How is Merkel cell carcinoma diagnosed?
Most skin cancers, including Merkel cell carcinoma (MCC), are brought to a doctor’s attention because of signs or symptoms a person is having.
If you have an abnormal area of skin that might be skin cancer, your doctor will do exams and tests to find out if it’s cancer or some other skin condition. If there’s a chance the skin cancer might have spread to other areas of the body, other tests may be done as well.
Medical history and physical exam
Usually the first step is for your doctor to take your medical history. The doctor will ask when the mark on the skin first appeared, if it has changed in size or appearance, and if it is causing any symptoms (pain, itching, bleeding, etc.). You may also be asked about your possible risk factors for skin cancer (including sun exposure and immune system problems) and if you or anyone in your family has had skin cancer.
During the physical exam, the doctor will note the size, shape, color, and texture of the area(s) in question, and whether they are bleeding, oozing, or crusting. The rest of your body may be checked for spots that could be related to skin cancer.
The doctor may also feel the nearby lymph nodes, which are bean-sized collections of immune system cells under the skin in certain areas. Merkel cell carcinomas (and some other skin cancers) can spread to lymph nodes. When this happens, the lymph nodes can become larger and might be felt as lumps under the skin.
If you are being seen by your primary doctor and skin cancer is suspected, you may be referred to a dermatologist (a doctor who specializes in skin diseases), who will look at the area more closely.
Along with a standard physical exam, some dermatologists use a technique called dermatoscopy (also known as dermoscopy, epiluminescence microscopy [ELM] or surface microscopy) to see spots on the skin more clearly. The doctor uses a dermatoscope, which is a special magnifying lens and light source held near the skin. Sometimes a thin layer of alcohol or oil is used with this instrument. The doctor may take a digital photo of the spot.
If the doctor thinks that a suspicious area might be MCC (or another type of skin cancer), he or she will remove a piece of skin from the area and send it to a lab to have it looked at with a microscope. This is called a skin biopsy.
There are different ways to do a skin biopsy. The doctor will choose one based on the suspected type of skin cancer, where it is on your body, its size, and other factors. Any biopsy is likely to leave at least a small scar. Different methods can result in different scars, so ask your doctor about possible scarring before the biopsy is done.
Skin biopsies are done using a local anesthetic (numbing medicine), which is injected into the area with a very small needle. You will probably feel a small prick and a little stinging as the medicine is injected, but you should not feel any pain during the biopsy.
Shave (tangential) biopsy
For this type of biopsy, the doctor shaves off the top layers of the skin with a small surgical blade. Usually just the epidermis and the outer part of the dermis are removed, although deeper layers can be taken if needed. Bleeding from the biopsy site is then stopped by applying an ointment, a chemical that stops bleeding, or by using a small electrical current to cauterize the wound.
A shave biopsy is useful in diagnosing many types of skin diseases, especially if the doctor thinks an abnormal area is unlikely to be a serious skin cancer such as MCC or melanoma. A thin shave biopsy is generally not used if the doctor strongly suspects MCC (or melanoma), because the biopsy often does not go deep enough to get below the tumor. On the other hand, a deeper shave biopsy can be useful if done properly.
For a punch biopsy, the doctor uses a tool that looks like a tiny round cookie cutter to remove a deeper sample of skin. The doctor rotates the punch biopsy tool on the skin until it cuts through all the layers of the skin, including the dermis, epidermis, and the upper parts of the subcutis. The sample is removed and the edges of the biopsy site are often stitched together.
Incisional and excisional biopsies
To examine a tumor that might have grown into deeper layers of the skin, the doctor may use an incisional or excisional biopsy. For these types of biopsies, a surgical knife is used to cut through the full thickness of skin. A wedge or sliver of skin is removed for examination, and the edges of the wound are usually stitched together.
An incisional biopsy removes only a portion of the tumor. An excisional biopsy removes the entire tumor, and is usually the preferred method of biopsy for a suspected MCC if it can be done. But it’s not always possible, so other types of biopsies may be needed.
