Understand melanoma

Melanoma is a rare form of skin cancer that can be dangerous because it’s more likely
to invade nearby tissues and spread to other parts of the body than other types of
skin cancers.

The information below is primarily based on information originally published by the National Cancer Institute (NCI), the US government’s main agency for cancer research. It may help you gain a better understanding of your melanoma diagnosis and what’s happening to your body. This may help when making important decisions with your doctor.

What is melanoma?

Melanoma is a disease in which malignant (cancer) cells form in melanocytes (cells that color the skin). The number of new cases of melanoma has been increasing over the last 30 years.

Melanoma can occur anywhere on the skin. In men, melanoma is often found on the trunk (the area from the shoulders to the hips) or the head and neck. In women, melanoma forms most often on the arms and legs.

Unusual moles, exposure to sunlight, and health history can affect the risk of melanoma. Being white or having a fair complexion increases the risk of melanoma, but anyone can have melanoma, including people with dark skin.

Stages of melanoma

Melanoma is categorized in 5 stages ranging from 0 to IV (4), with stage IV (4) being the most severe. The stages can also be broken down further into substages (A, B, C, or D).

Stage 0 melanoma

In stage 0, abnormal melanocytes are found in the epidermis. These abnormal melanocytes may become cancer and spread into nearby normal tissue. Stage 0 is also called melanoma in situ.

Stage I (1) melanoma

In stage I, cancer has formed. Stage I is divided into stages IA and IB.


Stage IA

  • The tumor is not more than 1 millimeter thick, with or without ulceration.

Stage IB

  • The tumor is more than 1 but not more than 2 millimeters thick, without ulceration.

Stage II (2) melanoma

Stage II is divided into stages IIA, IIB, and IIC.

Stage IIA

  • The tumor is more than 1 but not more than 2 millimeters thick, with ulceration.

Or stage IIA can be

  • The tumor is more than 2 but not more than 4 millimeters thick, without ulceration.

Stage IIB

  • The tumor is more than 2 but not more than 4 millimeters thick, with ulceration.

Or stage IIB can be

  • The tumor is more than 4 millimeters thick, without ulceration.

Stage IIC

  • The tumor is more than 4 millimeters thick, with ulceration.

Stage III (3) melanoma

Stage III is divided into stages IIIA, IIIB, IIIC, and IIID.

Stage IIIA

  • The tumor is not more than 1 millimeter thick, with ulceration, or not more than 2 millimeters thick, without ulceration. Cancer is found in 1 to 3 lymph nodes by sentinel lymph node biopsy.

Stage IIIB

  • (1) It is not known where the cancer began or the primary tumor can no longer be seen, and one of the following is true:
    • cancer is found in 1 lymph node by physical exam or imaging tests; or
    • there are microsatellite tumors, satellite tumors, and/or in-transit metastases on or under the skin.

Or stage IIIB can be

  • (2) The tumor is not more than 1 millimeter thick, with ulceration, or not more than 2 millimeters thick, without ulceration, and one of the following is true:
    • cancer is found in 1 to 3 lymph nodes by physical exam or imaging tests; or
    • there are microsatellite tumors, satellite tumors, and/or in-transit metastases on or under the skin.

Or stage IIIB can be

  • (3) The tumor is more than 1 but not more than 2 millimeters thick, with ulceration, or more than 2 but not more than 4 millimeters thick, without ulceration, and one of the following is true:
    • cancer is found in 1 to 3 lymph nodes; or
    • there are microsatellite tumors, satellite tumors, and/or in-transit metastases on or under the skin.

Stage IIIC

  • (1) It is not known where the cancer began, or the primary tumor can no longer be seen. Cancer is found:
    • in 2 or 3 lymph nodes by physical exam or radiologic imaging; or
    • in 1 lymph node and there are microsatellite tumors, satellite tumors, and/or in-transit metastases on or under the skin; or
    • in 4 or more lymph nodes, or in any number of lymph nodes that are matted (clumped) together found by physical exam or radiologic imaging; or
    • in 2 or more lymph nodes and/or in any number of lymph nodes that are matted together; and there are microsatellite tumors, satellite tumors, and/or in-transit metastases on or under the skin.

Or stage IIIC can be

  • (2) The tumor is not more than 2 millimeters thick, with or without ulceration, or not more than 4 millimeters thick, without ulceration. Cancer is found:
    • in 1 lymph node and there are microsatellite tumors, satellite tumors, and/or in-transit metastases on or under the skin; or
    • in 4 or more lymph nodes, or in any number of lymph nodes that are matted together; or
    • in 2 or more lymph nodes and/or in any number of lymph nodes that are matted together; and there are microsatellite tumors, satellite tumors, and/or in-transit metastases on or under the skin.

Or stage IIIC can be

  • (3) The tumor is more than 2 but not more than 4 millimeters thick, with ulceration, or more than 4 millimeters thick, without ulceration. Cancer is found in 1 or more lymph nodes and/or in any number of lymph nodes that are matted together; and there are microsatellite tumors, satellite tumors, and/or in-transit metastases on or under the skin.

Or stage IIIC can be

  • (4) The tumor is more than 4 millimeters thick, with ulceration, and:
    • the cancer is found in 1 to 3 nearby lymph nodes, which are not matted together, or
    • there are microsatellite tumors, satellite tumors, and/or in-transit metastases on or under the skin. It may or may not have reached 1 nearby lymph node.

Stage IIID

  • The tumor is more than 4 millimeters thick, with ulceration. Cancer is found:
    • in 4 or more lymph nodes, or in any number of lymph nodes that are matted together; or
    • in 2 or more lymph nodes and/or in any number of lymph nodes that are matted together; and there are microsatellite tumors, satellite tumors, and/or in-transit metastases on or under the skin.

Learn about a possible treatment option that may help prevent melanoma from coming back after surgery to remove the melanoma and the lymph nodes that contain cancer. This may also be a treatment option for certain patients with advanced melanoma that has spread or can’t be removed by surgery.

Stage IV (4) melanoma

In stage IV, the cancer has spread to other parts of the body, such as the lung, liver, brain, spinal cord, bone, soft tissue (including muscle), gastrointestinal (GI) tract, and/or distant lymph nodes. Cancer may have spread to places in the skin far away from where it first started.

Learn about a possible treatment option for certain patients with advanced melanoma that has spread or can’t be removed by surgery.