How Merkel Cell Carcinoma Is Diagnosed

Merkel cell carcinoma is a rare type of skin cancer with a high risk for spreading (metastasizing) to other areas of the body. This aggressive form of cancer requires early diagnosis and treatment to be treated effectively. 

Merkel cell carcinoma usually appears as a single, painless lump. The diagnostic process involves a physical exam, a thorough medical history, a skin biopsy, and possibly imaging studies. Because Merkel cell carcinoma tends to metastasize quickly, early diagnosis is an important part of treatment. This article will describe the diagnostic tests used to detect Merkel cell carcinoma, as well as signs that you can look for at home. 

Dermatologist examining patient for signs of skin cancer

kali9 / Getty Images

Self-Checks/At-Home Testing

One of the best ways to catch skin cancer early is to perform regular self-checks. Each month, check your skin for new growths or sores and see your doctor if you are concerned. 

To spot Merkel cell carcinoma, use the AEIOU method recommended by the Skin Cancer Foundation, which is:

  • A for asymptomatic: The growth is usually not painful.
  • E for expanding: The growth expands quickly.
  • I for immunosuppressed: People with compromised immune systems are at higher risk.
  • O for older: Individuals over age 50 are at higher risk of developing Merkel cell carcinoma.
  • U for UV (ultraviolet) rays: A tumor is more likely to grow on an area of the body that receives the most sun exposure. 

Physical Examination

When you see your doctor, the doctor will start by performing a physical exam, closely inspecting any new skin findings and the surrounding skin. Your doctor may use a tool called a dermatoscope to check your skin. This tool has a special light and magnifying glass; it does not hurt. 

The doctor will also look at your lymph nodes and feel them to determine if they are enlarged. Merkel cell carcinoma can quickly spread to surrounding lymph nodes. Be sure to see your doctor for an annual skin check. During this exam, the doctor or nurse will examine your entire body to check for signs of skin cancer. 

In addition to the exam, your doctor will ask you questions about your skin, including:

  • When did you first notice this new growth?
  • Is it painful?
  • Has it changed recently?
  • Does it ever bleed or itch?

Labs and Tests

Once your doctor has examined your skin, the next step is to obtain a skin biopsy. During this procedure, your dermatologist (a specialist in conditions of the skin, hair, and nails) will remove a tissue sample from your skin. The sample will be sent to a pathologist (a doctor who examines body tissues), who will examine the cells under a microscope and look for signs of cancer. 

Before your doctor performs a skin biopsy, they will numb your skin with a local anesthetic. The numbing medicine is injected into your skin with a very fine needle. You will likely feel a small prick. Once the medicine is injected, it may sting a little at first. After that, your skin will feel numb, and you should not feel pain during the biopsy. 

The different types of skin biopsies include:

  • Shave biopsy: The dermatologist shaves off the top layers of your skin with a small surgical blade, then stops the bleeding in the office with topical medication or cauterization (electrical current). 
  • Punch biopsy: The dermatologist removes a deeper sample of skin with a tool that looks like a small round cookie cutter. They will then stitch the edges of skin back together to prevent bleeding. 
  • Incisional biopsy: The dermatologist uses a surgical knife to cut out part of the tumor and then uses stitches to close the wound. 
  • Excisional biopsy: The dermatologist uses a surgical knife to cut out the entire tumor. This method is commonly used in Merkel cell carcinoma because of the cancer's ability to spread quickly.

Because Merkel cell carcinoma can quickly spread to the lymph nodes, your doctor may recommend a lymph node biopsy as well. The different types of lymph node biopsies include:

  • Sentinel lymph node biopsy: This procedure determines which lymph node is the first one that developed. The doctor will inject dye into this primary tumor. The dye then flows through the lymph ducts to the nodes. The first lymph node to receive dye is removed, and a pathologist studies it under a microscope.
  • Lymph node dissection: During this procedure, lymph nodes are removed, and a sample of tissue is viewed under a microscope. Your doctor may remove some or all of the nodes in the tumor area.
  • Core needle biopsy: A tissue sample is removed using a wide needle that is then viewed under a microscope.
  • Fine needle aspiration: A tissue sample is removed using a thin needle and then viewed under a microscope.
  • Immunohistochemistry: This test is used when the pathologist cannot determine if there are cancer cells simply by examining the sample. This test looks for certain proteins found on cancer cells and is used to diagnose Merkel cell carcinoma. 

Imaging

Your doctor may recommend an imaging study to determine if cancerous cells have spread to other areas of your body. Possible imaging tests to expect include:

  • CT scan: Computed tomography (CT) shows where the cancer has spread and is often used to visualize the lymph nodes.
  • PET scan: Positron emission tomography (PET) can detect malignant cancer cells in the body. The technician will inject a small amount of radioactive glucose (sugar) into a vein. Because cancer cells use more glucose than healthy cells, the test will show where the cancer has spread. 

