The most frequent tumours of the skin include basal-cell carcinomas, squamous-cell carcinomas, and melanomas. Melanomas are among the most malignant tumours, and can appear near to an existing pigmented birthmark. In most case, however, they appear on skin without birthmarks or other lesions.

Any birthmark that changes its colour or size over time, or that bleeds or appears ulcerated, should be examined by a dermatologist, typically using a dermatoscope. Additionally, all birthmarks should be examined by a dermatologist once a year. The most common skin cancers are basal-cell carcinomas, which very rarely metastasize and do not seriously threaten the patient’s life. They are most common among older people, where they appear on skin that has been exposed to the sun (on the head, hands, and face). The risk of basal-cell carcinoma is increased by sunburn in childhood, excessive exposure to the sun, and not using protective sunscreen. These carcinomas generally look like a noninflammed bump or ulcer surrounded by a small ridge.

Any nonhealing skin lesion that bleeds occasionally, especially if found on the face or head, should be examined by a dermatologist in order to be distinguished from basal-cell carcinoma. The treatment for a basal-cell carcinoma depends on its size and clinical presentation. The methods used include local preparations, such as Aldara (imiquimod), cryosurgery with liquid nitrogen, photodynamic therapy, or surgical removal.

Squamous-cell carcinoma is decidedly rarer than basal-cell cancer, but it is more malignant, and has a tendency to infiltrate and metastasize to nearby lymph nodes. It resembles a wart or ulcer, but infiltrates deeper. It often occurs on the border of the skin and mucous membrane (such as the lips).

Basal-cell carcinoma also frequently develops in the so-called precancerous state of cutaneous horns. Treatment typically involves surgery and a skin biopsy. In the case of skin cancer, prevention is extremely important: sunscreens with high SPF factors should be used, sunburn should be avoided—especially among children—and lesions that fail to heal and birthmarks should be looked at by a dermatologist.