How does sunlight cause melanomas on the soles of the feet?

This article in the Daily Mail caught my eye today. It concerns, as often seems to be the case at this time of year, the perils of sun exposure. It includes the comments of a consultant dermatologist who tells us that she wears factor 30 or higher sunscreen on her face throughout the year, “even in November when it’s raining”, and that she never sits with the sun in my face. Well, you can’t be too careful. Expect, maybe you can, seeing as there is evidence linking the use of sunscreens with an increased risk of skin cancers, including malignant melanoma
. But only whisper these findings as, well, they seem to put a major spanner in the works of the ‘sunlight causes melanoma’ rhetoric.

And I say rhetoric because, actually, quite a lot of evidence points away from the idea that melanoma is chiefly caused by sun exposure. Some of the most salient research was summarised by emeritus professor of dermatology Sam Shuster in the British Medical Journal [1]. You can read the full text of this piece here. You can read the counter arguments here.

In his piece, Sam Shuster points out that about 75 per cent of melanomas occur in relatively unexposed sites of the body. Elsewhere, he draws our attention to a form of melanoma known as acral lentiginous melanoma, where typical sites include the soles of the feet, palms of the hand and the inside of the mouth. The Daily Mail piece today even draws our attention to this paradox, including in its title which references ‘hotspots’ for cancers including the soles of the feet.

In his BMJ piece, Sam Schuster also draws our attention to the evidence showing that in Europe and the US, melanoma incidence and deaths due to melanoma fall as sun exposure increases.

Some say that it’s not sunlight per se that causes melanoma but intermittent sun exposure and/or burning, especially in early life (the counter-argument piece argues this position). Sam Shuster pours cold water on this theory though, by writing that this theory: “is easily excluded, because the melanomas would then occur at the burn sites; there is no evidence for this, and it is unlikely that any will be found, because sunburn occurs in sun exposed sites, and these are not the sites at which melanomas occur.”

The BMJ allows people to comment on articles on-line in the form of ‘rapid responses’. One of the rapid responses comes from surgeon who suggests that Sam Shuster has taken leave of his senses. Here’s an extract:

As a newly qualified doctor who spent 8 weeks last year studying at the Sydney Melanoma centre I was dumbfounded by the claims made against a link between sun exposure and melanoma. Having spent time with endless patients reporting hours of sun exposure and sunburn in earlier years and now presenting with cancerous lesions I believe that Dr Shuster may benefit from a similar “elective” in order to change his mind on the melanoma theory! Rubbishing the claims that the high incidences of melanoma do not occur in sun bathed areas seems bizarre.

Oh, dear. Notice the complete absence of reference to relevant science here. The observations this doctor made fit his pre-conceived beliefs so, voila, the answer is obvious (to him). There is a term for this sort of (faulty) thinking: confirmation bias. This doctor suggests Sam Shuster could do with an educational trip down-under, but I suggest he himself might take a trip to a relevant textbook or the internet to read about the scientific method.

But don’t lose complete faith in medical professionals just yet. Compare and contrast that first response with a later one which comes from another surgeon:

We most certainly do NOT know for sure that sunlight exposure is directly responsible for melanoma. As Schuster so rightly states, debate based on opinion is precisely what we don’t need in this area. We need hard evidence, and our profession isn’t always the best at offering advice on this basis.

Good to see this doctor thinking straight, and not falling into the trap of propagating ideas that seem to be based more on folklore than fact.

References:

1. Shuster S. Is sun exposure a major cause of melanoma? No BMJ 2008;337:a764

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29 Responses to How does sunlight cause melanomas on the soles of the feet?

  1. Julie 11 June 2013 at 2:36 pm #

    I remember reading once that, when you’re checking your body for strange moles and spots that could be skin cancer, that you should remember to check on your feet and in between your toes. For some reason, it didn’t seem odd at the time, but now that you mention it, it is quite strange, isn’t it? So if it’s not sun exposure that causes skin cancer, what does?

  2. Chris 11 June 2013 at 4:39 pm #

    It is interesting that the second bullet point beneath the Daily Mails headline suggests, ‘The number of cases has risen faster than almost any other kind of cancer.’ I’d be interested to see the rise plotted and to wonder within which times significant increases in incidence have occurred.

