What causes melanoma?
The skin is divided into an outer layer called the epidermis and a deeper layer called the dermis – these are shown in the diagram below. Melanocytes are found at the junction of the two layers.
Melanoma skin cancer starts when melanocyte cells begin to grow and multiply in an abnormal way. This can be at the site of an existing skin mole, or in an area that was previously mole-free. It is not known exactly why or how melanocytes become abnormal, but ultraviolet light from the sun plays a role by causing damage to cells’ DNA.
Melanomas grow in two phases. First is the radial phase, in which they grow horizontally within the epidermis. Later comes the vertical phase, where the melanoma grows down into the dermis. During the vertical phase it develops the ability to break away and spread to other parts of the body, a process called metastasis.
Types of melanoma
Melanoma usually develops on the skin, but in rare cases it can develop in the eye or on the mucus membranes (for example in the nose or inside the body). There are five different types of melanoma:
- Superficial spreading: accounts for 70% of melanomas. Often develops from an abnormal mole. It spreads along the surface of the skin for a period of months or years before starting to grow downwards. It often has an irregular border and variations in color. In women it is most common on the legs or upper back, and in men on the trunk.
- Nodular: accounts for 10-15% of melanomas. It is usually symmetrical and a uniform dark brown or black in color. It progresses quickly to the vertical growth phase, so it has often grown quite deeply into the skin by the time it is discovered.
- Lentigo maligna: accounts for 10-15% of melanomas. It develops from a type of skin mark called a lentigo, and is large and flat. Usually found on sun-exposed areas such as hand or face, and most common in elderly people. It is usually slow-growing with a long radial growth phase.
- Acral lentiginous: a rare melanoma that occurs on the palms of the hands, soles of the feet, and beneath the nails. It is the only melanoma type that is equally common in black and white people. It progresses rapidly from the radial to the vertical growth phase, and is often diagnosed late.
- Mucosal lentiginous: develops in the mucus membranes, for example of the nose, mouth or inside the body. Accounts for about 3% of melanomas. It appears to progress quickly, although this may be because it is often diagnosed later than melanomas of the skin.
