Merkel Cell Carcinoma, from the CureRays guide library.

Merkel cell carcinoma is a rare, fast-growing skin cancer; it is very sensitive to radiation, and surgery, radiation, and immunotherapy together give the best control.

What it is.

Merkel cell carcinoma is a rare but aggressive skin cancer that grows from Merkel cells, special cells in the top layer of the skin involved in the sense of touch. It usually appears as a firm, painless, fast-growing bump that may be red, pink, or purple, most often on sun-exposed skin of the head, neck, or arms in older adults. Because it can look harmless — sometimes mistaken for a cyst or insect bite — it is often diagnosed once it has already started to grow quickly. Two main things drive this cancer: long-term sun (ultraviolet) exposure and a common virus called the Merkel cell polyomavirus, and it is more likely in people whose immune systems are weakened. Merkel cell carcinoma tends to spread to nearby lymph nodes earlier than other skin cancers, so prompt, coordinated treatment matters. The encouraging news is that this cancer is very sensitive to both radiation and modern immunotherapy, which have greatly improved outcomes.

How radiation treats it.

Radiation uses focused high-energy x-rays to damage the DNA inside tumor cells so they can no longer grow and divide. Merkel cell carcinoma is one of the most radiation-sensitive cancers, which makes radiation a powerful tool against it. After surgery, targeted radiation to the tumor site — and frequently the nearby lymph nodes — substantially lowers the chance the cancer returns. When surgery isn't possible, radiation alone can control the disease, and even spots that have spread often respond quickly. Modern image-guided techniques concentrate the dose on the target while sparing surrounding healthy skin and tissue. Treatments are painless and brief, given over a series of short sessions. Side effects depend on the area treated and are usually temporary, such as skin redness, irritation, or fatigue. Increasingly, radiation is paired with immunotherapy, and together they offer strong control of this aggressive cancer.

The ways we can treat it.

Post-surgery (adjuvant) radiation. Targeted radiation to the area where the tumor was removed, and often the draining lymph nodes, greatly reduces the chance the cancer comes back, taking advantage of its strong sensitivity to radiation.

Definitive radiation. When surgery isn't possible or advisable, focused radiation alone can control the cancer, because Merkel cell carcinoma responds so well to it.

Palliative / spot radiation. Precise radiation can quickly shrink and relieve symptoms from deposits that have spread, often working well even at modest doses.

Questions we hear often.

How is Merkel cell carcinoma different from other skin cancers? It is rarer, grows faster, and spreads to lymph nodes earlier than common skin cancers like basal or squamous cell carcinoma. Because of this, it needs prompt, coordinated treatment — but it is also very sensitive to radiation and immunotherapy, which work well against it.

Why might I need radiation after surgery? Merkel cell carcinoma is highly sensitive to radiation, and giving it after surgery to the tumor site and nearby lymph nodes substantially lowers the chance the cancer returns. For this cancer, radiation is a standard and important part of curative treatment, not just a backup.

Does the virus mean it's contagious? No. The Merkel cell polyomavirus is extremely common and harmlessly present on many people's skin; it is not spread from person to person as cancer. Only rarely, and usually combined with sun damage and a weakened immune system, does it contribute to this cancer.

This guide is informational only. It is not medical advice — please confirm anything here with your care team.