Why is skin cancer more prevalent now than in ancient times?

Cecil's Columns/Staff Reports
41

The evidence about sunscreen causing melanoma is exactly as weak as the evidence that intermittent high UV causes melanoma: guilt by association.

I do not personally think sunscreen has a thing to do with melanoma. I am not trying to persuade you it is harmful. The evidence that intermittent strong UV exposure causes melanoma is the based on the observation that those individuals have a higher risk. They are exactly the same cohort as ones using sunscreen, so the evidence for either as the cause is exactly the same (and studies providing clarity equally problematic).

Based on an association of UV and lentigo maligna melanoma (which is really transition of actinic damage to a malignant form) it might make sense to implicate UV for the other 85% of “regular” melanomas and draw a conclusion that UV should be implicated for all melanomas.

However, the available data shows that prolonged exposure to UV is protective for melanoma, so the association of UV and (regular) melanoma is substantially weakened.

Here’s how all the advice tends to be heard by the typical lay person:
“Stay out of the sun. Use sunblock or sunscreen generously, or stay indoors.”

Here’s how I’d summarize a best practice:
“Avoid getting sunburned. Chronic exposure to the sun will age your skin and increase the risk of many skin cancers. There are health benefits to being in the sun, so the best advice is to have your skin regularly checked by a qualified clinician. There’s no evidence that being in the sun increases your overall chance of melanoma, and there is some evidence that overall health benefits may accrue despite the increased risk of some skin cancers.”

If I were asked about melanoma in particular:
“The evidence that UV causes lentigo maligna melanoma is pretty good. We think LMM is basically a skin-damage cancer, and we know UV causes skin-damage cancers. However, this is only about 15% of all melanomas. For the other 85%, we don’t know. Chronic sun exposure has been shown to be associated with a decreased risk, for reasons that are not known.”

If I were asked about the overall benefits of being out in the sun versus avoiding it:
“At least one study looking at all-cause mortality in women found that those who avoided the sun had significantly higher overall mortality. Don’t confuse lily white skin as a marker for overall health.”

42

See here is where I get confused …

From those articles:

(Now they call a 60 to 70% increased risk (1.6 to 1.7 OR) only “modest” but they do conclude that it is the most important risk factor.)

There is a solid correlation between strong intermittent sun exposure and melanoma. There is solid evidence that lighter skinned people who live in higher irradiance areas are more likely to get melanoma than those who do not and that those whose skin has more innate protection from UV get less skin cancers, including melanoma, within the same areas than not.

The problem though with these epidemiological studies is the usual one: correlations are not causations, could be confounders that travel with moves to other climates, other factors that travel with skin pigmentation … what we need to really say something with more confidence is a long term randomized prospective controlled study. And we have one!

This randomized prospective study documented that regular sun screen use decreased melanoma rates.

I cannot find any articles that control chronic sun exposure for skin type (possible reverse causation as those with paler complexions and at greater risk may avoid chronic sun exposure) but more sunscreen used regularly decreased melanoma rates when skin types and other behaviors were randomized and controlled for. Let me emphasize: a 73% decrease in invasive melanomas.

The problem with sunscreens is mostly that people use them incorrectly. They don’t put on enough and they don’t reapply often enough, but they do end up with a sense of false confidence and end up with more intense UV exposure.

Now it likely is that some regular sun exposure is better than none (and to be fair Australians applying sunscreen regularly are still getting some significant regular exposure). Likely there is a dose response curve and a sweet spot frequency/intensity of exposure for each skin type. Vitamin D and all that. Moderation … we’ve heard the drill.

But the evidence that melanoma is caused by UV is very solid.

43

Nice followup, DSeid, thanks.

I have never encountered any physician who ever counseled patients to “stay out of the sun” and who equated “lily white skin” with health. Even my mother (a dermatologist) who was personally sun-phobic never urged her family to avoid sun exposure.

Based on examination of a skin biopsy I had recently (for a benign lesion), I do have a little actinic damage (solar elastosis, which causes a homogenized, bluish tint in the dermis). The dermatologist who did the biopsy commented succinctly “A hat is your friend” (and I do wear a hat of some kind regularly when outdoors for extended periods, plus sunscreen most of the time).