    If anything the trend for humans is that many of us tend to spend more time working indoors than we did in the past and so it would be interesting to see incidence divied-up against occupations, be they essentially indoors or outdoors. We travel further to work in enclosed vehicles, and maybe don’t walk so often as we did.

    It shouldn’t be forgotten that melanin (the pigment that has us turn brown when we tan) absorbs UVB and is natures most natural sunblock, or that adequate sunlight is needed to keep levels of the highly antioxidant (and anticancer) marvel vitamin D up.

    Melanin is produced by cells in the skin, melanocytes, and the number of melanocytes can vary from person to person, but within ranges it is not the count of melanocytes that matters so much as how active or expressive the melanocytes are when it comes to producing melanin itself. Hormones bear upon the melanocytes and capacity for malaneogenesis (producing melanin). I guess the key to understanding if the melanocytes can be more responsive and productive under certain conditions or conversely less so under other conditions. Perhaps gene switching has some bearing.

    Experience tells me the bits of me that rarely if ever see the sun burn more readily than those that do, and the bits of me that have seen more sun on past occasions tend to tan more readily, so I’d hazard a guess that the responsiveness and abilities of my own melanocytes to express themselves and synthesise the all important melanin varies across my body.

    I think if we come to view our relationship with the sun as being essentially maladaptive then we have committed the sin of throwing out baby with the bathwater. In the past and prior to significant deforestation in Europe fair-skinned humans maybe experienced more habitual exposure to dappled or intermittent sunlight, where in us it has trended to all or nothing. So the way we get our sun could be a factor.

    Amidst all the uncertainty one thing is beyond doubt; the people who make and sell sunscreen have vested interests in massaging our opinions.

  3. Mark Struthers 11 June 2013 at 7:40 pm #

    I spent my fourth year medical student ‘elective ‘ period in Malawi. This was where I met my first malignant melanoma. The African patient had a large lesion on the sole of his foot and was about to be wheeled into the operating theatre to have his leg amputated. Since then I have been a little sceptical of the idea that sunshine causes melanoma.

  4. George @ the High Fat Hep C Diet 11 June 2013 at 8:39 pm #

    There was a case in New Zealand recently where a practitioner (I don’t remember if a GP or naturopath) was charged with failure to diagnose and refer an elderly Chinese man with melanoma on his heel. I am skeptical that this melanoma had anything to do with sunbathing, which is not a habit one associates with elderly Chinese here.

  5. Nigel Kinbrum 11 June 2013 at 9:43 pm #

    Is it not the case that melanoma cells could be created in one part of the body (below severely sunburned skin, say), circulate around for a while, adhere to a different part of the body (the sole of the foot, say) and grow into a tumour?

  6. Dr John Briffa 12 June 2013 at 7:01 am #

    Nigel

    I suppose anything’s possible, but are you aware of any evidence for such a phenomenon?

    I’ve never heard of anyone ever visualising melanocytes circulating in the blood.

    I think a more likely and plausible explanation is that sunlight is not a major cause of malignant melanoma, personally.

  7. jimmy 12 June 2013 at 11:29 am #

    Not sure if a British thing, however in this post a couple sentences down you use “Expect”, I would expect it to be “Except”. Accuracy in spelling does actually effect how accurate I think the overall Post is. Thanks!

  8. Nigel Kinbrum 12 June 2013 at 5:02 pm #

    Dr John Briffa 12 June 2013 at 7:01 am
    “Nigel
    I suppose anything’s possible, but are you aware of any evidence for such a phenomenon?”
    Melanomas readily metastasise, so I thought that a melanoma cell formed below a severe sunburn would be able to readily move around the body (in the blood or the lymph).

  9. André 14 June 2013 at 5:33 am #

    Life originated in the sun. The sun gives us vitamin D and energy (photo-electric effect), much like a solar panel. In our skin we have carbon nano tubes that create the energy. With an energy consuming brain, could this be the reason human lost most of their body hair?

    So nature increased sunexposure by losing that fur. And then some expert had the brilliant idea the sun is bad for us. As is cholesterol and saturated fat.

    One thing I know: if you don’t scare people enough, you can’t sell all these suncreens.

