Merkel cell carcinoma
What is Merkel cell carcinoma of the skin?
Merkel cell carcinoma of the skin is a rare form of skin cancer. It may be very aggressive, and often metastasises (spreads) to other parts of the body.
Merkel cell carcinoma is believed to arise from Merkel cells, which are pressure receptors in the skin.
How gets Merkel cell carcinoma?
Merkel cell carcinoma has an estimated incidence of 0.23 per 100,000 people in Caucasian populations. Increasing numbers of Merkel cell carcinomas have been reported by some centres in recent years. Merkel cell carcinoma is more common in the elderly, with most cases occuring after the age of 50. It is slightly more common in men, and occurs on parts of the body commonly exposed to sunlight, most often the head and neck. It is also more common and more serious in those that are immune suppressed, such as patients with solid organ transplants, human immunodeficiency virus (HIV) infection, haematological malignancy or on drugs such as azathioprine.
What are the clinical features of Merkel cell carcinoma?
Merkel cell carcinoma usually presents as a rapidly enlarging, solitary, irregular red nodule. It is often similar in appearance to other more common skin cancers such as basal cell carcinoma.
Merkel cell cancers spread through the lymphatic system and multiple smaller seedlings can develop around the main tumour (local recurrence). Merkel cell carcinoma may also spread to lymph nodes in the neck, axillae and groin. This is more likely in thicker tumours.
Merkel cell carcinoma
What causes Merkel cell carcinoma?
Merkel cell polyomavirus (MCPyV) has been detected in about 80% Merkel cell carcinomas tested. It is thought that the virus causes gene mutations leading to Merkel cell carcinoma when immune function is defective.
Ultraviolet radiation has been implicated as a factor in developing Merkel cell carcinoma, due to the occurrence of the tumour on sun exposed skin. Immunosuppression is an important factor for the development of Merkel cell carcinomas.
How is Merkel cell carcinoma diagnosed?
The main test is biopsy of the tumour. This shows characteristic Merkel cell carcinoma pathology. After general examination, including evaluation of local lymph nodes, staging imaging investigations may be arranged to determine whether the tumour has spread ot other sites.
Some patients may have further investigations to see if the Merkel cell carcinoma has spread beyond its original site in the skin. This may include:
- Sentinel node biopsy
- Lymph node ultrasound scan
- Imaging using X-rays, CT and MRI scans
A specific Merkel cell carcinoma staging system is pubished by the American Joint Committee on Cancer.
Longterm survival is likely if the lymph nodes do not contain tumour cells.
What is the treatment of Merkel cell carcinoma?
Surgical excision is the main treatment of primary Merkel cell carcinoma. A wide area surrounding the cancer is also removed.
After removal of large lesions (eg >2 cm), the site may also be treated with radiotherapy post-operatively. Radiation treatment leads to increased local and regional disease control and higher long-term survival rates.
The relevant lymph nodes may also be surgically removed or irradiated as a prophylactic measure.
Disease involving regional lymph nodes
If the cancer has spread to involve the lymph nodes, these may be surgically removed and/or treated with radiotherapy. In some cases, systemic chemotherapy may also be administered.
Distant metastatic disease
Distant disease is very serious and has a very poor prognosis. Treatment of metastatic disease is aimed at improving quality of life. In some cases radiotherapy and/or systemic chemotherapy may be administered for treatment. Positive response to PD-1 blockade has been reported using pembrolizumab for advanced merkel cell carcinoma.
In March 2017, US the Food and Drug Administration granted accelerated approval to another PD-1/PD-L1 agent, avelumab (Bavencio®) to treat metastatic Merkel cell carcinoma. Approval was based on a trial of avelumab that showed response in one third of patients.
Consensus treatment guidelines are published by the National Comprehensive Cancer Network.