Skin cancer is a disease in which cancer cells form in the tissues of the skin. The skin has several layers, but the two main layers are the upper layer, called the epidermis, and the lower or inner layer, called the dermis. Skin cancer begins in the epidermis, which is made up of three kinds of cells.

These three cells are squamous cells, which are thin, flat cells that form the top layer of the epidermis; basal cells, which are the round cells found under the squamous cells; and melanocytes which are cells that make melanin and are found in the lower part of the epidermis.

There are also cancers that can spread to the skin, but these are not skin cancers.

How does skin cancer develop?

Skin cancer develops when the cells of the skin are damaged. This causes cells to mutate, reproduce abnormally and form a mass of cancerous cells. The type of skin cancer that develops depends on the type of cells that are involved.

What different types of skin cancer exist?

Skin cancers come in three main types: basal cell skin cancer, squamous cell skin cancer and melanoma.

The most common skin cancers are non-melanoma skin cancers: basal cell carcinomas and squamous cell carcinomas. Both cancers are found on the parts of the body which are regularly exposed to the sun and their development are primarily related to one’s sun exposure. While these two types of skin cancers can spread to other parts of the body, they rarely do.

Melanomas develop from the cells that make up the skin’s pigment. These cells are called melanocytes. Melanomas can look similar to moles and can develop anywhere on the body. They are most likely to start in exposed areas such as the neck and face. It is also common in men for melanomas to start on the chest and back. For women, melanomas usually start on the legs. Melanoma skin cancer can be treated if caught early. There are several other types of skin cancer, including Merkel cell carcinoma, skin lymphoma, Kaposi sarcoma, skin adnexal tumours and sarcomas, but they make up a small percentage of all diagnosed skin cancers.

How does skin cancer spread?

Whether and how skin cancers spread depends on the type of skin cancer. Basal cell carcinomas and squamous cell cancers rarely spread. Melanomas are the most likely type of skin cancer to spread and metastasise. Melanoma can spread to any part of the body, but the most common areas it spreads to are the lungs, liver, bones, brain, abdomen and lymph nodes.

What causes skin cancer?

Damage to the skin cells’ DNA leads to the cells mutating. This damage can be caused by ultraviolet radiation from sunlight and from tanning beds.

Other factors may also contribute to the development of skin cancer since it can develop on parts of the body that are not normally exposed to these sources of ultraviolet light. Exposure to toxic substances or having a weakened immune system may also contribute to the development of skin cancer.

Treating and preventing skin cancer

There are several different options available to treat skin cancer depending on the type of skin cancer and its size, including surgery, radiation, photo dynamic therapy and topical medications.

An emerging body of research is indicating that knowing a tumour’s genomic profile could be more important for successful treatment than knowing its location or size. As each tumour’s genomic profile is unique, this approach is often referred to as personalised or precision medicine.

Skin cancer is one of the most preventable types of cancer. Limiting exposure to the sun and avoiding tanning beds is key in its prevention.