Lymph node biopsy
MCC often spreads to nearby lymph nodes early in the course of the disease, so it’s very important for the doctor to find out if these nodes contain cancer cells. If MCC has already been diagnosed on the skin, nearby lymph nodes will usually be biopsied to see if the cancer has spread to them.
The type of biopsy used depends on how likely it is that the cancer has reached the nearby lymph nodes. If the nearby lymph nodes look normal on physical exams and imaging tests, a sentinel lymph node biopsy is likely to be done. But if exams or imaging tests suggest that nearby lymph nodes might contain cancer (for example, if the nodes are larger than normal), then a needle biopsy is more likely to be done.
Sentinel lymph node biopsy (SLNB)
A sentinel lymph node biopsy can be used to find the lymph nodes that are likely to be the first place the MCC would go if it has spread. These lymph nodes are called sentinel nodes.
To find the sentinel lymph node (or nodes), the doctor injects a small amount of a radioactive substance into the area of the cancer. After waiting an hour or so for the substance to travel to the lymph node areas near the tumor, the doctor uses a special camera to “see” the radioactive substance where it collects in one or more sentinel lymph nodes. Once the radioactive area has been marked, the patient is taken to where the surgery will be done, and a blue dye is injected in the same place as the radioactive substance. A small incision is then made where the doctor has marked, and the lymph nodes are then checked to find which one(s) became radioactive and turned blue. These sentinel nodes are removed and looked at with a microscope for cancer cells.
If a lymph node near a Merkel cell carcinoma is abnormally large, a sentinel node biopsy is not usually needed. The enlarged node is simply biopsied with a needle instead.
A needle biopsy can be used for large lymph nodes near a tumor to find out if the cancer has spread to them. Needle biopsies are not as invasive as some other types of biopsies, but they may not always provide enough of a sample to find cancer cells.
There are 2 main types of needle biopsies.
- In a fine needle aspiration (FNA) biopsy, the doctor uses a syringe with a very thin, hollow needle (thinner than the ones used for blood tests) to withdraw (aspirate) cells and small pieces of tissue.
- In a core biopsy, a larger needle is used to remove one or more small cylinders (cores) of tissue. Core biopsies remove a larger sample than FNA biopsies.
With either type of biopsy, a local anesthetic is sometimes used to numb the area first. These biopsies rarely cause much discomfort and do not usually leave a scar.
If the lymph node is just under the skin, the doctor can often feel it well enough to guide the needle into it. For a suspicious lymph node deeper in the body, an imaging test such as ultrasound or a CT scan is often used to guide the needle into place.
Surgical (excisional) lymph node biopsy
This type of biopsy might be done if a lymph node’s size suggests the cancer has spread there but a needle biopsy of the node has not been done (or if it did not find any cancer cells, but the doctor still suspects the cancer has spread there).
In this procedure, the doctor removes an enlarged lymph node through a small incision (cut) in the skin. This can often be done in a doctor’s office or outpatient surgical center. A local anesthetic (numbing medicine) is generally used if the lymph node is near the surface of the body, but a person may need to be sedated or even asleep (using general anesthesia) if the lymph node is deeper in the body.
Lab tests of biopsy samples
Samples from any biopsies you have will be sent to a lab, where a pathologist (a doctor who is specially trained to diagnose disease) will look at them under a microscope for MCC (or other types of cancer). Often, skin samples are sent to a dermatopathologist, a doctor who has special training in looking at skin samples.
If the doctor can’t tell for sure if the sample contains MCC just by looking at it, special tests may be done on the cells to try to confirm the diagnosis.
The diagnosis of MCC is usually confirmed using a test called immunohistochemistry (IHC). For this test, very thin slices of the sample are attached to glass microscope slides. The samples are then treated with special proteins (antibodies) designed to attach only to a specific substance found in certain cancer cells. (The most important of these for MCC is called CK-20.) If the patient’s cancer cells contain that substance, the antibody will attach to the cells. Chemicals are then added so that antibodies attached to the cells change color, which can be seen under a microscope.
