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However, US men should not assume that immediate treatment is best, doctors warn, because the study was done before PSA testing became common, and a newer study found the opposite.
PSA blood tests are not recommended for screening by leading medical groups but are widely used in the US, leading to a dramatic increase in the number of prostate cancers found at a very early stage.
Read: Prostate cancer: to screen or not
Whether to treat them is a big question. Most tumours grow too slowly to threaten a man's life and there is no good way to tell which ones will. Treatments can cause impotence, incontinence and other problems.
Testing less common in Europe
In Europe, PSA testing is far less common, and most prostate cancers are found when they cause symptoms such as trouble urinating. Symptoms like this suggest a larger or more advanced tumour.
Read: Shape of urine can indicate prostate issues
The study involved 695 such men in Sweden, Finland and Iceland, diagnosed between 1989 and 1999. Half were given surgery to remove their prostates and the rest were closely monitored and given surgery only if symptoms or signs of cancer worsened.
Earlier results suggested that the surgery group fared better. That trend strengthened as time went on. By the end of 2012, 200 men in the surgery group and 247 in the group assigned to monitoring had died – 56% and 69%, respectively.
Immediate surgery most benefitted men younger than 65; it didn't reduce deaths in those over that age.
"This study tells us we should be very careful about treating anyone over the age of 65 . . . and anyone who has low-grade disease," said Dr Otis Brawley, chief medical officer of the American Cancer Society.
A good trend
In the US, fewer men are getting treatment right away, and "that's a good trend and we need to continue that," he said.
The US government, the Swedish Cancer Society, the Karolinska Institute, and two foundations paid for the study. Results appear in this week's New England Journal of Medicine.
Read more:
Rise in surgery for early prostate cancer
