A melanoma is a malignant tumour of the skin and is commonly called skin cancer. A benign tumour does not invade and destroy tissue, nor does it spread to other parts of the body. The term cancer is reserved for malignant tumours, which if left untreated are life-threatening. A melanoma arises from a cancer of melanin-forming cells called melanocytes.
Melanin is the pigment found the skin and the amount varies with race.
The first three types of melanoma grow in a similar fashion, in that they initially spread superficially but do not invade deeper tissue. This is called the superficial or radical growth phase. Nodular melanoma differs from the others in that it lacks this radical growth phase and instead jumps straight to the invasive or vertical growth phase. This means that nodular melanoma is capable of metastasis very early in its development. A melanoma will spread to other parts of the body via the bloodstream or lymphatic system.