People with darkly pigmented skin very rarely get skin cancer.
Those of a Celtic constitution, when exposed to strong sunlight in countries such as Australia, get skin cancer very readily.
Australia has the highest incidence of skin cancer in the world, with 140 000 new cases per year, and 1200 deaths per year, mainly from melanoma.
It is therefore important to understand that there is a variation in skin sensitivity to sunlight.
This is rated from one to six (Fitzpatrick classification).
Skin type one subjects have red hair and do not tan, burn very easily in the sun and develop skin cancer readily, whereas skin type six subjects have black skin (with an inbuilt sun protection factor of 10) and very rarely develop skin cancer.
This is a useful guide in assessing the risk of sun damage and in determining the dose of ultraviolet B in treatment.
Ultraviolet radiation
There are three types of ultraviolet radiation the short wavelength ultraviolet C (100–280 nm), ultraviolet B (290– 320 nm), and long wavelength ultraviolet A (320–400 nm).
Beyond this is visible light then infrared, and radiowaves. ultraviolet C does not penetrate beyond the stratosphere as it is absorbed by the ozone layer.
Ultraviolet B is very important in both sunburn and the development of skin cancer.
Ultraviolet A is thought to be of increasing importance in the development of skin cancer, and causes tanning but not sunburn.
It is also important in people with photosensitivity.
The effects of ultraviolet radiation may be classified as short term (sunburn, photosensitivity) or long term (skin cancer, wrinkling, solar elastosis, solar keratoses, seborrhoeic warts).
There is general awareness that the sun causes cancer in the skin, with some people becoming obsessively fearful of any exposure to sun.
A sensible approach with emphasis on reasonable precautions is called for. Useful points are:
• Most moles are entirely harmless.
• Detecting the changes in moles or early melanoma enables the diagnosis to be made at an early stage with a good chance of curative treatment.
• The non-melanotic, epidermal cancers basal cell and squamous cell carcinomas grow slowly and are generally not life threatening.
But squamous cell carinoma arising at sites of trauma, on the extremities, or in ulcers may metastasise.
Exposure to sun has usually occurred many years previously.
Prevention of sun damage and skin cancer
Prevention of sun damage and skin cancer will depend on reducing exposure to ultraviolet radiation.
This can be achieved in a number of ways:
• Covering the skin with clothes. It must be remembered however that light clothes such as shirts or blouses may only have a sun protection factor of four.
A wide-brimmed hat is essential to protect the face and neck.
• Sunscreens will greatly reduce sun exposure for exposed parts such as the face and hands.
Sunscreens are much more efficient than previously, particularly those with a sun protection factor greater than 30; they are now water resistant, and most have a broad spectrum, protecting against ultraviolet B and ultraviolet A.
This is important because there is now increasing evidence that ultraviolet A is important in the development of skin cancer.
• Exposure to midday sun, particularly in tropical or subtropical latitudes, should be avoided.
At this time of the day the sunlight passes vertically through the atmosphere and there is less filtering of dangerous ultraviolet light.
So remember the adage: “Between eleven and three, stay under a tree” in the summer months.
Development of skin cancers
Sun-damaged skin
A number of different features characterise sun-damaged skin, which is often seen in the elderly particularly if they have lived in a sunny climate such as Australia.
The skin has many fine wrinkles and often has a sallow yellowish discoloration particularly on the face and other exposed parts of the body.
Hyperpigmentation occurs as result of recent sun exposure, which may be diffuse or localised in the form of solar lentigo.
In some areas there may be hypopigmentation, particularly where solar keratoses have been treated with liquid nitrogen (cryotherapy).
There may be marked telangiectasia and numerous blood vessels are seen. In some, there may be thickening and a yellow hue of the skin, particularly of the neck, due to elastin deposition in the upper dermis; this is known as solar elastosis.
Forms of skin cancer
There are three common forms of skin cancer caused by ultraviolet light: basal cell carcinoma, squamous cell carcinoma, and malignant melanoma.
Whereas there seems to be a direct relationship with the amount of ultraviolet exposure and basal cell carcinoma and squamous cell carcinoma, the relationship with ultraviolet exposure and melanoma is more complex and it seems likely that intermittent exposure to ultraviolet light is the main factor (for example, exposure to sunlight on holidays).
These different types of neoplastic change that occur in the skin are discussed in chapters 13 and 15.
Photosensitivity
Exposure to sun in non-pigmented races causes inflammation in the skin, depending on the skin type and amount of exposure.
In some individuals there is an abnormal sensitivity to sunlight.
This may arise because of an idiopathic reaction to sunlight or allergic reaction that is activated by sunlight.
Some chemicals seen to induce photosensitivity without causing an allergy.
Other causes are metabolic diseases and inflammatory conditions that are made worse by sun exposure.
Polymorphic light eruption
This is the most common of the idiopathic photosensitive rashes and occurs predominantly in women.
It is due to both the shorter (ultraviolet B) and longer (ultraviolet A) wavelength types of sunlight.
The eruption occurs from hours to days after exposure and varies in severity from a few inflamed papules to extensive inflamed oedematous lesions.
There may be only a few trivial lesions initially, but increasingly severe reactions can develop restricting the patients ability to venture outside.
A useful measure of seventy is to ask the patient if they cross to the shady side of the street to avoid the sun.
Treatment includes topical or systemic steroids for the acute rash and prevention by using sunscreens.
Desensitisation by narrow waveband phototherapy before exposure is effective.
Solar urticaria
This is a much less common condition and may be induced by longer wavelength (ultraviolet A) and visible radiation as well as ultraviolet B.
It is characterised by rapidly developing irritation and in the exposed skin is followed by urticarial wheals.
It can occur as part of a photoallergic reaction, in which case avoidance of the relevant allergen will prevent the condition.
Treatment is with antihistamines and sunscreens.
In some cases phototherapy with ultraviolet B, narrow waveband or psoralen with ultraviolet A (PUVA), is helpful.
