Skin cancer. Scar from a basal cell carcinoma after radiotherapy treatment. Basal cell carcinomas, also known as rodent ulcers, are the most common form of skin cancer among fair-skinned people, mostly affecting the elderly. They usually appear on the face and are thought to be caused by the ultraviolet radiation in sunlight. Although they slowly destroy surrounding tissues, they seldom spread to other parts of the body. Treatments include surgery, cryosurgery and radiotherapy.