Understanding the stages of Merkel cell carcinoma (MCC) helps you and your physician know how far the cancer has grown or spread. These stages also guide decisions about the treatment options your physician gives you. Because MCC is aggressive and can spread quickly, accurate staging is important.
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How Staging Works
The most widely used system in the United States is the TNM (Tumor, Node, Metastasis) staging system. It is based on these three factors:
- T = Tumor: The main (primary) skin tumor is checked for size and to see if it grows into nearby tissues
- N = Nodes: The physician tests to see if the cancer has reached any nearby (sometimes called “regional”) lymph nodes
- M = Metastasis: Cancer cells are also tested to find out if the disease has spread (metastasized) to organs or distant parts of the body beyond nearby lymph nodes
Once this information is known, the disease can be staged as 0, I, II, III or IV.
Stage 0: Cancer in the Skin Only (Carcinoma in Situ)
Stage 0, or “in situ,” means abnormal Merkel cells are found only in the top layer of skin. They have not invaded deeper tissue and have not spread.
Stage I (1): Small Tumor, No Node or Distant Spread
A Stage I tumor is 2 centimeters or smaller. Importantly, stage I also means there is no evidence of cancer in the lymph nodes or distant parts of the body.
Stage II (2): Larger Tumor, Still Local
Stage II means there is a tumor bigger than 2 centimeters. However, like stage I, the cancer still does not show any evidence of spreading to lymph nodes or other parts of the body. Sometimes, physicians further divide stage II into two more categories, IIA and IIB:
- Stage IIA: The tumor is bigger than 2 centimeters, but it has not invaded deep tissues like the muscles or cartilage
- Stage IIB: The tumor has grown into deeper connective tissue, muscle, cartilage or bone, but still has not spread to the lymph nodes or distant parts of the body
Stage III (3): Spread to Nearby Lymph Nodes
Stage III means the cancer has spread to lymph nodes or tissues near the original tumor. It is sometimes further divided as follows:
- Stage IIIA: Cancer may be found in the lymph nodes, but it can only be detected through a biopsy and is not visible on an imaging test
- Stage IIIB: The cancer includes the lymph nodes and can be seen or felt without the need for an imaging test. It also may have spread to nearby connective tissue, bone, muscle or cartilage.
Stage IV (4): Distant Metastasis
Stage IV means MCC has metastasized (spread) beyond the nearby lymph nodes to distant organs or areas of the body, such as the lungs, liver, bones or the brain.
Why Staging Matters
Staging isn’t just a label. It helps your physician better understand your health and plan the best treatment for you. If your MCC is in one of the lower stages (when the tumor is small and the cancer has not spread), generally you will have better outcomes.
One of the most promising emerging treatment options for MCC is a class of drugs called immune checkpoint inhibitors (ICIs), which are a type of immunotherapy. Immunotherapy harnesses the body’s own immune system to fight cancer. Learn more about Merkel cell carcinoma immune checkpoint inhibitors.