Cancer is a group of many related diseases. All cancers begin in cells, the building blocks that make up tissues. Tissues make up the organs of the body.

Normally, cells grow and divide to form new cells as the body needs them. When cells grow old and die, new cells take their place. Sometimes this orderly process goes wrong. New cells form when the body does not need them, and old cells do not die when they should.

The extra cells form a mass of tissue, called a growth or tumor. Tumors can be either benign (not cancerous) or malignant (cancerous). Benign tumors do not spread to other parts of the body, and they are rarely a threat to life. Malignant tumors can spread (metastasize) and may be life threatening.

Cancer can begin in any organ or tissue of the body. The original tumor is called the primary cancer or primary tumor. It is usually named for the part of the body or the type of cell in which it begins.

Metastasis means the spread of cancer. Cancer cells can break away from a primary tumor and enter the bloodstream or lymphatic system (the tissues and organs that produce, store, and carry the cells that fight infections). That is how cancer cells spread to other parts of the body.

Cancer cells may spread to lymph nodes (rounded masses of lymphatic tissue) near the primary tumor (regional lymph nodes). This is called lymph node involvement, positive nodes, or regional disease. Cancer that spreads to other organs or to lymph nodes far from the primary tumor is called metastatic disease or distant disease.

When cancer cells spread and form a new tumor in a different organ, the new tumor is a metastatic tumor. The cancer cells in the metastatic tumor are like those in the original tumor. That means, for example, that if breast cancer spreads to the lung, the metastatic tumor in the lung is made up of abnormal breast cells (not abnormal lung cells). The disease in the lung is metastatic breast cancer (not lung cancer). Under a microscope, breast cancer cells look the same whether they are found in the breast or have spread to another part of the body.

To determine whether a tumor is primary or metastatic, a pathologist examines a sample of the tumor under a microscope. A pathologist can obtain this sample by performing a biopsy (the removal of cells or tissues from the body for examination). In general, cancer cells look like abnormal versions of cells in the tissue where the cancer began. A pathologist can usually tell where the cancer cells came from.

Metastatic cancers may be found before or at the same time as the primary tumor, or months or years later. When a new tumor is found in a patient who has been treated for cancer in the past, it is more often a metastasis than another primary tumor.

No. A metastatic tumor always starts from cancer cells in another part of the body. Sometimes, however, the cancer is discovered only after the metastatic tumor causes symptoms. For example, a man whose prostate cancer has spread to the bones in his pelvis may have lower back pain (caused by the cancer in his bones) before experiencing any symptoms from the primary tumor in his prostate.

In most cases, when a metastatic tumor is found first, the primary tumor can be found. The search for the primary tumor may involve lab tests, x-rays, and other procedures. However, in a small number of cases, a metastatic tumor is diagnosed but the primary tumor cannot be found, in spite of extensive tests. The pathologist knows the tumor is metastatic because the cells are not like those in the organ or tissue in which the tumor is found. Doctors refer to the primary tumor as unknown or occult (hidden), and the patient is said to have cancer of unknown primary origin (CUP). Because diagnostic techniques are constantly improving, the number of cases of CUP is going down.

When cancer has metastasized, it may be treated with chemotherapy, radiation therapy, biological therapy, hormone therapy, surgery, cryosurgery, or a combination of these. The choice of treatment generally depends on the type of primary cancer, the size and location of the metastasis, the patient’s age and general health, and the types of treatments the patient has had in the past. In patients with CUP, it is possible to treat the disease even though the primary tumor has not been located.

Yes, many new cancer treatments are being developed. To develop new treatments, the National Cancer Institute sponsors clinical trials (research studies) with cancer patients in many hospitals, universities, medical schools, and cancer centers around the country. Clinical trials are a critical step in the improvement of treatment. Before any new treatment can be recommended for general use, doctors conduct studies to find out whether the treatment is both safe for patients and effective against the disease. The results of such studies have led to progress not only in the treatment of cancer, but in the detection, diagnosis, and prevention of the disease as well. Patients interested in taking part in a clinical trial should talk with their doctor.