Staging describes the extent or severity of a cancer diagnosis based on information about the tumor. Staging is important, because:

• It helps the doctor plan your treatment
• The stage can be used to estimate your prognosis (likely outcome or course of the disease).
• It helps your doctor identify clinical trials that may be suitable for you.

The common elements considered in most staging systems are:

• Location of the primary tumor,
• Tumor size and number of tumors,
• Spread of cancer into lymph nodes,
• Cell type and tumor grade (how closely the cancer cells resemble normal tissue)
• Presence or absence of metastasis (the spread of the cancer).

The TNM system is one of the most commonly used staging systems. It is based on the extent of the tumor (T), the extent of spread to the lymph nodes (N), and the presence of metastasis (M). A number is added to each letter to indicate the size or extent of the tumor and the extent of spread.

For example, breast cancer T3 N2 M0 refers to a large tumor that has spread outside the breast to nearby lymph nodes, but not to other parts of the body. Prostate cancer T2 N0 M0 means that the tumor is located only in the prostate and has not spread to the lymph nodes or any other part of the body.

For many cancers, TNM combinations correspond to one of five stages. Criteria for stages differ for different types of cancer. For example, bladder cancer T3 N0 M0 is stage III; however, colon cancer T3 N0 M0 is stage II.

 Cancers of the blood or bone marrow, including most types of leukemia, do not have a clear-cut staging system.

The summary staging system is used for all types of cancer. The categories are:

• In situ - early cancer that is present only in the layer of cells in which it began.
• Localized - cancer that is limited to the organ in which it began, without evidence of spread.
• Regional - cancer that has spread beyond the original (primary) site to nearby lymph nodes or organs and tissues.
• Distant - cancer that has spread from the primary site to distant organs or distant lymph nodes.
• Unknown - cases for which there is not enough information to indicate a stage.

The tests used to determine staging may include:

• Physical exams. A doctor examines the body by looking, feeling, and listening for anything unusual. The physical exam may show the location and size of the tumor(s) and the spread of the cancer to the lymph nodes and/or to other organs.
• Imaging studies. These produce pictures of areas inside the body. Procedures such as x-rays, computed tomography(CT) scans, magnetic resonance imaging (MRI) scans, and positron emission tomography (PET) scans can show the location of the cancer, the size of the tumor, and whether the cancer has spread.
• Laboratory tests. These studies of blood, urine, other fluids, or tissues can provide information about the cancer.
• Pathology reports. A biopsy (the removal of cells or tissues from the body for examination under a microscope) may be performed to provide information for the pathology report. Cytology reports also describe findings from the examination of cells in body fluids.
• Surgical reports. These describe the size and appearance of the tumor as observed during surgery and often include observations about lymph nodes and nearby organs.

Source: National Cancer Institute