Staging

Once you have been diagnosed with Merkel cell carcinoma, the medical team will conduct testing to determine the stage of your cancer. Staging shows how advanced your cancer is and how much it has metastasized throughout the body. Merkel cell carcinoma stages include:

  • Stage 0: Also known as carcinoma in situ, stage 0 describes the finding of abnormal Merkel cells in the top layer of the skin. These cells are not yet cancerous but are at risk of becoming malignant.
  • Stage I: The cancerous tumor is 2 centimeters wide or smaller.
  • Stage IIA: The tumor is wider than 2 centimeters.
  • Stage IIB: The tumor has spread to nearby tissues.
  • Stage IIIA: The tumor has spread to nearby tissues, and a lymph node can be felt during the physical exam.
  • Stage IIIB: The tumor has spread to nearby tissues, a lymph node can be felt during the physical exam, and cancer cells are found in the lymph node.
  • Stage IV: The tumor has spread to distant areas of the body, including the liver, lungs, bones, or brain.

Merkel cell carcinoma can also be graded using the TNM system. This method includes three criteria:

  • T for tumor: What is the size of the original tumor and what is its growth rate?
  • N for nodes: Have cancer cells spread to the lymph nodes?
  • M for metastasis: Has the cancer spread o distant lymph nodes and other organs?

Differential Diagnosis

Diagnosing Merkel cell carcinoma can be very challenging. The tumor often appears as a benign (harmless) skin condition like a pimple or insect bite. The cancerous Merkel cells also spread quickly, which means that by the time the cancer is diagnosed, it may have already spread to other parts of the body. 

According to the Skin Cancer Foundation, 56% of Merkel cell carcinoma tumors are at first misdiagnosed as benign conditions by physicians. In fact, a 2017 study found that by the time patients are diagnosed with Merkel cell carcinoma, 30% have already experienced metastasis. 

Recap

Merkel cell carcinoma can be challenging to diagnose because it often appears as a benign skin condition like a pimple or insect bite. 

Summary

Merkel cell carcinoma is a rare and aggressive form of skin cancer. Early diagnosis and treatment can improve the prognosis. Merkel cell carcinoma is diagnosed with a physical exam, lab tests, and imaging studies. Lab tests to expect include a skin biopsy and lymph node biopsy. A CT scan or PET scan may also be performed to determine if cancer has spread or metastasized. 

A Word From Verywell

If you suspect that you have Merkel cell carcinoma, the first step is to talk with your doctor. This is an overwhelming diagnosis, and your dermatologist will be able to walk you through the process. Expect to undergo a physical exam, lab tests, biopsies, and imaging studies. 

Once you are diagnosed, your dermatologist will determine your cancer’s stage and then make a treatment plan. 

Frequently Asked Questions

Frequently Asked Questions

  • Can Merkel cell carcinoma be prevented?

    Merkel cell carcinoma cannot be prevented, but you can reduce your risk. Exposure to ultraviolet (UV) rays increases your risk, so always protect your skin in the sun with sunscreen, lightweight clothing, and a wide-brimmed hat. Never use an indoor tanning bed. 

  • Does Merkel cell carcinoma hurt?

    Merkel cell carcinoma is usually painless, but the tumor may feel tender or sore when pressed on. As the tumor grows, it may open and bleed. 

  • How is Merkel cell carcinoma diagnosed?

    Merkel cell carcinoma is usually diagnosed with a physical exam, a detailed history, and a skin biopsy. Your doctor may also recommend imaging studies to determine if the cancerous cells have spread beyond the primary tumor. 

Was this page helpful?
9 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. Ramahi E, Choi J, Fuller CD, Eng TY. Merkel cell carcinoma. Am J Clin Oncol. 2013;36(3):299-309. doi:10.1097/COC.0b013e318210f83c

  2. Skin Cancer Foundation. Merkel cell carcinoma

  3. American Cancer Society. What is Merkel cell carcinoma?

  4. Skin Cancer Foundation. Merkel cell carcinoma warning signs and images.

  5. American Academy of Dermatology Association. Skin cancer types: Merkel cell carcinoma diagnosis & treatment.

  6. American Cancer Society. Merkel cell skin cancer early detection, diagnosis, and staging.

  7. National Cancer Institute. Merkel cell carcinoma treatment

  8. Skin Cancer Foundation. Merkel cell carcinoma stages

  9. Becker JC, Stang A, DeCaprio JA, Cerroni L, Lebbé C, Veness M, Nghiem P. Merkel cell carcinoma. Nat Rev Dis Primers. 2017 Oct 26;3:17077. doi:10.1038/nrdp.2017.77