Neuroendocrine carcinoma of the skin, also known as Merkel cell carcinoma, is a rare and aggressive form of skin cancer that requires careful and timely diagnosis to guide treatment decisions and improve outcomes for patients.

Introduction: Who Should Seek Diagnostic Testing

If you notice a new lump or bump on your skin that grows quickly, especially on areas exposed to the sun like your face, head, neck, or arms, it is important to see a doctor as soon as possible. Neuroendocrine carcinoma of the skin, often called Merkel cell carcinoma, is a very rare but serious type of skin cancer that forms when certain specialized cells in the skin begin growing out of control.^[3]

This cancer most often affects people over the age of 50, particularly those with fair skin who have had significant sun exposure throughout their lives. Men are diagnosed more often than women. People who have weakened immune systems, such as those who have had organ transplants or live with conditions like HIV, are at higher risk and should be especially watchful for unusual skin changes.^[3][11]

The first sign is usually a single, painless bump on the skin. This growth tends to appear suddenly and grow rapidly over a period of weeks or months. The bump is often firm and may look pink, red, purple, or the same color as the surrounding skin. Because the growth is usually painless, people sometimes delay seeking medical attention, thinking it is harmless. However, early diagnosis is critical because this cancer can spread quickly to nearby lymph nodes and then to other parts of the body.^[3][4]

Anyone who notices a rapidly growing skin lump, especially in sun-exposed areas, should seek medical evaluation without delay. This is particularly important for older adults, people with a history of extensive sun exposure, and those with weakened immune systems. Even if you are unsure whether the change is significant, it is always better to have a healthcare professional examine it. Early detection can make a meaningful difference in treatment options and outcomes.

Classic Diagnostic Methods

Diagnosing neuroendocrine carcinoma of the skin begins with a careful examination of the skin and a review of your medical history. Your doctor will ask about when the lump first appeared, how quickly it has grown, and whether you have any other symptoms. They will also ask about your history of sun exposure, any previous skin cancers, and whether you have conditions that weaken your immune system.^[11]

A full-body skin examination is an important part of the diagnostic process. During this exam, the doctor carefully checks your entire skin surface for any unusual bumps, spots, or areas that look abnormal in color, size, shape, or texture. The doctor will also feel the lymph nodes in areas near the lump, such as the neck, armpits, or groin, to check for swelling or firmness that might suggest the cancer has spread.^[11]

The most important step in diagnosing Merkel cell carcinoma is a skin biopsy. This procedure involves removing a small piece of the suspicious skin growth so it can be examined under a microscope. The biopsy is usually done in the doctor’s office using local anesthesia, which numbs the area so you do not feel pain during the procedure. The tissue sample is then sent to a laboratory where a specialist called a pathologist examines it closely.^[10][11]

Under the microscope, Merkel cell carcinoma has a very distinctive appearance. The cancer cells are small, round, and packed tightly together, with a characteristic pattern in how their genetic material (chromatin) is arranged, often described as having a “salt and pepper” look. The tumor is usually located in the deeper layers of the skin, in an area called the dermis, and often extends down into the fatty tissue beneath. Importantly, the tumor typically does not connect with the outermost layer of skin, the epidermis.^[4][6]

To confirm the diagnosis, pathologists use a technique called immunohistochemistry. This involves applying special stains to the biopsy tissue to detect specific proteins that are characteristic of Merkel cell carcinoma. The tumor cells usually test positive for proteins that are found in both nerve cells and epithelial cells, reflecting the unique nature of neuroendocrine cells. One of the most helpful markers is a protein called cytokeratin 20, which shows a distinctive dot-like pattern inside the cancer cells in more than 90% of cases. Other markers, such as synaptophysin and chromogranin, help confirm the neuroendocrine nature of the tumor.^[4][6]

In some cases, doctors may also use electron microscopy, a more advanced imaging technique that can reveal tiny structures inside the cells. This can provide additional confirmation of the diagnosis by showing specific features that are typical of neuroendocrine cells.^[6]

Once the diagnosis of Merkel cell carcinoma is confirmed, additional tests are needed to determine whether the cancer has spread beyond the original site. Ultrasound imaging of the lymph nodes near the tumor is often the first step. This painless test uses sound waves to create images of the lymph nodes, allowing doctors to check their size and appearance for signs of cancer spread.^[4]

