Skin cancer. Light micrograph of a section through an epidermoid carcinoma, a type of skin cancer. At centre, the cells are large and irregular. This cancer arises in epithelial cells, those which line a tissue or organ. It is common on the skin, where it can form in response to long-term overexposure to sunlight. It is a malignant tumour, and can spread to other parts of the body (metastasis). If this has happened, treatment is usually unsatisfactory, but if the tumour is detected early, surgical removal can effect a cure. Magnification unknown.