Merkel cell carcinoma

This page was reviewed under our medical and editorial policy by

Maurie Markman, MD, President, Medicine & Science

This page was updated on June 1, 2022.

Merkel cell carcinoma is a rare but aggressive type of skin cancer.

It’s also known as:

  • Trabecular carcinoma of the skin
  • Cutaneous neuroendocrine carcinoma
  • Primary small cell carcinoma of the skin
  • Toker tumor
  • Malignant trichodiscoma

It’s the second most common cause of skin cancer-related death after melanoma.

Merkel cell carcinoma originates from Merkel cells that have become cancerous. Merkel cells, which are skin cells that convey the sensation of touch, are located at the junction of two skin layers, the dermis and epidermis.

People at high risk of developing Merkel cell carcinoma are those who:

The rates of Merkel cell carcinoma are higher in white and male patients. Merkel cell carcinoma can spread easily to another region of the body and has high recurrence rates—with one review finding that about 30 percent of patients whose cancers were caught in early or mid stages had a recurrence on the skin, according to the National Cancer Institute (NCI).

Although Merkel cell carcinoma is still considered a rare cancer, the number of new cases has dramatically increased since the early 2000s, likely due to both improvements in detection and diagnosis and an aging U.S. population. Seniors are more susceptible to development of Merkel cell carcinoma. The median age at diagnosis is 75 to 79 years old. In addition, increased sun exposure due to a depleted ozone layer may also drive the Merkel cell carcinoma rates up.

This article will cover:

Merkel cell carcinoma causes

About 80 percent of Merkel cell carcinoma cases are caused by a skin virus called Merkel cell polyomavirus (MCPyV), and the rest likely by extensive UV damage.

Although MCPyV infection is common among adults, not every individual infected with MCPyV develops Merkel cell carcinoma. Most healthy people can control MCPyV. However, immunocompromised individuals—such as seniors or patients with human immunodeficiency virus (HIV) or other diseases affecting the immune system—may lose control of the virus. A weakened immune system may lead to virus activation, which in turn may transform normal skin cells into cancerous ones.

Merkel cell carcinoma symptoms

Merkel cell carcinoma most often appears on sun-exposed parts of the body, such as the:

  • Face
  • Neck
  • Upper extremities

And to a lesser extent on the:

  • Lower extremities
  • Trunk

Very rarely, Merkel cell carcinoma can develop inside the body (for example, in the nose or esophagus).

Merkel cell carcinoma appears as a painless but fast-growing lump on sun-exposed skin, and it can range in color from reddish to deep purple. Its initial appearance is unremarkable, but the rapid growth of the primary lesion may send patients to seek medical help. Merkel cell carcinoma often forms additional lesions on the skin next to the primary tumor site, called satellite lesions.

Merkel cell carcinoma diagnosis

During a medical visit, an oncologist will physically examine (and may photograph) the patient’s primary tumor. The oncologist may examine lymph nodes that are close to the site of the tumor with his or her hands, as well as the entire skin surface. This is important, as Merkel cell carcinoma often generates satellite lesions and Merkel cell carcinoma patients often develop other types of skin cancers.

A biopsy of the tumor may also be performed. The goal of the biopsy procedure is to excise the entire tumor surrounded by the healthy tissue to ensure that no tumor tissue is left. This is referred to as a clean margin biopsy.

Sometimes this isn’t possible because the tumor has grown too large or it’s located on the face, where complete removal would lead to significant disfigurement. In that case, the oncologist may perform a smaller biopsy to get a tumor sample for further analysis instead. The biopsied sample is sent to a pathologist, who will study it under a microscope, using specific dyes to look at the shape, size and appearance of cells.

In order to differentiate from other types of skin tumors—such as melanomas, skin lymphomas or small-cell lung cancers that have metastasized into skin—the pathologist uses a technique called immunohistochemistry to detect the presence of proteins that are characteristic of Merkel cell carcinoma.

The tumor biopsy results will help determine accurate tumor staging, which is used to determine how much a tumor has developed within the body. This information helps the oncologist understand the possible future behavior of a tumor and the patient’s prognosis.

If Merkel cell carcinoma is confirmed, the doctor may suggest additional tests to determine whether it has spread away from the original site. A care team may recommend a procedure called sentinel lymph node biopsy (SLNB) to surgically remove the lymph node closest to the primary tumor site. After this surgical removal, the pathologist can examine the lymph node for the presence of cancerous cells. SLNB is an important procedure used to determine how far a tumor has progressed.

If cancerous cells are detected in the lymph nodes near the primary tumor site, the oncologist may recommend diagnostic imaging tests, such as a computed tomography (CT) scan or magnetic resonance imaging (MRI), to see whether cancer has metastasized (spread) throughout the body.

