Merkel Cell Carcinoma

Merkel cell carcinoma is a very rare skin cancer (40 times less common than melanoma and 650 times less common than BCC and SCC) that occurs most often in the sun-exposed areas of the skin, particularly the face. It typically occurs in Caucasians between the ages of 60 and 80 and is twice as common in men than women.

A Merkel cell carcinoma occurs when a Merkel cell becomes cancerous. Merkel cells are specialised cells that function with sensory (feeling) nerve fibre endings to enhance the body’s perception of shapes and textures. The interesting thing is that although Merkel cell cancer occurs most commonly in sun damaged skin it is not sun damage per se that causes them. It has been found that 80% of Merkel cell carcinomas contain a virus called the Merkel cell polyomavirus, which has become integrated into the cancerous Merkel cells. It is the virus itself (this is known as an oncovirus) which causes the Merkel cells to mutate and turn cancerous.

Most Merkel cell carcinomas start as firm, painless pink or flesh-coloured lumps (nodules) in the skin. They can grow quite rapidly reaching sizes of 2cm in diameter in a few months. The problem with Merkel cell carcinomas is that once they start growing into the dermis of the skin, they have a strong tendency to metastasise (spread). Most commonly Merkel cell carcinomas will first metastasise to the local lymph nodes (much like melanoma) before spreading further afield via the blood stream to the lungs, liver, brain and bone.

Once the diagnosis of a MCC is made it is necessary for the patient to have a thorough examination of the regional lymph nodes to rule out obvious spread (enlarged firm lymph nodes). Thereafter it is important to do a whole body CT scan to assess whether there has been spread to any of the internal organs. If there are no obvious metastases, then the next step is to perform a surgical excision including a generous safety margin of normal looking skin (wide excision) around the MCC. The safety margins employed are the same as one uses for melanoma. At the same time as doing a wide excision of the Merkel cell carcinoma it is recommended that a sentinel lymph node biopsy be done as well. If spread to the lymph nodes is found, then removal of all the remaining lymph nodes in that area is recommended to clear any additional diseased nodes.

Despite doing a wide excision of normal-looking skin, Merkel cell carcinoma has a strong tendency to regrow close to its original site. To minimise the risk of this happening it is recommended that all patients with Merkel cell carcinoma receive radiotherapy to the skin excision site. Radiation is also given to areas where there has been spread to the lymph glands. This also minimises the risk of recurrence.

Chemotherapy is only used when the MCC has metastasised to distant organs. Unfortunately, the available treatments offer little hope of cure.