One randomized trial has been done to date that compares different ways of using sunscreen and physical barrier methods to protect against common non-melanoma skin cancers. The quality of evidence to date is low, but the review team urge caution in changing behaviour on the basis of the review findings.

Keratinocyte cancer is a more common but less serious type of skin cancer than melanoma. It comprises basal cell carcinoma (BCC) and cutaneous squamous cell carcinoma (cSCC). The main risk factor for keratinocyte cancer is exposure to ultraviolet radiation, which is a component of sunlight. The incidence of this type of cancer has increased since the 1960s, and costs of treating it are high because as much as 95% of skin cancers are of this type. Applying sunscreen and physical barrier methods, such as sun-protective clothing, hats, sunglasses, and avoiding direct exposure to sun by  searching for shade when outdoors, are often advised to stop this type of cancer from developing.

A team of Cochrane authors based in Colombia and the United Kingdom worked with Cochrane Skin to investigate how different strategies such as topical sunscreen and physical barrier methods prevent the development of BCC and cSCC of the skin in adults and children, when compared with any type of control.

The team found one randomized controlled trial from Australia that evaluated two different strategies. In one group, people were instructed to apply sunscreen daily; in the other group, they were instructed to use it when they wanted. The study had 1621 participants who were monitored for 4.5 years for new cases of BCC or cSCC. The authors found low quality evidence that the strategies had similar effects  in the number of people who developed BCC (about 8% in each group) or cSCC (about 3% in each group) over the time period of the trial.  The authors found no studies that evaluated advising people to adopt other sun protection measures, such as the use of sun-protective clothing, sunglasses, or hats, or seeking the shade when outdoors.

Lead author Guillermo Sanchez, MD, MSc, PhD from Instituto de Evaluación Tecnológica en Salud- IETS, Fundación Universitaria de Ciencias de la Salud in Bogotá, Colombia answers a few questions about this review and what the evidence means:   
 
The review set out to assess sunscreen versus other methods of sun protection such as sun-protective clothing, hats, sunglasses, or shade for preventing keratinocyte skin cancer. But based on extremely limited evidence, with just one study of occasional versus daily sunscreen, the review didn't find evidence to support these preventive measures. What’s the take-home message for patients or consumers from this lack of evidence?

“Nobody should see our review as a reason to stop protecting themselves against spending too much time in the sunlight or to avoid using sunscreen when they are outdoors. There are good reasons to avoid excessive exposure to sunlight apart from non-melanoma skin cancers. One of the key features of this review is the question we set out to address; namely how much of an effect do different skin protection strategies have on non-melanoma skin cancer risk (i.e. basal and squamous cell cancer of the skin)? We need to draw attention to this because some of the media coverage of this review challenges advice that public health agencies currently provide about reducing the risks of skin cancer, namely using sunscreen and avoiding excessive exposure to sunlight. Our most important finding relates to the lack of evidence on different approaches to reducing non-melanoma skin cancer risk by how people are advised to use sunscreen. Because further high-quality research is very likely to have an important impact on our conclusions, it is necessary to carry out additional research about sunscreen effectiveness, as well as effectiveness of other sun protection strategies (such as wearing sun-protective clothing, glasses, or hats, or seeking the shade when outdoors) for preventing of these types of cancer.

Lack of high-quality experimental evidence should not be equated with evidence that such interventions are ineffective. It is important that patients and consumers do not stop protecting their skin until better quality evidence emerges. Patients and consumers in general need to consult health professionals to obtain specific advice about the need of specific preventive measures, according with their age, skin color, occupation, and presence of other risk factors for skin cancer, among other factors.”

This review focused on more common but generally less deadly types of skin cancer - based on your knowledge of other research on skin cancer prevention, do you think there is also insufficient evidence to suggest that sunscreen or other sun protection protects against melanoma?

“Our review was focused on keratinocyte cancer, sun protection for preventing basal cell and squamous cell skin cancer, so our knowledge is limited to this issue. However, we know that being exposed to ultraviolet radiation is a risk factor for all skin cancers, and this can be avoided or reduced. Several high-quality scientific publications have addressed the evidence about melanoma, and also have been focused on population at high-risk of skin tumors. ”

 If you had a patient ask you about sun protection to minimize their skin cancer risk, what would you tell them to do?

“Given the risk factors for developing skin cancer, avoiding them and increasing protective factors are likely to play a role in lowering our risk of skin cancer. Sun exposure is one of the risk factors that can be avoided or reduced, especially in children. However, as health professionals, we need more information to decide which strategy (sunscreen type, frequency, clothing, or seeking shade) or which combination of them is more effective in the prevention of non-melanoma skin cancer. Consumers should consult qualified health professionals to obtain advice about the need of specific preventive measures, according to their age, occupation, and presence of other risk factors for skin cancer. Those with a previous history of a skin cancer and those at higher risk of skin cancer (such as people with fair skin or on immunosuppressive drugs for organ transplant) should also seek specialist advice and continue with a range of sun-protective measures until better evidence emerges.”

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Selected media coverage:
"Sunscreen may not prevent skin cancer, says new study: What to do now" on Medical Daily

"Scientists rehash evidence on sunscreen and skin cancer" on Reuters UK, Yahoo! UK and Ireland, Business Insider

"Not much evidence sunscreen prevents skin cancer: Review" on NewsMax.com

"There's almost no evidence daily sunscreen use can prevent skin cancer" on Huffington Post