A sentinel lymph node biopsy is recommended for nearly all patients diagnosed with Merkel cell carcinoma. This procedure helps determine whether cancer cells have traveled to the nearest lymph nodes. During the procedure, a small amount of radioactive tracer or blue dye is injected near the tumor site. This substance travels through the lymphatic system to the first lymph node or nodes that drain fluid from the tumor area. The surgeon then removes these “sentinel” nodes and sends them to a pathologist for examination. If cancer is found in these nodes, it indicates that the disease has started to spread, which affects treatment decisions and prognosis.^[10][11]

To check for spread to distant parts of the body, doctors often order whole-body imaging. A positron emission tomography (PET) scan combined with a computed tomography (CT) scan is commonly used. This test involves injecting a small amount of radioactive sugar into a vein. Cancer cells, which use more energy than normal cells, absorb more of this sugar and appear as bright spots on the scan. This helps doctors identify cancer that has spread to lymph nodes, lungs, liver, bones, or other organs.^[4]

The information gathered from all these tests allows doctors to assign a stage to the cancer. Staging describes how much cancer is in the body and where it is located. Merkel cell carcinoma is staged using the American Joint Committee on Cancer (AJCC) system, which ranges from Stage I (cancer confined to the skin) to Stage IV (cancer that has spread to distant organs). Knowing the stage is essential for determining the most appropriate treatment plan.^[4]

Diagnostics for Clinical Trial Qualification

Clinical trials test new treatments or approaches for Merkel cell carcinoma and may offer access to promising therapies that are not yet widely available. To participate in a clinical trial, patients must meet specific criteria established by the researchers conducting the study. These criteria ensure that the trial results are meaningful and that participants are likely to benefit from or safely tolerate the treatment being tested.

One of the most fundamental requirements for enrolling in a clinical trial is confirmed diagnosis through a biopsy. Researchers need pathology reports that clearly document the presence of Merkel cell carcinoma, including results from immunohistochemistry staining that show the characteristic markers of the disease. This ensures that all participants in the trial truly have the same type of cancer.^[4][11]

Accurate staging is also critical for trial enrollment. Many clinical trials are designed for patients at specific stages of the disease. For example, some trials focus on patients with early-stage disease who have not yet received treatment, while others are designed for patients with advanced or metastatic disease that has spread to lymph nodes or distant organs. To determine eligibility, patients typically need to undergo comprehensive staging evaluations, including sentinel lymph node biopsy, imaging studies such as PET-CT scans, and sometimes additional tests to assess the extent of disease.^[11]

Blood tests are often required before and during participation in clinical trials. These tests assess overall health and organ function to ensure that patients can safely undergo the experimental treatment. Common blood tests include a complete blood count, which measures red blood cells, white blood cells, and platelets, and blood chemistry tests, which evaluate kidney function, liver function, and electrolyte levels. These tests help identify any underlying health problems that might make the trial treatment unsafe.^[5]

Some trials may also require testing for specific biomarkers or genetic features of the tumor. For instance, researchers might test for the presence of Merkel cell polyomavirus (MCPyV), which is found in the majority of Merkel cell carcinomas and may influence how the cancer responds to certain treatments. Other trials might require testing for specific protein markers or genetic mutations in the tumor tissue.^[4][10]

Performance status is another important consideration. This is a measure of how well a patient can carry out daily activities and is often assessed using standardized scales. Trials typically require that participants have a certain level of physical function to ensure they can tolerate the treatment and complete the study as planned.

Some clinical trials exclude patients who have received certain prior treatments or who have other active cancers. This is because previous therapies or coexisting cancers might affect how the body responds to the experimental treatment, making it difficult to interpret the trial results. Detailed medical history and documentation of all previous treatments are essential parts of the screening process for trial enrollment.

Emerging diagnostic tools are also being explored in the research setting. For example, circulating tumor DNA (ctDNA) testing is being studied as a way to monitor treatment response and detect recurrence early. This blood test looks for tiny fragments of DNA released by cancer cells into the bloodstream. While not yet standard practice, ctDNA testing may become an important tool in clinical trials and eventually in routine care.^[10]