What is skin cancer?
Skin cancer is a condition where the cells in the skin mutate to an abnormal type and divide uncontrollably. Skin cancer is the most common cancer in the fair-skinned populations in many regions of the world. Men and women are affected equally [1, 2].
The most common types of skin cancers are non-melanoma cancers, including basal cell carcinoma and squamous cell carcinoma, representing approximately 30% of new skin cancers diagnosed every year [3]. Melanomas, on the other hand, are less common but can be more dangerous due to the potential to metastasise and spread to other parts of the body [1].
The primary cause or skin cancer is exposure to sunlight, which contains UV radiation. This causes DNA mutations in the skin cells, transforming them into cancerous cells. Interestingly, UV damage induces a well-defined set of genetic changes, which is referred to as mutational signature, which can be used to support the diagnostic process [1, 2]. Besides UV radiation, skin cancer can be caused by exposure to chemicals, certain viruses, as well as a genetic predisposition [4].
As early detection of suspicious skin changes improves the patient's prognosis, it is important to routinely visit the dermatologist to examine moles and skin changes. Thanks to improvements in diagnostics and treatment, the patient prognosis is improving [1].
What is merkel cell carcinoma (MCC)?
Merkel cell carcinoma (MCC) is a rare, but very aggressive type of skin cancer, which is typically found in older patients. It presents as a red or bluish node on the skin (cutaneous) or under the skin (subcutaneous), most usually located on the extremities or the head and neck region [5, 6]. As with every cancer, MCC is a heterogeneous disease, and there are multiple presentations and risk factors. It is often associated with immunosuppression (decreased functionality of the immune system), chronic UV light exposure, and infection with the Merkel cell polyomavirus, a type of virus that infects and integrates into the DNA of neuroendocrine cells of the skin (cells that excrete neurotransmitters and regulate hormones) [7].
MCC originates from a malignant transformation of Merkel cells. Merkel cells are cells in the epidermis, the outermost layer of the skin. They have roles associated with the nervous system and the endocrine system, particularly with detection of light tactile stimuli and transmitting the information through the nervous system. Additionally, the cells secrete peptides to facilitate signalling in the skin to maintain its homeostasis (the function to maintain a stable environment).
How is MCC diagnosed?
The diagnosis of MCC is challenging at first, as it can be mistaken for other skin conditions due to its appearance. MCC often resembles cherry angioma, a benign (non-cancerous) lesion of the skin. The diagnosis is made by biopsy (taking a small sample) or excision (removing the node completely) of the suspected node, and its histological evaluation and immunohistochemical tests. These involve slicing the node in very thin slices and staining with specialised dyes that bind to certain cell types and/or molecules, so that they are visible under the microscope. Common histological findings include the presence of uniform round cells expressing cytokeratin 20, neurofilament, synaptophysin, chromogranin, and neuron-specific enolase [8].
Prognosis and Treatment of MCC
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