If you want to be impressed with what unprotected sun exposure does, spend some time in Texas, where you’ll see lots of leathery-looking people who’ve prematurely aged badly. The guy who cut our hay crop had had multiple facial surgeries for skin cancers, and it appeared only scar tissue was holding what remained of his face together. I think we can do without that kind of “healthful” exposure.

44

From here:

From here:

I’m not sure why what I said carries any mystery. Perhaps you misunderstood me: I said “sun damage from UV”, not “sun exposure to UV”. Sun damage consists of changes to the DNA of the cell due to mutations.

Okay, looking at the plots in Fig 1 and Fig 2:

Fig 1 shows that low sun exposure group has about 93% survival at 15 years, versus the high sun exposure group that has about 96% survival. That is a small but measurable level of survivability due to sun exposure, and the intermediate exposure tracks an intermediate curve, which would be consistent.

Fig 2 shows that the three curves for low sun, moderate sun, and high sun exposure track consistently and parallel with low sun showing highest mortality rate and high sun the lowest mortality rate. The low sun has 2 times higher mortality rate than the high sun group.

These two plots do seem to show what you state, that more sun exposure = better survivability.

I do note this from the “Strengths and Weaknesses” section:

Wait right there. They just said this study has no ability to distinguish between the consequences of an unhealthy lifestyle and avoidance of sun exposure. In other words, were the people avoiding sun exposure to reduce their cancer risk, or were they avoiding sun exposure because they were too weak/fat/old/unhealthy to do much physical activity including outdoor activity? I would fully expect people who are bedridden or essentially crippled to have much less time in the sun. That seems to be a pretty big shortcoming.

45

First, thanks for the article from the J of CO. One of the best I’ve seen in favor of a beneficial effect for sunscreen, presumably for the intermittently-exposed who are the cohort using sunscreen.

Regarding the other articles in post 13. You are right that there is an accepted relative risk increase for intermittent exposure. But what you didn’t quote is the fact that chronic sun exposure is protective. Protective. It reduces risk, at least in studies trying to evaluate this so far. Intermittent sun exposure increases risk; chronic sun exposure reduces it. If the risk is just related to exposure to UV it’s difficult to reconcile those two items. If what’s happening instead is that burned skin has a higher incidence of becoming dysplastic, then making UV the culprit as “the cause of melanoma” is not a very accurate way of presenting the issue. (Yes; the UV burns the skin, but is not necessarily creating the dysplasia per se. 2% of burn scars might develop squamous cell carcinoma, but we don’t say “exposure to heat causes skin cancer.”)

It is much more accurate to say that the data suggests (probably high-dose) intermittent sun exposure might cause a higher incidence of melanoma, but that chronic sun exposure is protective against it. IOW, “Don’t get sunburned.”

This is a substantially different message from “Don’t get exposed to UV. It causes melanoma.”

The message we have gotten out there instead suggests something similar to thinking of smoking as carcinogenic:
A little is probably bad, a lot is definitely bad; the whole association is sort of dose-related so that chronic smoking is really bad, and you would be better off health-wise not smoking at all.

This is not at all analagous to the data for UV and melanoma, and in fact besides just being protective against melanoma, chronic exposure to the sun may be protective against other causes of death.

46

I agree with your description of the chronically burnt up guys. I’m just trying to stick to what the literature says to avoid anecdotes. Maybe that guy will outlive us both. Me anyway.

Re the average polloi’s takeaway wrt to recommedations about UV exposure. Perhaps I am over-reading into the recommendations or have a circle of more panicky mom’s. Here’s a typical set of recommendations,and I leave it to you to decide what a typical inference would be from it:

*Sun Safety Tips

Check the U.S. Environmental Protection Agency’s UV Index before you spend time outdoors and plan your sun protection accordingly, using these tips—

Seek shade, especially during midday hours.
Cover up with clothing to protect exposed skin.
Wear a hat with a wide brim to shade the face, head, ears, and neck.
Wear sunglasses that wrap around and block as close to 100% of both UVA and UVB rays as possible.
Use sunscreen with broad spectrum (UVA and UVB) protection and sun protective factor (SPF) 15 or higher.
Remember to reapply sunscreen at least every 2 hours and after swimming, sweating, or toweling off.

