Doctors say contrary to conventional wisdom, radiation therapy to the whole brain doesn't improve survival, and harms memory, speech and thinking skills.

A major study could change care for many of the hundreds of thousands of people each year who have cancer that spreads to the brain from other sites.

Contrary to conventional wisdom, radiation therapy to the whole brain did not improve survival, and it harmed memory, speech and thinking skills, doctors found.

"This is the classic question: Which is worse, the disease or the treatment?" said one study leader, Dr Jan Buckner of the Mayo Clinic in the US.

Radiation helped control the cancer, "but at the cost of cognitive decline."

For patients, the study is not necessarily the bad news it may seem.

It shows that in this case, quality of life is better with less treatment, and many people can be spared the expense and side effects of futile care.

An estimated 400,000 patients in the United States alone each year have cancer that spreads to the brain, usually from the lungs, breast or other sites.

That is different from tumours that start in the brain, like the one that just killed Joseph "Beau" Biden III, the vice president's son.

Cancer that spreads to the brain is usually treated with radiosurgery - highly focused radiation with a tool such as the Gamma Knife, followed by less intense radiation to the whole brain.

The latter treatment can cause hair loss, dry mouth, fatigue and thinking problems.

Dr Paul Brown of the University of Texas in Houston led a study of 213 patients with one to three tumours in the brain to see whether the risks of whole brain radiation were worth its help in controlling cancer.

Half of the patients had the usual radiosurgery and the rest had that followed by whole brain radiation.

Three months later, 92 per cent of patients who got both treatments had cognitive decline versus 64 per cent of those given just radiosurgery.

"The negative effects far outweigh any benefits" of the combo treatment, Brown said.

Doctors probably will use the combo less frequently because of this study, but certain patients still may benefit from it, said Dr Andrew Lassman of Columbia University and New York-Presbyterian Hospital.

The work should spur research on different ways to give radiation that may not harm thinking skills as much.