Stages of Merkel Cell Carcinoma

Reviewed by: HU Medical Review Board | Last reviewed: May 2017.

Cancer staging is a way of describing how extensive a cancer is. The stage is related to tumor size. It also relates to whether the cancer has spread from the original tumor to other parts of the body. The cancer stage helps you and your doctor to develop a treatment plan. It also provides information about survival.

Since 2010, Merkel cell carcinoma (MCC) has been staged using a system called TNM.1 This cancer staging system is used by most hospitals and medical systems.2 Once your doctor has categorized the T, N, and M, these values are combined to assign a cancer stage.

Before 2010, the criteria for other non-melanoma skin cancers were used.3 This was not ideal, because MCC has a different course than other skin cancers, such as basal cell carcinoma. Therefore, separate criteria for MCC were developed.

TNM staging system for MCC

T - Primary Tumor: Size of the tumor, presence of high-risk features, invasion of facial or skeletal structures
N - Regional Lymph Nodes: Presence, location, and number of lymph nodes with cancer cells and the size of the metastatic tumor
M - Metastasis: Spread of cancer to other parts of the body1

T Categories

The T categories describe the main tumor based on its size and the structures in has invaded.1 Your doctor begins to gather this information by doing a skin biopsy. Your doctor may do imaging tests as part of the initial work-up for MCC.

  • Size. The longest dimension of the tumor is measured in centimeters (cm).
  • Invasion. The tumor is categorized by whether it is invasive. The T4 category indicates that the tumor has invaded bone, muscle, fascia (the tissue around a muscle), or cartilage.

T Categories1

  • TX - Primary tumor cannot be assessed
  • T0 - No evidence of a primary tumor
  • Tis - Carcinoma in situ (the tumor is only in the epidermis)
  • T1 - Longest dimension is ≤ 2 cm
  • T2 - Longest dimension is >2 cm and ≤ 5 cm
  • T3 - Longest dimension is >5 cm
  • T4 - Tumor has invaded the bone, muscle, fascia, or cartilage.

N Categories

The N categories indicate whether cancer cells have spread to nearby lymph nodes.1

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N categories can be “clinically staged,” which means staged before lymph node biopsy or surgery. These stages are based on the physical examination and imaging tests.1

There are additional categories that can be used when your doctor has a tissue sample. Your doctor obtains tissue either with a needle biopsy or lymph node surgery.1 This is called “pathologic staging.” A “micrometastasis” is when the MCC is in the lymph nodes, but it is so small it can only been seen under the microscope. Micrometastasis is labeled with an “a.” The opposite is MCC in the lymph nodes that is big enough to be seen on imaging or felt during the physical exam. This is called “macrometastasis” and labeled with a “b.”

An “in-transit” metastasis is a lesion that is

  • Between the primary tumor and regional lymph nodes OR
  • Separate from the primary tumor and located further from your body’s core relative to the primary tumor. Another term for this is “distal” to the primary tumor.

N Categories

  • NX - Regional (nearby) lymph nodes cannot be assessed
  • N0 - No metastasis in nearby lymph nodes
  • cN0 - No sign of cancer in the lymph nodes observed during physical examination or on imaging tests
  • pN0 - A sample of tissue has been taken from the lymph nodes and no cancer was found
  • N1 - Cancer found in nearby lymph nodes
  • N1a - Micrometastasis
  • N1b - Macrometastasis
  • N2 - In-transit metastasis

M Categories1

The M categories indicate whether the cancer has spread (metastasized) to distant parts of the body.1 Simply, 0 means no metastasis and 1 means metastasis. Letters are added at the end to indicate the location of the metastasis.

M Categories

  • M0 - No distant metastasis
  • M1 - Metastasis beyond the regional (nearby) lymph nodes
  • M1a - Metastasis to the skin, tissues under the skin, or distant lymph nodes
  • M1b - Metastasis to the lung
  • M1c - Metastasis to any other internal organs

Putting it all together: assigning a stage

Once the cancer is characterized, the categories are combined to assign a stage.1 Generally speaking, the lower the stage, the better the chance for long-term cure and survival.

Anatomic Stage

T Cat
N Cat
M Cat
Stage 0
Stage IA
Stage IB
Stage IIA
Stage IIB
Stage IIC
Stage IIIA
Any T
Stage IIIB
Any T
Stage IV
Any T
Any N