All skin cancers and actinic keratoses are linked to excessive exposure to UV radiation. Sunlight comprises about 5 per cent UVB (wavelengths 290-315nm) and 95 per cent UVA (wavelengths 315-400nm):

• UVB radiation is responsible for direct DNA damage and the redness and inflammation of sunburn. UVB is also required for synthesis of vitamin D in the skin
• UVA radiation penetrates deeper into the skin, causes indirect DNA damage and is responsible for the ageing effects of sun exposure such as sagging and wrinkling of the skin

Chronic sun exposure leads to UV-induced mutations in the DNA of skin cells.

In a normal situation, when DNA is damaged, endogenous mechanisms quickly repair the damage. If the damage cannot be repaired, then apoptosis (programmed cell death) is triggered and the damaged cell is removed, thereby preventing further damage and protecting the body.

However, UV radiation not only induces DNA damage but also influences the repair and apoptotic mechanisms in two important ways:

• It impairs the local (cutaneous) immune function and the lowered immune status is associated with an increased risk for the development of SCC
• It is also known to increase the expression of viral genes that have anti-apoptotic activities and delay DNA repair mechanisms. The human papilloma virus (HPV), for example, is believed to play a role here. As a result, skin cells with damaged and potentially cancerous DNA persist in the skin

The skin protects itself by producing melanin in melanocytes. This is then fed to the surrounding keratinocytes to protect the cell nuclei. Melanin in the skin functions as a broadband UV absorbent. This is the process of tanning.

Older people and people who had frequent episodes of sunburn as children are believed to be at higher risk of developing skin cancers because they have had longer to collect potentially cancerous changes in the skin.

Table 1 (next screen) summarises the different types of skin cancer, their features and treatment.

Identifying skin cancers

A booklet to help identify skin cancers and actinic keratosis can be downloaded from the Skcin website. Skcin is a charity dedicated to education, prevention and early detection of skin cancer.