MODEL RELEASED. Basal cell carcinoma on the skin of the shoulder of a 66-year-old man. Basal cell carcinoma is one of the most common forms of skin cancer, and is most often caused by over-exposure to sunlight. The cancer starts as a small flat nodule and grows slowly, eventually breaking down at the centre to form a shallow ulcer that persistently bleeds. Basal cell carcinomas rarely spread to other parts of the body. Treatment involves the surgical removal of the tumour or radiotherapy.