The Role of Sunlight in the Causation of Skin Cancer

It is generally believed that ultraviolet radiation (UVR) from the sun is the major cause of “skin cancer” however skin cancer is not just one disease but a group of different type of tumours and the causative factors for each are different.

The strongest link with UVR as a cause is for Squamous Cell Carcinoma of the skin. The mechanism by which the UVR damages the skin cells DNA (genetic blueprint) is understood. SCC related to UVR most commonly occur on the most sun exposed areas of the body. We see a spectrum of disease that progresses with increasing exposure to UVR (we call this a dose response relationship). Initially there is skin damage resulting in thinning of the skin with reddening or pigmentation, later solar keratoses (sun spots) appear, some of these can progress to early cancer confined to the outer or epidermal layer of the skin (known as Intraepidermal Carcinoma, Squamous Cell Carcinoma in Situ, or Bowens Disease). Later still the Squamous Cell Carcinoma can invade to the deeper dermal layer of the skin and ultimately to distant body sites such as the lymph glands. A large number of other factors apart from UVR can be involved in causing an SCC in an individual case and these include genetic susceptibility, the human papilloma virus, chemical carcinogens (hydrocarbons, solvents, arsenic for example), ionizing radiation (such as used in radiotherapy), and chronic inflammation (SCC arising in scars and ulcers).

Whilst UVR is probably the main causative agent for BCC, there are some inconsistencies. Genetic susceptibility most commonly manifest by a fair skin type is very important. BCC seems to be more common with intermittent sun exposures and sunburns than with chronic or continuous exposure. Certain patients also seem to develop a disproportional number of lesions on the trunk which is generally not the most sun exposed area of the body. An important factor for some patients with BCC is exposure to arsenic which was previously used in some asthma treatments and other remedies.

Believe it or not the weakest evidence for UVR as the main causative agent is for melanoma skin cancer. Here genetic susceptibility would appear to be the main issue and UVR a secondary factor. Melanoma is more common in the fairer skin types, family history is important and some families know as melanoma prone kindred have had specific susceptibility genes identified. The commonest body sites for melanoma (back and shoulders in men and back and legs in women) are not the most frequently sun exposed. Intermittent sun exposure and sunburns are more strongly linked than continuous exposure. The adoption of widespread sunscreen use has not influenced melanoma incidence which continues to rise. There are various reasons this may be the case and it is hotly debated. We know that many people don’t use sunscreen as it is meant to be used, that is they don’t apply sufficient or apply it early or frequently enough to all exposed skin. Furthermore the availability of sunscreens has given some a false sense of security; they seem to believe that they can slip slop and slap then spend all day in the sun safely. The section on melanoma describes different types of melanoma and for the most dangerous and difficulty to detect “rapidly growing” type it appears sun exposure is not relevant at all. This causes a dilemma because the thrust of even our most recent skin cancer preventive campaigns in Australia is toward sun protection, and the evidence is that this has not worked.

Our belief is that sun protection is important because UVR is important in the cause of SCC and BCC, and plays a role in some melanomas. Additionally it causes photo damage and premature skin aging.

However until we fully understand the genetic basis of melanoma the best way to prevent melanoma deaths is to educate the population to identify skin changes early, to closely follow up those at increased risk, to encourage regular skin checks and to train doctors in the use of current and evolving technologies that enable earlier diagnosis.