Skin cancer. Light micrograph of a section through an epidermoid carcinoma, a type of skin cancer. Many of the cells (dark pink) 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 (metastasize) to other parts of the body. If the tumour is detected early, surgical removal can effect a cure. However, if the cancer has already spread treatment is usually ineffective and the prognosis poor.