Sun exposure makes people both more and less likely to die of melanoma. How can that be?

Quick quiz question: two people are diagnosed with melanoma — Sarah Sunburn, an adamant sun-worshipper, and Paula Pale-All-The-Time, a fanatical sun-avoider. Who is more likely to die of the disease?

The answer is pale-faced Paula. Surprised? Let me unpack this mystery and explain why sun exposure simultaneously kills people, while making the cancers they are diagnosed with appear to be less life-threatening.

I will start with what you probably know already. Melanoma is a potentially life-threatening skin cancer. It occurs usually as a result of genetic risk, including fair complexion and sun exposure. (I lost my beloved redheaded aunt to melanoma when she was tragically young.) All else equal, people who spend more time in the sun, or in tanning booths, are more likely to die of melanoma, with people of fair complexion especially at risk.

Now here is the part that you might find confusing: among people diagnosed with melanoma, those with a history of lots of sun exposure and fair complexion are less likely to die. That’s right: sun exposure makes people both more and less likely to die of melanoma. How can that be?

It’s the difference between the rate of death from melanoma and the death rate of people diagnosed with melanoma.

The rate of death from melanoma is the proportion of people in a population who die of the disease. By this measure, people with lots of sun exposure are more likely to die of the disease.

Here are some imaginary numbers. Imagine there are 100 sun-worshippers, 6 of whom get diagnosed with melanoma. Imagine there are also 100 sun-avoiders, only 2 of whom get diagnosed with melanoma. By this measure (the rate of melanoma diagnoses) sun exposure is bad, as we all know. Looking more closely at these two imaginary groups, let’s suppose that two sun-worshippers die of melanoma versus only one sun-avoider. Once again: Sun exposure is bad. But here is the twist: Out of the six sun-worshippers with melanoma, only two died, for a fatality rate of 33%; by contrast, 1 of the 2 sun-avoiders diagnosed with melanoma died of that disease, for a fatality rate of 50%.

The key here: if we only look at people diagnosed with melanoma, it will look like sun-worshipping is protective, cutting the death rate from 50% to 33%. But that’s not the right way to look at the situation. We need to look at the population as a whole, a view that confirms the well-known hazards of too much time in the sun.

The death rate of people diagnosed with melanoma is a conditional probability: among people diagnosed with melanoma, this number shows how many will die. By this measure, as discussed above, people with lots of sun exposure appear (misleadingly) to be less likely to die of melanoma.

Think of it this way: People with lots of sun exposure are more likely to see dermatologists, for treatment of all the sun damage on their skin. While at those appointments, the dermatologists find melanomas that otherwise might have gone undetected. More importantly, some of those melanomas might be relatively mild, and if undetected might have never become life-threatening. My wife, for example, is a redhead, whose family is chock full of people who have been diagnosed with melanoma. So she has been diligent about seeing dermatologists regularly. One of those doctors removed a bad looking mole a few decades ago: the pathology revealed a melanoma. But it wasn’t a bad melanoma. It was a borderline case. In fact, a decade later, a melanoma expert at the University of Pennsylvania reviewed her pathology slides and downgraded her diagnosis to “severely dysplastic nevus.”If she had never seen a dermatologist in her life, it’s possible that dysplastic nevus would have never progressed, and she would have never carried a melanoma diagnosis. It’s also possible it would have become a life-threatening melanoma.

The bottom line? It’s bad news that sun exposure reduces melanoma death rates. Sun exposure kills. Melanoma kills. People should be prudent about exposing themselves to the sun and quick about going to dermatologists when they exhibit suspicious moles. There is no mystery about that – that’s life-saving advice.

Peter Ubel is a physician and behavioral scientist who blogs at his self-titled site, Peter Ubel and can be reached on Twitter @PeterUbel. He is the author of Critical Decisions: How You and Your Doctor Can Make the Right Medical Choices Together. This article originally appeared in Forbes.

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