Melanoma more common on left side of the body

Skin cancer is more common on the left side of the body, scientists have discovered. And sun bathing facing south may be to blame. New research shows that there are up to 19 per cent more skin melanomas on the left sides of people in Britain than on the right.

Similar differences were also found in four other countries by the researchers who looked at the sites of almost 100,000 tumours.In the first study of its kind, researchers from the from the NHS, the US National Institutes <corr> of Health and other agencies, looked at cases of melanoma in England, Scotland, Australia, The Netherlands, and the US.

In England, there were 10,345 on the left side, and 9,267 on the right in England, showing a 12 per cent left side bias. In Scotland, there were 19 per cent more cases on the left - 1,667 compared to 1,410. Australia and the US were lowest, with an eight per cent left side bias

In all countries they found a left side bias in melanoma, the most serious form of skin cancer whose incidence has doubled in the last 20 years to around 8,000 cases, with 1,800 deaths linked to spread of the disease. The research, being published in the European Journal of Cancer this week, shows that, overall, there were 51,338 melanomas on the left, and 46,883 on the right. The difference ere found in both sexes.

Ultraviolet light exposure is the major cause of all types of skin cancer, but why there should be more melanomas on the left than the right is not clear.

The researchers say there are several possible reasons for such a bias. They reject the idea that it is a chance findings because the same trend was found in all cancer registries, and for the same reason they reject a possibly of reporting errors.

They looked at the idea that handedness - 11.5 per cent of the population are left handed - might have an effect on the way sunblocks are applied, but reject that too: "While handedness might result in differential application of sunscreen to the arms, this argument is unlikely to apply to the legs or to young children who have sunscreen applied by their parents," they say. "The fact that there is an excess of left-sided melanomas on the lower as well as the upper limb makes handedness a less convincing explanation."

They say the left-sided excess on the lower limbs, and the fact that most vehicles in the UK are driven on the right side, also makes it unlikely that drivers expose their left arms through an open vehicle window.

One theory is that melanocytes, cells in the skin responsible for producing melanin, the brown pigment that helps screen against the harmful effects of ultraviolet light, may for some unknown reasons develop differently on the two sides.

Another theory is that the way people sunbathe may be involved: "If people sunbathing in the northern hemisphere tend to lie facing south in the morning, the left side of their body might be exposed and sunburned as the sun moves from east to west, making them more likely to cover up by afternoon," they say.

That would not explain the left-sided excess of melanoma in Australia, although the excess was lowest there.

"The excess seems unlikely to be explained by chance or recording bias. The most likely explanations would appear to be either differences in sun exposure or asymmetry of melanocyte distribution or characteristics arising at the time of embryological development," say the researchers.

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