    By the way : vitamin D protects against sunburn (and skin cancer). Most people are D deficient. That’s what causes the cancer. I think.

  10. George @ the High Fat Hep C Diet 14 June 2013 at 5:44 am #

    Nigel, wouldn’t there still be a primary melanoma somewhere more obvious? Melanomas are not supposed to be occult, that’s why we have mole maps?

  11. m.penney 14 June 2013 at 5:49 am #

    Polyunsaturates were once used to suppress immune function in transplant patients until it was found that they developed cancer. Australia uses mostly unsaturated fats (even taking cream out of milk and replacing it with vegetable oil) Sunscreens contain carcinogens, too.

    Food for thought here?

  12. @lowcarb_zealot 14 June 2013 at 7:12 am #

    I wonder how many people bother to apply sunscreen to the soles of their feet before lying down on the beach all day, pointing their feet at the sun?

    Just playing devil’s advocate, I think the whole sunlight=melanoma hypothesis is flawed.

  13. Mark I 14 June 2013 at 8:25 am #

    Everyone seems to be arguing about “the” cause – could there not be multiple causes?

    @jimmy, Dr Briffa does make a few typos, but I imagine the effort in eliminating them would be significant, so I think we should excuse him (sorry, don’t mean to be patronising). Also, you said “Accuracy in spelling does actually effect”, should be “affect” :-).

  14. Paul 14 June 2013 at 8:41 am #

    “I remember reading once that, when you’re checking your body for strange moles and spots that could be skin cancer, that you should remember to check on your feet and in between your toes. For some reason, it didn’t seem odd at the time, but now that you mention it, it is quite strange, isn’t it? So if it’s not sun exposure that causes skin cancer, what does?”

    Well, as a non-medical professional, and having read about Michael Douglas’ unfortunate throat cancer, perhaps there is a connection between HPV and melonoma. Having personally suffered from verrucae outbreaks on my feet, I shall be checking my feet more frequently from now on.

  15. Sam Mackrill 14 June 2013 at 9:09 am #

    Sam Shuster BMJ link should be : http://www.bmj.com/content/337/bmj.a764

  16. Chris 14 June 2013 at 9:24 am #

    Andr&eacute,

    The loss of more heavy body hair in the evolution of the human from primate to modern human (naked ape) is interesting. It’s often put down to changes in climate, alterations to habitat, more grassy savannah, a decline in forestation, more time spent out on the savannah, and more time spent ranging and walking upright.
    Did skin colour darken on the African continent as we shed our heavy body hair? I dunno, I’d speculate it did.
    Did skin colour turn paler as early homos began to venture out of Africa and into the more temperate latitudes? The A to that Q is clear, I think.

    I’d like to know more about the skin acting as the equivalent of a photo-voltaic cell. Do you know more about this and can you direct to sources that can explain it?

    In the Wonders of Life series Prof Brian Cox was at pains to point out that life, including ‘us’, and biology, cannot breach the fundamental laws of physics. I am trending to think life cannot breach the fundamental nature of substance either, and so biology must conform to the less well understood laws of quantum physics.

    In cosmology, I learned as late as this week, a small minority of people are shunning the mass, matter, and gravity centric models of the universe and turning to plasma, or energy, centric models. Here’s the piece that informed me.

    In the last month I had a brainwave: Does biology, as a general rule, issue more ions, or ionised species of biochems, to the immediate environment as it takes in? Then the second question; do anions (the -ves) get a slight boost from some weak force as compared to cations (+ves) so, in other words, does a slight pressure gradient act upon anions such that under certain conditions more anions are issued than cations?
    Those conditions, btw, are the exception, man-made, and arise when a species ceases to have permanent or frequent (habitual) and intermittent contact with ground. For under these conditions an unhealthy +ve charge builds up on the body and leads to complications, so the theory goes.

    I don’t know how this pressure gradient would arise, if it exists it would have living things draw a weak current from ground, and it seems they do. Then If I had to guess I would say quantum theory as may apply to those endless reactions, or transactions, involving oxidation and reduction in biology may hold the key, as may the fact that all life on Earth experiences three forcefields, one being gravity, the second being electrostatic (stemming from the global electrostatic phenomenon) and the third being displacement (buoyancy) in water or air.

