Skin cancer. Light micrograph of a section through the skin of a patient affected by basal cell carcinoma (BCC). The nuclei of the tumour cells (left) are stained dark purple. The edge of the tumour is marked by a row of cells (lower left to upper right) while the internal structure is less ordered. 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 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.