Medical Encyclopedia by MedlinePlus, National Library of Medicine, U.S. National Institutes of Health. Provides basic information on basal cell carcinoma, its medical definition, causes, risk factors, symptoms, diagnosis, treatment, prognosis, prevention, and when to contact a medical professional. Basal cell carcinoma is a type of non-melanoma skin cancer, and is the most common form of cancer in the United States. According to the American Cancer Society, 75% of all skin cancers are basal cell carcinomas. Non-melanoma skin cancers have a relatively high rate of cure, if treated early. The principal cause of basal cell carcinoma is exposure to sunlight.