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 white people between the ages of 60 and 80 and is slightly more common in men than women.

What is a Merkel Cell Carcinoma?

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 one'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 (MCC) 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 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.

How are Merkel Cell Carcinomas Treated?

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. Thereafter, it is important to do a whole body CT scan or PET scan (if the medical aid will pay for it) 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 used to be the only option for treating metastatic Merkel cell carcinomas and it was not very effective, rarely resulting in a cure and generally only buying a little bit of extra time. In March 2017, however, the FDA approved Avelumab for the treatment of metastatic MCC. Avelumab is an antibody that blocks the PD-ligand 1 receptor. The PD-ligand 1 receptor is produced by melanomas and merkel cell carcinomas on their cell membranes and it binds with the PD-1 receptor on T-cells to switch off the T-cell. This de-activates the T-cell, which is then unable to kill the cancer cell. The Avelumab antibody attaches to the PD-ligand 1 receptor and prevents it from attaching to the PD-1 receptor so that the Merkel cell cannot switch off the the T-cell. There are no long term results yet for Avelumab but it significantly lengthens the time that patients have before the Merkel cell carcinoma recurs. There are ongoing trials assessing the effectiveness of Pembrolizumab and Nivolumab against MCC. Preliminary results should be available in the next 2 years.