cancers don't form masses or lumps, such as those that affect the blood, such as leukemia.


Symptoms vary widely depending on the type, location, size, and extent of the cancer.

Some cancers can exist for many years without causing any symptoms or decrease life expectancy. Common examples are small, non-aggressive prostate and breast cancers.

Other cancers can be relatively small and localized to one spot but cause significant pain. Widespread cancer can often lead to marked fatigue, weakness, loss of appetite, and weight loss.

Some cancers release substances into the blood that can cause symptoms unrelated to the location or size of the original tumor. For example, one type of lung cancer produces chemicals that trigger unusual neurological symptoms.


Your doctor will review your medical history and perform a thorough physical examination. He or she will ask about your family history of cancer.

In the initial phase of diagnostic evaluation, blood tests and CT scans of the chest, abdomen, and pelvis are performed. Depending on the results, follow-up tests may include MRI and PET scans.

Almost always, a biopsy is necessary to confirm the diagnosis and to provide information that will guide prognosis and therapy.

Expected Duration

How long cancer lasts depends on its type and response to treatment. Some cancers grow so slowly that they persist, but never cause harm. Other cancers can grow and spread quickly despite therapy.


The number one way to prevent the most dangerous types of cancer is to not smoke cigarettes or use other tobacco products.

Other ways to help prevent cancer include:

  • getting certain vaccines to prevent cancer related viral infections, such as the human papilloma virus (HPV) vaccine to prevent cancer of the cervix and other cancers, and the hepatitis B vaccine to prevent cancer of the liver
  • getting screened for colon cancer starting at age 50, or younger if you have personal risk factors
  • practicing safe sex to avoid HIV infection
  • using alcohol in moderation, if you do drink
  • eating a diet rich in fruits and vegetables
  • staying physically active and exercising regularly.


For some cancers, no treatment may be necessary because they are very slow-growing. Low-grade prostate cancer in men over the age of 70 is a good example of a cancer that often does not require immediate treatment.

Most cancers do require active treatment. If the cancer is confined to only one spot in the body, surgery offers an excellent chance of cure.

Cancers of the bone marrow and lymph nodes are treated with chemotherapy and sometimes bone marrow transplant. The chances of cure for many of these types of cancer have improved dramatically.

Many other types of cancers are also treated with chemotherapy. The goal may be to control the cancer, rather than cure it.

Other treatments for cancer may include:

  • Radiation therapy. Radiation kills most cancer cells. The area of the body exposed to radiation is carefully planned to avoid damage to normal tissues.
  • Hormone therapy. Certain cancers such as breast cancer and prostate cancer grow faster in response to sex hormones (such as estrogen, progesterone, or testosterone). By blocking those hormones, cancer cell growth can slow or stop.
  • Immune therapy. These therapies use the body's own immune system to attack and destroy cancer cells.
  • Targeted therapy. This therapy uses a special chemotherapy drug that only kills cancer cells with a certain trait. For instance, targeted therapy would work on a person's lung cancer only if the cancer cells contained that specific target.

Treatment options

The following list of medications are in some way related to or used in the treatment of this condition.

View more treatment options


The outlook for people diagnosed with cancer continues to improve every year.

External resources

National Cancer Institute (NCI)

American Cancer Society (ACS)

Further information

Always consult your healthcare provider to ensure the information displayed on this page applies to your personal circumstances.