  17. Nigel Kinbrum 14 June 2013 at 3:00 pm #

    George @ the High Fat Hep C Diet 14 June 2013 at 5:44 am #
    “Nigel, wouldn’t there still be a primary melanoma somewhere more obvious? Melanomas are not supposed to be occult, that’s why we have mole maps?”
    That sounds logical. I don’t know exactly where a melanoma cell is first formed. If it’s on the surface of a mole, that kills my theory. What if it’s under a mole? If it doesn’t travel, the mole becomes a melanoma. If it does travel, the melanoma forms elsewhere.

    I don’t understand why a melanoma would spontaneously form on the soles of the feet if there’s no stimulus to make it form.

  18. Rita 14 June 2013 at 4:17 pm #

    Sun definitely does cause skin cancers, although whether melanoma specifically is caused is an open question. My mother, a very fair-skinned person, spent decades playing golf and gardening without sunscreen. Starting in her late 60′s, she had numerous basal cell and squamous cell carcinomas removed from her face and arms, the parts of her body most affected by sun.

  19. Missnoma 14 June 2013 at 4:45 pm #

    “Effect” is correct in this case.

  20. Dr John Briffa 14 June 2013 at 5:27 pm #

    Nigel

    I don’t understand why a melanoma would spontaneously form on the soles of the feet if there’s no stimulus to make it form.

    Well, couldn’t you make the same argument about any type of cancer? What’s the stimulus for those? Sunlight? Probably not, right? So, why can’t it be the same for melanoma?

  21. Sandy Angove 14 June 2013 at 11:28 pm #

    Interesting discussion. I’ve had a couple of ‘patches’ of Bowens Disease, also supposed to be sun connected… but they were both on the inside of my knee.. Hardly an area where one would expect to see excess exposure to the sun!

  22. Steve 16 June 2013 at 5:07 pm #

    30 years ago , when I was 33, I developed a malignant growth, called a ‘rodent ulcer’ in those days if I remember correctly, just below my left eye. This was surgically removed. I am light-skinned and had never been a person who liked lying in the sun. It was recommended that I use a strong (factor 60 or 50) sun cream whenever I ventured out. Over the years I gradually grew out of the habit of using the stuff but noticed that despite spending long hours exposed to sunlight on holiday I never got burned. With the benefit of hindsight I now attribute this apparent immunity to the fact that over the last 20 years or so I have regularly eaten tomatoes, almost on a daily basis, and am now firmly of the opinion that they confer considerable benefit in tandem with sunlight.

  23. André 16 June 2013 at 6:48 pm #

    Chris,

    This site should give you some insights. Hope Dr Briffa doesn’t mind I refer to other sites. Anyway, I do not benefit from it ;-)

    http://www.jackkruse.com/quantum-biology-9-photosynthesis/

  24. m.penney 17 June 2013 at 10:54 am #

    One answer to the ‘naked ape’ question is that our species went through an aquatic phase. Sir Alistair Hardy first noticed the similarities between us and the aquatic mammals in the 1960s, and Elaine Morgan has written several very persuasive books on the subject.
    This hypothesis answers all the questions about why we are so different from our closest primate relatives.

  25. Chris 19 June 2013 at 5:53 pm #

    André,

    Thank you so much for that link to Jack Kruse. His theories and propositions are not short on substance are they? I shall follow him attentively in future.

    His piece, “QUANTUM BIOLOGY 9: PHOTOSYNTHESIS”, from 6th June 2013 deserves a thorough and attentive read through so I’ll be returning. In that piece he clearly levels the proposition that under the ‘rules’ of quantum biology photons from sunlight have the capacity to repair DNA, subject to intensity and ‘dose’.

    m.penny,

    Aquatic mammals have been linked to carnivorous land mammals who ventured into shallows in search of food. Professor Ian Stewart gives this theory an airing in <a Href=”http://www.bbc.co.uk/programmes/p01993gc”.Rise of the Continents (ep1/6) and it has something to do with a major geologic event way back when Africa was closely associated with Pangea. (It was late and I confess to nodding off.)

