Skin Cancer risk factors

The main preventable risk factor for skin cancer is excess exposure to ultraviolet radiation (UVR),1 the principal source of which is the sun (Figure 4.1).

A study published in December 2011 estimated that around 86% of malignant melanomas in the UK in 2010 were linked to exposure to UVR from the sun and sunbeds, with a higher proportion in men (around 90%) than women (around 82%).82 It is estimated that 90% of non-melanoma skin cancers are caused by sun exposure.1 Table 4.1 gives incidence rates by ethnic group in the USA.3

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Risk of melanoma is most strongly linked to intermittent exposure to high-intensity sunlight, often resulting in sunburn rather than to chronic exposure, typical of that received by people with outdoor occupations. 4 A history of sunburn doubles the risk of melanoma. 55, 56

Sunburn, especially in childhood, also increases risk of basal cell carcinoma. 57-62 Risk of squamous cell carcinoma is strongly related to long-term occupational exposure to sunlight. 57-59, 63

Sunbeds and sunlamps are a source of artificial UVR. Before the 1980s these appliances primarily emitted UVB and sometimes UVC. 8Since then, commercial salons using mainly UVA lamps (these lamps may also emit some UVB) have become popular. 8,9 This variable spectral output over time and also between appliances, complicates the measurement of risk. 10

In 1999 a quarter of men and a third of women in Britain reported using a sunbed or tanning machine in the previous six months and there was even higher use amongst young people (Figure 4.2). 11 Any impact of sunbed use is therefore likely to be greater in the future.

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The most recent meta-analysis concluded that the use of sunbeds increases risk of malignant melanoma, especially when used before the age of 35. 64 Use of a sunbed at any age doubles the risk of squamous cell carcinoma. 64 In June 2009, the International Agency for Research on Cancer raised the classification of sunbed use to Group 1 "carcinogenic to humans". 74 There is evidence from a case-control study that using a sunbed without ever burning can increase the risk of malignant melanoma.81

One UK study using modelling techniques estimated that sunbed use caused one hundred deaths a year from melanoma in the UK. 14

The possibility that younger people and those with high risk skin types are at greatest risk is widely recognised and in 2003 the International Commission on Non-Ionizing Radiation Protection (ICNIRP) 10 and the World Health Organisation (WHO) 15 recommended that certain categories of people should not use sunbeds as shown in Table 4.2. Legislation was passed in 2010 banning the use of sunbeds in under-18s in England and Wales (Scotland already had this legislation) following surveys commissioned by Cancer Research UK in 2008 and 2009, which showed that 6% of 11-17-year-olds in England had used a sunbed.80

The ICNIRP also concluded that anyone using suntanning appliances is likely to raise their risk of skin cancer, eye damage, photodermatosis, photosensitivity and premature skin ageing. 10

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( see also Molecular Biology section)

People with light eyes, skin or hair, or who sunburn easily or do not tan, have an increased risk of skin cancer. 57, 59, 62, 63, 65 In addition, people with a large number of moles, or just one or more unusually shaped or large moles, have an increased risk of melanoma. 66 Risk increases with the number of moles. People with very high numbers (100+) of common moles on their bodies have nearly seven times the risk compared to people with very few (0-15 moles). 16

People with dysplastic moles and a family history of melanoma (dysplastic mole syndrome) have a 500-fold increased risk of developing melanoma. 17Development of moles is caused by exposure to sunlight as well as being genetically determined and most are acquired during childhood. 18,19

Chronic sun exposure is the most important environmental factor determining development of moles rather than number of sunburn episodes. 19 A UK study of moles in twins concluded that the emergence of moles in adolescents is under strong genetic control. 20 Freckling is associated with an increased risk of melanoma, independent of number of moles. 19

In terms of hair colour, people with red and light hair have the highest risk for melanoma. 7 The risk for people with red hair is as much as four times higher than for people with dark brown or black hair while people with blond hair had twice the risk of dark-haired individuals. 7,21

There is an approximately three-fold risk increase for melanoma associated with very pale skin compared to people with the darkest white skin. There are similar differences in risk for those with skin that does not tan compared with those whose skin develops a strong tan. 2

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There is divergent evidence for a protective role of sunscreen in melanoma aetiology. Data from in vitro studies have shown significantly higher DNA damage in skin not treated with sunscreen versus treated skin 22, and a randomised trial carried out in the 1990s found children randomised to SPF30 sunscreen application developed fewer moles over a three-year period. 23

The most recent review considered 18 case-control studies and reported no association overall between sunscreen use and risk of melanoma but it is possible that people use sunscreen in order to spend longer in the sun or to avoid using protective clothing, which would cancel out any protective effect. 24, 67, 68

It has been known for many decades that the only thoroughly established beneficial effect of solar ultraviolet radiation on the skin is the synthesis of vitamin D and its role in maintaining bone health. But evidence is emerging that suggests that sunlight exposure, and the resulting cutaneous synthesis of vitamin D, might have a beneficial influence for certain major cancers, most notably colorectal cancer. 25-29.

However, the picture is by no means clear and more research is needed before exposure to solar UVR could be recommended as a means of reducing the incidence or mortality of cancer.

People with a family history of melanoma have roughly double the risk of developing the disease compared to people without a family history. 65 Rare families exist in which 3 or more cases occur. In these families a significant proportion have a hereditary susceptibility gene such as CDKN2A and as gene carriers are at considerable lifetime risk of melanoma. People with a family history of squamous cell carcinoma or basal cell carcinoma have an increased risk of non-melanoma skin cancer. 60, 62, 69

US registry studies report an increased risk of melanoma in women previously diagnosed with breast cancer and in people previously diagnosed with non-Hodgkin lymphoma. In both cases, the association was bi-directional, supporting a shared genetic or environmental factor but further studies are needed to confirm these findings. 32,33.

People with a previous non-melanoma skin cancer have a much higher risk of developing a second one. 70-72 People with a previous melanoma have a nine-fold increased risk of developing a second melanoma,75-77 and a three-fold increased risk of developing non-melanoma skin cancer.78 Risk of second melanoma is particularly high where an individual also has a parent who has been diagnosed with melanoma (>30-fold).75

People who have had renal transplants have been shown to have a 33-fold increased risk of non-melanoma skin cancer and a higher risk of melanoma also. 73

An increase in risk of melanoma and non-melanoma skin cancer has been shown for people with atopic dermatitis, the most common form of eczema.79