Sunshine Superman

Sun exposure reduces prostate cancer risk, but increases the risk for sun-induced cancers.

by Alicia Di Rado

In the largest such study to date, a research team from three cancer centers—including USC/Norris Comprehensive Cancer Center—has found that increased exposure to sunlight may decrease the risk of prostate cancer.

Researchers reported in the June 15 issue of Cancer Research that men with high sun exposure had half the risk of prostate cancer than did men with low sun exposure.

The team was led by epidemiologist Esther John, Ph.D., from the Northern California Cancer Center, and included Sue Ingles, Ph.D., assistant professor of preventive medicine at the Keck School of Medicine, and Gary G. Schwartz, Ph.D., associate professor of cancer biology and public health sciences at Wake Forest University.

The group reported that the reduction in risk was modified by certain gene variants that reduced risk in some men even further, to as much as 65 percent.

“We believe that sunlight helps to reduce men’s risk of prostate cancer because the body manufactures the active form of vitamin D from exposure to sunlight,” John says.

Ingles notes that the vitamin D findings, if borne out by future studies, could have significant impact on public health. “Prostate cancer is the most common non-skin cancer among American men, with about 230,000 new cases and 30,000 deaths occurring each year,” says Ingles, a researcher at USC/Norris. “Since there are few known modifiable risk factors for prostate cancer, our finding that a modifiable factor, vitamin D, may reduce risk is important.”

Previous research by Schwartz and colleagues showed that the prostate uses vitamin D to promote the normal growth of prostate cells and inhibit the invasiveness and spread of prostate cancer cells to other parts of the body.

“The genes involved are those that determine the type of vitamin D receptors a person has,” Schwartz says. “These receptors, which function with vitamin D like a lock and key, vary in their ability to bind vitamin D, and thus to influence cell behavior.”

The researchers also stressed that sunlight is not the only source of vitamin D, and that men should not try to reduce their risk of prostate cancer by sunbathing because that increases the chances of getting sun-induced skin cancers, especially melanoma.

“If future studies continue to show reductions in prostate cancer risk associated with sun exposure, increasing vitamin D intake from diet and supplements may be the safest solution to achieve an adequate level of vitamin D,” they wrote in Cancer Research.

The researchers compared 450 non-Latino white patients in the San Francisco Bay area who had advanced prostate cancer with a matched control group of 455 men who did not have prostate cancer. They defined advanced prostate cancer as cancer that had penetrated through the prostate capsule, spreading either to the same region of the body or to distant sites.

Using a reflectometer, the scientists measured sun exposure by comparing pigmentation of underarm skin, which is usually not exposed to sunlight, with forehead pigmentation, which is often exposed to the sun.

Because it is hard for sunlight to reach the underarm area, researchers saw no statistical difference in the pigmentation measurement between men with prostate cancer and those in the control group. But when forehead color was compared to underarm color, the control group had significantly darker pigmentation than the cancer patients.

Increasing forehead color darkness was associated with decreasing risk of prostate cancer. The scientists also obtained a sun exposure history from each participant so they could track outdoor activity. The researchers conclusion: “Reduced risk of advanced prostate cancer was associated with high sun exposure determined by reflectometry and high occupational outdoor activity.”

David Van Den Berg, Ph.D., research assistant professor of urology at the Keck School, also participated in the study. The Cancer Research Fund of the California Department of Health Services supported the research.

 

Improving Evaluation

Oncologists monitor how well patients are responding to prostate cancer treatment by measuring the levels of prostate-specific antigen, or PSA, in patients’ blood. An elevated PSA level can mean that cancer is growing. But PSA is far from a perfect cancer gauge: Other non-cancerous conditions may elevate PSA, while some cancers do not raise PSA. Imaging tests such as bone scans also are not sufficiently accurate and cannot show soft-tissue disease.

A more advanced method to monitor patients was needed. Enter Hossein Jadvar, M.D., Ph.D., associate professor of radiology at the Keck School. Jadvar believes that combined positron emission tomography-computed tomography, or PET-CT, imaging systems will provide physicians with better information about patients’ progress during treatment. He also hopes it will help better predict patients’ outcomes, providing more realistic expectations about the success of therapy.

“Prostate cancer will only become a bigger health problem in the future, as life expectancy is increasing,” Jadvar says. “We need to improve the way we evaluate prostate cancer patients.”

And the National Cancer Institute agrees, so much so that it awarded Jadvar nearly $3.4 million over five years to study PET-CT imaging in the monitoring of metastatic prostate cancer during treatment.

Radiologists use PET to get a unique image of the whole body. When used in patients with cancer, it can detect places where cancerous cells have spread, even if they are far from the original tumor.

For this study, Jadvar will use PET together with a chemical called [F-18] fluorodeoxyglucose, or FDG, a form of sugar marked with a radioactive tracer.

Radiologists inject the FDG into the patient, which enables the PET scan to show if there are any hot spots in the patient’s body where the sugar is being metabolized quickly. These spots may indicate cancer, Jadvar explains, because cancerous cells use sugar more than healthy cells.

Together, the technology is called FDG-PET, which Jadvar will use in conjunction with CT to provide even more detailed information.

Jadvar says, “I’m hoping FDG-PET will help give patients the best information we have, and ultimately enhance overall survival.”