Fast Facts About Skin Cancer

When you’re having fun outdoors, it’s easy to forget how important it is to protect yourself from the sun. Unprotected skin can be damaged by the sun’s UV rays in as little as 15 minutes. Yet it can take as long as 12 hours for skin to show the full effect of sun exposure.
Even if it’s cool and cloudy, you still need protection. UV rays, not the temperature, do the damage.
Tanned skin is damaged skin. Any change in the color of your skin after time outside—whether sunburn or suntan—indicates damage from UV rays.
Anyone can get skin cancer, but some things put you at higher risk.
Indoor tanning exposes users to both UVA and UVB rays, which damage the skin and can lead to cancer.
A change in your skin is the most common symptom of skin cancer. This could be a new growth, a sore that doesn’t heal, or a change in a mole.*
I have patients who, if told this, would take away that 15 minutes is too much sun, and that a tan is harmful. It’s not that simple.

47

Actually, I did address that. Again, the issue is the fundamental limitation of epidemiology based studies: controlling for confounders. You are making the standard error of assuming causation from a correlation when there is no report of even attempting to control for confounders. Actually the lack of even attempts to control for known confounders kind of shocks me. “Chronic sun exposure within geographic population groups is associated with less melanoma risk” is not equal to “is protective from …” It suggests that as one hypothesis but alternate hypotheses are equally viable, such as but not limited to:

  • Lifestyle factors associated with chronic sun exposure are protective, such as diet and exercise.
  • Selection bias such that those who are more at risk for melanoma, such as because of skin type, are, within a particular geographic cohort, less likely to have chronic sun exposure, be that by choice based on perceived risk, or by socioeconomic variables that travel with pigment (many societies, not only America, are stratified such that lighter pigmented people are of higher SES and more likely to be exposed to less sunlight chronically).
  • The apparent protective effect is exclusively by way of decreased risk of sunburns resulting in decreased risk outweighing a hypothetical smaller increased associated with chronic exposure.

Randomized prospective studies, when available, thus trump epidemiological studies as evidence, and what we can currently state is that such a study has demonstrated that decreased chronic sun exposure by regular use of sun screen (in a high irradiance environment at least) is protective (not just correlated with) significantly decreased risk of invasive melanoma.

Could that be way of decreased sunburn incidence in the regular sunscreen use group, yes, but clearly more chronic exposure was NOT more protective.

No question there is room for discussion on how we message sun exposure and protection. Sun exposure is not like lead or smoking. Sun exposure is instead an issue of having some right amount without really knowing what that right amount is.

FWIW the Australia Cancer Council does try to strike that balance in their messaging.

They even discuss how much regular exposure is likely a good idea (using vitamin D levels as their proxy) based on skin type and location.

In America it is is sort of like the circumstance with alcohol - messaging that a little might be good is felt to be too subtle for our audiences and is avoided out of fear that such a message will increase the risks of too much. IMHO.

48

Anyone notice that Cecil posted an update to this column addressing some of the objections raised in this thread?
Powers &8^]

49

Thanks for that. I note that Cecil’s reply is essentially the things we’ve said in this thread.

You are right, it’s not that simple, but it is true that a tan is an indication of harm. You keep focusing on malignant melanoma, which is admittedly the most severe cancer, but it is not the only one, and more importantly, it is not the only form of sun damage.

A tan is a sign of damage. Melanin response is the body’s response to UV, it is a defensive response. UV affects the skin, not just with DNA damage that leads to cancers, but with thickened skin, deep set red undertone to the tan, wrinkles and creases, freckles, and liver spots.

You know where the term “redneck” comes from? The chronic exposure to sun of a certain class of rural population including farmers, oilfield workers, etc, who wear shirts doing their work but have their necks exposed. The sun tans the back of the neck with a deep tan, but it has a red undertone. Thus, “red neck”.

Have you seen women in their 30s who bought into the tanning hype when they were teenagers and now their chests have that same deep red undertone, along with freckles and leathery texture? It’s ironic they were going for that “healthful tanned look” and now their skin is less attractive.

Sure, some sun exposure is good - 20 minutes a day. Sure, chronic exposure is protective to melanoma above intermittent exposure. If you work outside all day, every day, soak up that sun and not increase your melanoma risk. Your non-melanoma risk will go up, and the other damage effects will build up, but you get plenty of vitamin D and your risk of melanoma will plateau.

But that doesn’t mean you have zero risk of melanoma. Oops. So you could do that and feel confident you have a lower risk than intermittent exposure, and still end up with malignant melanoma. Oh well.