If MCC is found, the pathologist will also look at certain important features such as the tumor thickness, mitotic rate (the portion of cells that are actively dividing), and whether it has invaded the tiny blood vessels or lymph vessels in the sample. These features could help determine a person’s prognosis (outlook).
Imaging tests use x-rays, magnetic fields, or radioactive substances to create pictures of the inside of the body. They are often used to see if MCC has spread to lymph nodes or other organs in the body.
Imaging tests can also be done to help determine how well treatment is working or to look for possible signs of cancer coming back (recurring) after treatment.
Computed tomography (CT) scan
The CT scan uses x-rays to make detailed, cross-sectional images of your body. Unlike a regular x-ray, CT scans can show the detail in soft tissues (such as internal organs). This test can help tell if any lymph nodes are enlarged or if other organs have suspicious spots, which might be due to the spread of MCC.
Instead of taking one picture, like a regular x-ray, a CT scanner takes many pictures as it rotates around you while you lie on a table. A computer then combines these pictures into detailed images of the part of your body that is being studied.
Before the scan, you may be asked to drink a contrast solution and/or get an intravenous (IV) injection of a contrast dye that helps better outline normal and abnormal areas in the body. You may need an IV line through which the contrast dye is injected. The injection can cause some flushing (a feeling of warmth, especially in the face). Some people are allergic and get hives or, rarely, more serious reactions like trouble breathing and low blood pressure. Be sure to tell the doctor if you have any allergies (especially to iodine or shellfish) or have ever had a reaction to any contrast material used for x-rays.
A CT scanner has been described as a large donut, with a narrow table that slides in and out of the middle opening. You need to lie still on the table while the scan is being done. CT scans take longer than regular x-rays, and you might feel a bit confined by the ring while the pictures are being taken.
CT-guided needle biopsy: CT scans can also be used to help guide a biopsy needle into a suspicious area within the body. For this procedure, you stay on the CT scanning table while the doctor moves a biopsy needle through the skin and toward the suspicious area. CT scans are repeated until the needle is in the mass. A needle biopsy sample is then removed and looked at under a microscope.
Magnetic resonance imaging (MRI) scan
Like CT scans, MRI scans give detailed images of soft tissues in the body. But MRI scans use radio waves and strong magnets instead of x-rays to create pictures. A contrast material might be injected, just as with CT scans, but is used less often.
This test can help tell if any lymph nodes are enlarged or if other organs have suspicious spots, which might be due to the spread of MCC. MRI scans are also very helpful in looking at the brain and spinal cord.
MRI scans take longer than CT scans – often up to an hour – and are a little more uncomfortable. You may have to lie inside a narrow tube, which is confining and can upset people who are afraid of enclosed spaces. Newer, more open MRI machines can sometimes be used instead, but the images might not be as sharp in some cases. The MRI machine also makes loud buzzing and clicking noises, so some centers provide earplugs to help block this noise out.
Positron emission tomography (PET) scan
A PET scan can help show if the cancer has spread to lymph nodes or other parts of the body. This test looks for areas where cells are growing quickly (which might be a sign of cancer), rather than just showing if areas look abnormal based on their size or shape.
For this test, you are injected with a radioactive substance (usually a type of sugar known as FDG). The amount of radioactivity used is very low and will pass out of your body over the next day or so. Because cancer cells in the body tend to grow faster than other cells, they absorb more of the radioactive sugar. After about an hour, you are moved onto a table in the PET scanner. You lie on the table for about 30 minutes while a special camera creates a picture of areas of radioactivity in the body. The picture is not as detailed as a CT or MRI scan, but it can provide helpful information about cancer in your whole body.
Many centers have special machines that can do both a PET and CT scan at the same time (PET/CT scan). This lets the doctor compare areas of higher radioactivity on the PET scan with the more detailed appearance of that area on the CT scan.
For more information on these imaging tests, see Imaging (Radiology) Tests.
Last Medical Review: 04/13/2015
Last Revised: 02/09/2016