    Dr Ancel Keys and Uri Geller incline me to retain some ambivalence to theories that emerged between the 1950s and 1970s – so I do not think my (several times) great-grandmother was a mermaid if that’s what you’re indicating what Hardy and Morgan each proposed. But ..

    .. the business of hair loss, climate change, alterations to habitat, and human migration is thoroughly intriguing, and for a while yet the best answers we can hope for are ‘best-guesses’.

    In a comment beneath the link André provides Jack Kruse answers pedro. He discusses the evolution of the bigger brain in the emerging human, and registers sentiments that show his favour that a seafood diet sourced from coastal margins provided brain specific nutrients that helped us on our way. I can see the logic in this. I think it is relevant, if not in isolation.

    I favour the climate induced ‘wimp’ hypothesis that perceives a contraction in the habitat that supplied the former ape-diet for our ancestors had them reduced to scavenging a carcass after a true predator had had its fill, and then sourced marrow from bones and brains from skulls. This transition phase changed properties of the diet in ways that would permit a shrinking gut and an enlarging brain, it also permits emergence and evolution of sociological traits too, and allows for trending in steps to becoming intelligent tool-carrying hunters.

    The importance of coastal and riverine habitats is not lost on Curtis Marean and Rick Potts of the Smithsonian. Marean is the director at the Pinnacle Point digs. The inhabitants at Pinnacle Point were sophisticates who adopted cyclical practices in keeping with tidal ranges. When the tides were their lowest the ancestors harvested seafood, and then when low tides were not so low they may have ranged inland in search of land animals and plants to eat. Then there is the thorny issue of migration.

    Alice Roberts examined various theories about the routes of migration out of Africa in the Incredible Human Journey (book and TV). The most significant advances could have been made when sea-levels were lower, land-bridges would be better, and migrating ancestors could have utilised coastal routes and marine resources for food. Therein resides a problem for the bone collectors, since those margins are now far out to sea and under significant depths of water.

    Kruse answers pedro, “Higher levels of Vitamin D cause hair loss in the primate clade. Another fact the bone collectors got wrong.” I read into this that Kruse thinks food richer in vitamin D could have us lose hair. Loss of hair and loss of deep pigmentation could have us adapt to being better able to synthesise vitamin D. I think the loss of hair and the prevalence of sweat glands on a human suggests adaptation to spending more time ranging under a hot sun in far more open grassland simply because it helps keep an increasingly accomplished biped cooler.

    Foods that tend to be richer in Vitamin D tend to be cold-water fish, cos the beginnings of vitamin D sythesis start back with cold-water zooplancton and their place in the marine food chain. That has to be a boon in temperate and sub-temperate lattitudes.
    It would be intersting to become informed how vtamin D synthesis and utilisation differs in a primate and modern human.

    The answers are the kernels to questions that are not easy nuts to crack.

  26. Chris 19 June 2013 at 5:55 pm #

    <a href=”http://www.bbc.co.uk/programmes/p01993gc”.Rise of the Continents

  27. Chris 19 June 2013 at 5:56 pm #

    .Rise of the Continents

  28. André 20 June 2013 at 6:25 am #

    One other thing Jack states is that humans are able to hydrate through the feet. So walking in the sea hydrates us. As seawater contains salt, the salt has to be reversed. That could be the reason only humans have sweaty feet. (apes don’t!). Can’t deny we seem to be exceptionally suited for a life along the shorelines. At the same time time that provides ‘grounding’; we need to ‘earth’ for proper energy creation. That’s why walking bare foot or on leather soles is so much better than rubber or plastic.

  29. Thomas Keryk 29 June 2013 at 1:30 pm #

    Has anyone considered what we put on our feet? Flip flops, sneakers, shoes, socks treated with triclosan and other antimicrobials could be a contributor. Leathers treated with chrome, rubber materials manufactured with who knows what. Chemicals being boiled to the surface of the flip flops by the sun, soaked up by the soles of the feet when done sunbathing. Vitamin D is made from cholesterol. What do these cholesterol lowering drugs do? Lower cholesterol, lower vitamin D (and low cholesterol is related to increased cancer risk). Question: do people who develop these melanomas have naturally (or induced) low levels of cholesterol to begin with? High cholesterol levels limits cancer development.

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