MELANOCYTES are funny things. They sit in the bottom layer of the EPIDERMIS (which is the outer covering of the skin; the rest of the skin is the DERMIS) and their job is to make melanin, which they pass onto the cells on either side of them. Melanin protects cells from damage to their DNA by UV light. DNA damage can lead to cancer. People with pigmented skin do not have more melanocytes, just more melanin.
They make moles that are benign and evolve from purely intraepidermial (‘junctional’) to mix epidermal and dermal (‘compound’) and eventually purely (‘intradermal’). ie they move downwards. They may be disfiguring, or become inflamed, or be a nuisance, but they are BENIGN, although they may become malignant.
Dysplastic naevi are not entirely benign and may progress to cancer; therefore, they need to be excised completely and with a sufficient margin of normal tissue.
Melonoma (with or without the prefix ‘malignant’) is NEVER benign. Once diagnosed and excised, it is the job of the pathologist to tell the dermatologist all the technical factors which influence how it will behave and therefore what further treatment is required. The most important thing the pathological assesses is how thick the melanoma is: less than 1 millimetre (see how thin that is?) 1-2 millimetres thick, 2-3 millimetres thick, 3-4 millimetres thick or over 4 millimetres thick. The thicker they are, the worse it is.
Further treatment may include a wider local excision (WLE) of up to 3cm around the original melanoma to mop up any satellite deposits. That’s a lot, believe me.
Melanomas can spread, but they may spread in a funny way. The most usual is to the local lymph glands (or nodes). Sentinel node biopsy is a way of determining if the local nodes are involved or not. Melanoma may spread to the liver or lungs or anywhere, though. Also melanoma may occur in funny locations that never see the sun.
All cancers have abnormal genes. Some melanomas have BRAF mutations that may be used as a therapeutic target – ie if the mutation is present, certain drugs may be of help.
Lastly, people can occasionally live a long time with melanoma that has spread. Usually, though it is an aggressive cancer that kills quite quickly.