What causes skin cancer besides the sun

My friend was diagnosed with a melanoma on the bottom of her foot. I thought melanoma was caused by exposure to sunlight so how is this possible?"

Melanoma is a type of cancer which usually occurs on skin that has been overexposed to the sun. However, melanomas can also occur on parts of the body that have never been exposed to the sun. Like all cancers, melanomas are the result of changed or damaged genes that lead to cancer cells being able to grow and invade other tissues. UV rays can cause these genetic changes, but so can other factors. 

Around 10 per cent of melanomas occur in people with a family history. In these people, the odds that a melanoma will appear on a hidden body part are higher than for others because of the genes they’ve inherited. However, melanomas can also occur in people, even where there isn’t any obvious family history. It is possible a small proportion of melanomas unrelated to sun might also arise because of factors like viruses, chemicals and environmental pollutants. More research is needed to confirm this. Regardless of the time you spend in the sun or your family history, it’s important to get to know your skin – all of it, not just the parts exposed to the sun – and report any changes to your GP.

If sun exposure causes skin cancers, how is it that some skin cancers grow in body parts that never see the light of day?

Between the toes, on the soles of the feet, even around the genitals ... skin cancers can appear on body parts that rarely or never see the sun. What's going on?

It's true the sun's ultraviolet light has the power to damage our genes and this is far and away the major cause of skin cancers, says Cancer Council Australia CEO Professor Ian Olver.

But if there's any downside to the success of skin cancer awareness campaigns, it's perhaps the misperception that the sun is the only cause of skin cancer. It's not.

The cancer trigger

Like all cancers, skin cancers are the result of changed or damaged genes that lead to cells being able to grow and invade other tissues, Olver says.

Left unchecked, this can be deadly as key tissues that perform vital body functions may ultimately get "taken over" by the invading cells and become unable to do their job.

But no cancer we know of is triggered by just one genetic change. There's usually a sequence of changes required and these may occur over long periods of time. So as Olver puts it, "you might need four or five 'hits' to [ultimately] cause cancer".

Sunlight on your skin could be responsible for more than one of those "hits" over time; in some people, sun might trigger most of them, he says.

"But we know it's not the sole factor at play because we do find cancers in non-sun-exposed areas [of the body]."

While such skin cancers are rare – no-one's sure exactly how rare – if you notice one, you shouldn't ignore it.

"We don't want people [who find a spot] to think, just because it's on the soles of their feet or behind their ears, 'it can't be melanoma'. It can be."

In fact a skin cancer in a hidden part of the body is actually more likely to be a melanoma, which is the most dangerous type of skin cancer. But non-melanoma skin cancers – basal cell and squamous cell cancers – can sometimes occur in non-sun-exposed areas too. Non-melanoma cancers are less deadly because they are slower to spread, but they nonetheless still require prompt treatment.

The role of genes

So if it's not sun that's triggered a skin cancer, what is it?

"The strongest 'non-sun' factor is a bad gene you've inherited," Olver says.

About 10 per cent of melanomas occur in people with a family history of melanoma. In these people, the odds a skin cancer will be in a hidden body part are higher than for others because of the "bad genes" they have inherited.

"If you've got a cancer-causing gene, you've already got some or most of the changes needed [to trigger a cancer] because you've inherited them," he says.

But melanomas can occur in hidden parts of the body even where there isn't any obvious family history.

It's possible a small proportion of skin cancers unrelated to sun might also arise because of factors like viruses, chemicals and environmental pollutants. But more research is needed to confirm this.

The bottom line is, whether it's a sun-exposed body part or not, you need to be aware of changes that might be a cancer, Olver says.

While skin checks by doctors are important, don't ignore things you notice yourself at other times.

"You need to get to know what your skin looks like and if you think anything's changing – getting darker, changing in pigmentation, size, shape, itching, or bleeding – then report it immediately."

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