The stage of a cancer is how much and how far the cancer has spread in your body. Your doctor uses exams and tests to find out the size of the cancer and where it is. They can also see if the cancer has spread into nearby areas, and if it has spread to other parts of your body. The stage of a cancer is one of the most important things to know when deciding how to treat the cancer.

Melanoma starts in the top or outermost layer of the skin. As it grows, it can go through to the deeper layers of the skin. Then, like all cancers, it can spread (metastasize) to other parts of the body.

Doctors use different systems to measure the thickness of a melanoma and to stage the disease. These systems summarize the extent of your cancer. Be sure to ask your doctor to explain the stage of your cancer.

The most commonly used system to stage melanoma is the TNM system from the American Joint Committee on Cancer. Here's what the letters stand for in the TNM system:

• T tells how far the main tumor has spread into the layers of the skin. It's based on the melanoma's thickness and whether it's ulcerated. The thickness of the melanoma is called the Breslow depth. Ulcerated means that the layer of skin covering the melanoma is gone. There may be bleeding with ulceration.
• N tells if the lymph nodes in the area of the original tumor have cancer in them.
• M tells if the cancer has spread (metastasized) to distant organs in the body, such as the liver, lungs, or brain.

Numbers or letters after T, N, and M provide more details about each of these factors.

Stage groupings are determined by combining the T, N, and M values from the TNM system. These groupings give an overall description of your cancer. A stage grouping can have a value of 0 or of Roman numerals I through IV (1 through 4). The higher the number, the more advanced your cancer is.

These are the stage groupings for melanoma and what they mean:

Stage 0. The melanoma is only in the top layer of skin (called the epidermis). This is also called melanoma in situ.

Stage I. The cancer is no more than 2 millimeters (mm) thick, and may or may not be ulcerated. It has not spread to the lymph nodes or distant parts of the body.

Stage II. The cancer is at least 1.01 mm thick and may be more than 4 mm thick. It may or may not be ulcerated and has not spread to lymph nodes or distant parts of the body.

Stage III. The cancer has not spread to distant parts of the body and is divided into these groups:

• Stage IIIA. The cancer is no more than 2 mm thick. It may or may not be ulcerated. It has spread to 1 to 3 nearby lymph nodes, but the nodes are not enlarged and the melanoma is only found when looked at under a microscope.
• Stage IIIB. In this stage, one of these is true:
  • There's no sign of the main (primary) melanoma, but melanoma is found in 1 nearby lymph node or small melanoma tumors are found in nearby skin or lymph vessels (but not in lymph nodes). Lymph vessels are like blood vessels, but they carry a fluid called lymph all through the body. Lymph vessels are part of the immune system.
  • The cancer is no more than 4 mm thick, may or may not be ulcerated, and is found in 1 to 3 nearby lymph nodes or small melanoma tumors are found in nearby skin or lymph vessels (but not in lymph nodes).
• Stage IIIC. In this stage, one of these is true:
  • There's no sign of the main (primary) melanoma, but melanoma is found in at least 2 or more nearby lymph nodes or cluster of lymph nodes or small melanoma tumors are found in nearby skin or lymph vessels (but not in lymph nodes).
  • The cancer is no more than 4 mm thick, and may or may not be ulcerated. It has also spread to 4 or more nearby lymph nodes or cluster of lymph nodes or small melanoma tumors are found in nearby skin or lymph vessels (but not in lymph nodes).
  • The cancer is between 2.1 mm and 4 mm thick and is ulcerated or is thicker than 4 mm but not ulcerated. It has also spread to at least 1 nearby lymph node or cluster of lymph nodes or small melanoma tumors are found in nearby skin or lymph vessels (but not in lymph nodes).
  • The cancer is more than 4 mm thick and is ulcerated. It has also spread to no more than 3 lymph nodes, or small melanoma tumors are found in nearby skin or lymph vessels. (It might or might not have reached 1 nearby lymph node).
• Stage IIID. The cancer is more than 4 mm thick and is ulcerated. It has also spread to 4 or more nearby lymph nodes or a cluster of lymph nodes, or small melanoma tumors are found in nearby skin or lymph vessels. And it has spread to at least 2 nearby lymph nodes or lymph nodes that are clustered together.

Stage IV. The cancer has spread to distant lymph nodes, or other organs in the body, like the lungs, liver, or brain.