23 July 2011
The inspiration for this week’s article is not from one person, but from a culmination of many people asking me “dumb” questions about cancer and forcing myself and others to re-examine the assumed, the dogma, and the textbooks. If B leads to C, and C leads to D, and A hinders B, does A inhibit D?
It’s common knowledge that sunscreen helps prevent UV exposure to the skin. It’s well established that UV rays cause DNA damage. It’s also well established that DNA damage leads to carcinogenesis and cancer. But does sunscreen really prevent melanoma, the most dangerous and lethal of skin cancers?
Above: Sunscreen has been applied to the right half (left to us) of this man’s face. On the right is an image taken in the visible spectrum of light (what we see) and on the right is one taken with the ultraviolet spectrum. Image source: Wikipedia Commons
After all, common knowledge is meant to be questioned, and any assertion should be backed up by experimentation whenever possible. In biological systems, transitive theorems can run into trouble because of unforeseen confounding factors. Whenever possible, one needs to see if, say, A inhibits D.
I was surfing PubMed (link) the other morning and found this interesting study.
Here is the abstract: PURPOSE: Regular sunscreen use prevents cutaneous squamous cell carcinoma long term, but the effect on melanoma is highly controversial. We evaluated whether long-term application of sunscreen decreases risk of cutaneous melanoma. Participants and METHODS: In 1992, 1,621 randomly selected residents of Nambour, a township in Queensland, Australia, age 25 to 75 years, were randomly assigned to daily or discretionary sunscreen application to head and arms in combination with 30 mg beta carotene or placebo supplements until 1996. Participants were observed until 2006 with questionnaires and/or through pathology laboratories and the cancer registry to ascertain primary melanoma occurrence. RESULTS: Ten years after trial cessation, 11 new primary melanomas had been identified in the daily sunscreen group, and 22 had been identified in the discretionary group, which represented a reduction of the observed rate in those randomly assigned to daily sunscreen use (hazard ratio [HR], 0.50; 95% CI, 0.24 to 1.02; P = .051). The reduction in invasive melanomas was substantial (n = 3 in active v 11 in control group; HR, 0.27; 95% CI, 0.08 to 0.97) compared with that for preinvasive melanomas (HR, 0.73; 95% CI, 0.29 to 1.81). CONCLUSION: Melanoma may be preventable by regular sunscreen use in adults. For those that do not speak sci-geek, please let me translate: During a four-year period in Queensland, Australia a group of 1600 volunteers were randomly split into two groups: test and control. The test group was told to use a UV-blocking sunscreen every day for four years. The control group was given a dummy sunscreen that looked and smelled like the real stuff but had no UV protection.
People in the control group had a 2X higher incidence of melanoma. Those that had UV-blocking sunscreen did not develop melanoma as frequently as the control group. Also, the most aggressive forms of melanoma were not as common among the UV-protected group as well. So, sunscreen lessens both the risk of melanoma, and further still lessens the risk of the more aggressive forms of melanoma.
However, some people who received the UV-protecting cream still had some incidence of melanoma. The rate of melanoma was reduced, but not ablated. This underscores the importance of regular check-ups and avoiding harsh, direct sunlight whenever possible. For instance, if you’re an athlete and love running, it’s still a good idea to avoid running in the middle of the day, even if you use sunscreen liberally.
Ryon
References: http://www.ncbi.nlm.nih.gov/pubmed/21135266