Merkel cell carcinoma is most likely to spread to distant lymph nodes or other skin areas, as well as to the lungs, brain, bones, or other organs.

Merkel cell carcinoma stages

The comprehensive diagnostic approach described above helps oncologists accurately stage Merkel cell carcinoma. Current Merkel cell carcinoma staging was developed by the American Joint Committee on Cancer (AJCC), and it divides this disease into seven categories (0, 1, 2a, 2b, 3a, 3b and 4).

Staging is defined by the TNM factors:

  • T (tumor size)
  • N (node)
  • M (metastasis)

The AJCC classification is as follows:

Stage 0

Can include: Tis, N0, M0

Stage 0 Merkel cell carcinoma means:

  • The tumor is considered in situ (still at its primary location).
  • There are no regional lymph nodes involved.
  • There is no metastasis.

Stage 1

Can include: T1, N0, M0

Stage 1 Merkel cell carcinoma means:

  • The maximum tumor diameter measures 2 cm or smaller.
  • There are no regional lymph nodes involved.
  • There is no metastasis.

Stage 2a

Can include: T2-T3, N0, M0

Stage 2a Merkel cell carcinoma means:

  • The tumor is larger than 2 cm. T
  • There are no regional lymph nodes involved.
  • There is no metastasis.

Stage 2b

Can include: T4, N0, M0

Stage 2b Merkel cell carcinoma means:

  • The primary tumor has spread to underlying tissue such as cartilage, muscle and bone.
  • There are no regional lymph nodes involved.
  • There is no metastasis.

Stage 3a

Can include: T0-T4, N1a-b, M0

Stage 3a Merkel cell carcinoma means:

  • The tumor size includes all of the sizes described for previous categories.
  • Metastasis is detected in regional lymph nodes and tissues.
  • No distant metastasis is detected.

Stage 3b

Can include: T1-4, N1b-N3, M0

Stage 3 Merkel cell carcinoma means:

  • The tumor size includes all of the sizes described for previous categories.
  • Metastasis is detected in regional lymph nodes or in transit toward the lymph node.
  • No distant metastasis is detected.

Stage 4

Can include: T0-T4, any N, M1

Stage 4 Merkel cell carcinoma means:

  • The tumor size includes all of the sizes described for previous categories.
  • Lymph node status includes all described for previous categories.
  • Metastasis is detected in distant sites in the body.

Merkel cell carcinoma treatment

Early diagnosis is critical for better outcomes, and treatment options depend on the stage of the Merkel cell carcinoma at the time of diagnosis.

  • For stages 1, 2 and 3, the first line of therapy is surgery of the primary tumor with wide excision to ensure complete removal of cancerous tissue and to help reduce the chance of recurrence at the original site. If cancer cells were found in the lymph nodes, the oncologist may recommend removal of lymph nodes to help reduce the chances of cancer returning.
  • In addition to surgical treatments, doctors may recommend radiation therapy of the primary tumor site and nearby lymph nodes. This is especially the case when it isn’t possible to achieve complete surgical removal of the tumor or when Merkel cell carcinoma has already spread into lymph nodes.
  • For some patients who have inoperable tumors due to the size and/or location, or who are not candidates for surgery for other medical reasons, radiation therapy may be the only option for local control of Merkel cell carcinoma.
  • Patients who meet certain criteria may be eligible to participate in clinical trials studying potential new treatment options.

For patients with metastatic disease (stage 4 cancer), immune-based therapies are offering promising evidence-informed treatment options. Immune-based therapies harvest the potential of the body’s immune system to attack cancer cells. Bavencio® (avelumab), a drug developed as part of immune-based therapy, is approved by the U.S. Food and Drug Administration as therapy for metastatic Merkel cell carcinoma.

Merkel cell carcinoma survival rates

The prognosis for Merkel cell carcinoma patients is influenced by many factors, such as:

  • How much cancer is in the body (tumor load)
  • The presence or absence of metastasis

The NCI's Surveillance, Epidemiology, and End Results (SEER) Program tracks cancer survival rates.

According to the SEER database, the overall five-year relative survival rate for Merkel cell carcinoma is 64 percent.

The detailed breakdown for five-year relative survival rates is based on the stage of the disease at the time of diagnosis.

  • Localized (Merkel cell carcinoma that was caught early before spreading): 76 percent
  • Regional (patients with spread to nearby tissues or lymph nodes): 56 percent
  • Distant (patients with cancer that has spread to distant parts of the body): 23 percent

It’s important to note that these numbers are estimates and that new treatments may have emerged, providing a better outlook. Other factors, such as the patient’s age and overall health, also may play a role. Early detection is a key factor in the prognosis of many cancers. That’s why regular checkups with a dermatologist can be critical for finding and treating Merkel cell carcinoma.

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