SAN FRANCISCO — Men who show signs of balding at age 20 are twice as likely to develop prostate cancer in their lifetimes as those who keep their hair or lose it later, according to a French study.

Neither balding at 30 or 40 years of age, nor the pattern of hair loss, pointed to any increased risk of developing the disease, the researchers wrote in the medical journal Annals of Oncology. Early hair loss wasn’t associated with an earlier onset of cancer or a more severe course of the disease, the study found.

Researchers have long known that men who more readily convert the male hormone testosterone into a form called DHT have a higher risk of prostate cancer and are more likely to go bald.

Understanding that relationship better may help doctors identify men who may be helped by cancer screening, said Michael Yassa, a radiation oncologist who helped conduct the study.

“There’s a big debate about who can really benefit from prostate cancer screening’’ with the PSA test that’s used to predict risk of the malignancy in men, said Yassa, who now works at Maisonneuve-Rosemont Hospital in Montreal.

Studies have shown that men who have elevated levels of the protein measured by the PSA test don’t always have prostate cancer.

Some may end up getting unnecessary procedures and surgeries based on the test results. Those findings have raised questions about whether PSA testing is useful and which men should get the test.

“We’re saying, ‘OK, if we can’t have consensus on the general population, can we get a consensus on people who are at higher risk?’ ’’ Yassa said in a telephone interview. “With balding we don’t need to do any tests. When they walk into the clinic, we can say, ‘Were you bald at 20 or 30? If so, you may be at higher risk of getting prostate cancer.’ ’’

Giraud and his colleagues questioned 669 men, of whom 388 had prostate cancer, about their hair status at different ages.

The men classified their hair pattern according to a modified version of a widely used measure of baldness called the Hamilton-Norwood scale, and the researchers asked physicians about details of the participants’ illness.

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