50

I find this acceptance of the hypothesis that intermittent sunlight is bad but long exposure is good to be too focused on a single aspect of the topic. There are many studies showing that common sunscreen ingredients and other common chemicals are or are suspected of increasing cancer, that some healthy food can reduce cancer, and that some unhealthy food can increase cancer.

Even though the original question basically asked for a comparision of modern and ancient skin cancer rates, a better way to ask the question is “What are we doing in modern times that is increasing our cancer risk unnecessarily?”

Carcinogenic chemicals, poor diet, not enough exercise, above or below optimal amount of sun exposure or times of sun exposure given a person’s genetics (UVA and UVB are different at different locations, weather conditions, angle of the sun, etc.), and according to some maybe damage to the ozone layer or air pollution.

51

No veramis, there is not any evidence that the chemicals in common sunscreens cause cancer. In fact the prospective randomized study I cited provides some solid evidence against that: being randomized into a regular sunscreen use group resulted in less cancer than being randomized into routine care. More sunscreen resulted in less cancer.

“What are we doing in modern times that is increasing our cancer risk unnecessarily?” (Other than smoking) primarily what we are doing is living longer.

Cancer is more than anything else “[a disease of older people](a disease of older people, with incidence rates increasing with age for most cancers), with incidence rates increasing with age for most cancers …”

That said cancer deaths and incidence are both still steadily decreasing.

Cancer is also not a modern pestilence, or even a human one.

That does not mean that there are not actions to be taken that can reduce that risk. In much of the world (much of the world being poor) indoor pollution from open fire cooking, heating, and leaky stoves, is responsible for 17% of all lung cancer deaths worldwide (in particular in women). Hepatocellular carcinoma is worldwide often caused by neonatal infection with Hepatitis B which is preventable with immunization. No question less inactivity, less obesity, more fiber rich foods, less processed meats intake, so on, all could help decrease cancers overall. (Not as much as dying earlier of other causes first would of course.)

52

From an article in today’s N.Y. Times (focusing on tanning bed risks to young people, with commentary on the association between uV exposure and melanoma risk):

*"Evidence of the link between melanoma and ultraviolet exposure may have been inconclusive a decade ago, but recent research, including fresh data from the Cancer Genome Atlas, a federally funded program that is cataloging genetic mutations responsible for cancer, bolsters the case for the link.

Dr. Jeffrey E. Gershenwald, a leader of the melanoma Atlas project, said studies to date showed that a majority of melanomas initially arising on the skin contain mutations associated with ultraviolet exposure. As for burning, one recent study controlled for that, and still found an increased risk from indoor tanning.

“There’s no longer a question of whether UV is important,” said Dr. Gershenwald, the medical director of the Melanoma and Skin Center at the University of Texas MD Anderson Cancer Center. “Genomics has been transformative in our understanding of melanoma.”*

53

I’ve read so many lofty proclamations in this thread, and much as I would like to have the topic be so black and white, the proclamations seem to be mostly coming from people who have little or no knowledge of the readily available information on the topic besides taking one or two studies and accepting their hypotheses as truths.

I don’t want to get into a debate with such people, so I will just put this link here for those who actually care about the information that is available on the topic which makes it obvious that the black and white explanations given in this thread are at best misguided.

54

The link provided by veramis doesn’t provide much support for the proposition that sunscreens are harmful (and I can’t find a single mention there of any purported cancer risk whatsoever).

The article states “Experts caution that the unintentional exposure to and toxicity of active ingredients erodes the benefits of sunscreens (Krause 2012, Schlumpf 2010).”

If you do a search using those researchers’ names and the topic of sunscreens in PubMed, you do find papers they co-authored in 2012 and 2010. I don’t see any mention in the abstracts of those papers about how sunscreen benefits are “eroded”. The abstracts cite the presence of sunscreen compounds in urine and/or breast milk, which the authors apparently find alarming, but they do not attempt to equate their presence with any human disease. If there is any good evidence of actual pathogenicity, I’ve yet to see it.
[QUOTE=veramis]
I will just put this link here for those who actually care about the information that is available on the topic
[/QUOTE]
Caring about the topic does not require agreeing with you.

I’ve seen a lot of bad effects of excessive uV exposure. I care about preventing them.

55

This is one of the reasons I love reading the SDMB. It is almost impossible to BS your way through a debate, even with cites from reputable sites. Nice work Jackmannii.