MONDAY, June 6 (HealthDay News) -- People with Parkinson's disease may have twice the risk of developing the deadly skin cancer melanoma, a new report confirms.

Parkinson's disease, which affects about one million people in the United States, is a progressive neurological disorder that causes tremors, stiffness and difficulty with movement, coordination and walking. Melanoma, with nearly 70,000 new cases diagnosed each year in the United States, is the most dangerous type of skin cancer.

"Over the past decade there has been accumulating evidence that Parkinson's disease patients are less likely to have most types of cancers, particularly smoking-related cancers; however, they were more likely to have melanoma," said lead researcher Dr. Honglei Chen, head of the Aging & Neuroepidemiology Group at the U.S. National Institute of Environmental Health Sciences.

Not only are people with Parkinson's disease more likely to develop melanoma, but people with melanoma are more likely to develop Parkinson's disease. "It appears the association is bidirectional," he said.

Scientists have theories about why these conditions seem to co-occur, but no firm answers. They know it's not the drugs Parkinson's patients take, Chen said.

"Both conditions may have common genetic pathways or environmental factors or both," he said. "We understand very little about this possible relationship."

Ultraviolet radiation from the sun can cause skin cancer, including melanoma. No association was found between Parkinson's disease and skin cancers other than melanoma, however, the investigators said.

The report, which was supported by the National Institute of Environmental Health Sciences, is published in the June 7 issue of Neurology.

For the study, Chen's team analyzed 12 epidemiologic studies dating from 1965 through 2010 that looked at a possible association between Parkinson's disease and melanoma. This is a process called a meta-analysis, in which data from various studies are pooled together to try to find a consistent statistical pattern.

While most of these studies reported fewer than 10 cases where both conditions occurred, the researchers found that men with Parkinson's disease were twice as likely to develop melanoma as men without Parkinson's disease.

For women, the risk of melanoma was one-and-a-half times greater if they had Parkinson's disease than if they didn't, Chen's group found.

However, since both conditions are relatively rare, the odds of having Parkinson's disease and melanoma are only about 4 percent, Chen said. "The absolute risk is not that high," he added.

Still, "it is prudent for Parkinson's disease patients to be a little cautious about their skin health," Chen said. "Be prudent, but do not be alarmed."

Limitations of the study include the fact that most of the studies were not originally designed to evaluate the association between melanoma and Parkinson's disease, and the analysis is based on a very small number of cases, the authors noted.

The authors also acknowledged that their research is preliminary and said more studies are needed to explore the relationship between the brain disorder and the potentially lethal skin cancer.

Joyce Oberdorf, CEO of the National Parkinson's Foundation, said an association between the two conditions is becoming increasingly evident, but most patients and doctors are unaware of the possible connection.

"Drawing attention to the risk is definitely required," Oberdorf said, adding that the foundation is launching a campaign to alert patients.

"We are making the recommendation that every person with Parkinson's wear sunscreen and receive an annual screening by a dermatologist for melanoma," she added.

More information

For more information on Parkinson's disease, visit the U.S. National Institute of Neurological Disorders and Stroke.