Articles
Skin cancer: there is more than one risk factor
Everyone is well aware of the risk of sun exposure (ultra-violet radiation [UVR] to the formation of skin cancers. It is the primary cause but other factors may also play a role. These are either personal or environmental.
There is often an overlap of multiple risks and these may each carry an individual risk or be added together and become cumulative.
The following applies to non-melanoma skin cancers.
Personal factors
1. Phenotype or personal characteristics:
The ability to tan is an important defence against developing skin cancers.
This is directly proportional to melanin (brown pigment) content in the skin cells and determines sensitivity to sun exposure. Melanin absorbs ultra-violet radiation. The number of melanin containing cells is genetically determined – you have what you are born with and cannot change this amount.
Skin is classified into six grades. Grades of white (always burns) through browns to black (tans easily). The pattern of sun exposure is also important. Regular sun exposure (outdoor occupations) is a risk for squamous cell carcinoma (SCC) while irregular, holiday type exposure predisposes to basal cell carcinoma (BCC). Childhood exposure is considered a risk for all carcinomas.
2. Genetic syndromes
Albinism is a well known risk for skin cancers. Xeroderma pigmentosa is a rare condition characterized by an intolerance of the skin and eyes to sun exposure. Naevoid basal cell syndrome not only predisposes to multiple BCCs but is also associated with a number of other abnormalities. Most other genetic conditions are rare.
3. Pre-existing lesions
Actinic keratoses are common sunspots and indicate chronic or long term sun exposure. Once formed they may resolve, remain stable or progress. About 10% become skin cancers; treatment prevents this progression. They are scaly, irregular, rough spots of various colours, commonly on the forearms and back of the hands. Treatment is recommended.
Naevus sebaceous are raised, hairless, yellowish nodules on the scalp or face, present from birth. They increase in size at puberty. About 10% progress to form skin cancers, mostly BCCs.
Cutaneous horns are hard, raised growths sticking up from fairly normal looking skin. They are longer than wide and treatment is recommended because 10% are associated with SCCs at the time of diagnosis.
4. Immune status
The immunity of a person is important in preventing the development of skin cancers. Chronic sun exposure also affects the skin’s own immunity.
This damages the skin cells’ DNA and also interferes with the cells’ ability to repair any damaged DNA. Diseases that affect immunity or medication taken to reduce immunity (transplant medication) result in a higher risk of skin cancers.
Environmental Factors
1. Ultraviolet radiation
UVR is the primary cause of non-melanoma skin cancers. The ultraviolet radiation is part of an electromagnetic spectrum that includes radio waves, infrared rays, visible light, ultraviolet rays, x-rays, gamma rays and cosmic rays.
UVR is made up of UVA, UVB and UVC. UVC is absorbed by the ozone layer. Without this layer the UVC would contribute to skin damage. About 95% of the UVR reaching the earth’s surface is UVA. It is UVB that is most likely to cause skin cancers. It directly damages skin DNA, interferes with DNA repair and reduces immunity. UVA potentiates UVB.
2. Ionising Radiation
This causes damage to skin cells by the cumulative dose. This is borne out by the higher incidence of skin cancers in uranium miners and after radiotherapy. This is more common with high dose therapy or older, less specific treatments. The radiation is considered a co-factor to developing skin cancers.
3. Chemicals
These may directly affect skin cells or enhance the effects of UVR. Atmospheric pollutants and chronic arsenic exposure fall into this category.
In Australia the role of UVR is the most important. Other factors play a smaller role and the risk increases the more variable that occur together.
In the next article we will discuss sunscreens and preventative measures.