The 10-year survival rates were also nearly identical, at just over 63 percent in both groups.
"It looks like when you get to 18 months, you reach a threshold where you are not going to improve survival by treating for a longer time," Nabid said.
Roth, who was not involved in the research, said the study might possibly change the standard of care. But he noted that findings presented at medical meetings are generally considered preliminary until all the data can go through peer review prior to publication in a journal.
Once that happens, Roth said, "I would hope that this changes people's minds."
The findings described at the meeting focused on patients' survival. Nabid said his team is still analyzing the data they collected on side effects and quality of life.
The presumption is both would be better in the long run. Roth said that when a man is on hormonal therapy for three years, the chances of his testosterone levels returning to normal afterward are low. A shorter duration of therapy boosts those odds.
Dr. Otis Brawley, chief medical officer for the American Cancer Society, urged caution in interpreting the results of a single study -- particularly before publication in a journal.
"I would not recommend that men choose to have 18 months of therapy based only on this study," Brawley said. However, if a man does stop at that point because he can't tolerate the side effects, this study offers some reassurance that it won't hurt his survival, he added.
In other news from the same meeting, a large study of U.S. men found that blacks and men aged 75 and up were at relatively greater risk of having more-aggressive tumors diagnosed through PSA (prostate-specific antigen) screening.
In general, although the risk was still small, black men were 80 percent more likely than white men to have high-risk cancer -- those most likely to progress and threaten a man's life -- while elderly
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