WASHINGTON - Despite huge advances in prevention and treatment, cancer is poised to become the leading cause of death worldwide as people refuse to ditch bad habits and the population ages, experts said Tuesday.

In the United States, medical advances and education campaigns have helped slash the death rate from cancer by nearly 16 percent in 20 years, American Cancer Society epidemiologist Susan Gapstur told reporters at the launch of a cancer-themed edition of the Journal of the American Medical Association.

But cancer still struck 1.5 million people and killed 560,000 in the United States in 2009 and experts predict it will this year edge out heart disease to become the most deadly disease worldwide.

One reason is because cancer usually strikes people later in life, from age 55 upwards, so as people live longer and the population ages, the risk of being diagnosed with cancer has risen: nearly half of men and a third of women will be diagnosed during their lifetime with cancer, Gapstur said.

Another reason is the poor lifestyle choices people make.

With more and more people around the world taking up smoking, Gapstur predicted a "worldwide cancer epidemic" that will help cancer to "overtake ischemic heart disease as the number one cause of mortality in the world this year."

Quitting smoking helps to reduce the risk of getting or dying of lung cancer, but only a fraction of patients kick the tobacco habit, even after they have been diagnosed with cancer, said Robert Timmerman, who led a study, published in JAMA, on radiation therapy for patients with inoperable lung cancer.

"You would think that one of the biggest motivators to quit smoking would be a life-threatening illness like cancer, but I?m told that fewer than 50 percent of patients quit smoking, even with a cancer diagnosis," Timmerman told AFP.

"At least 20 percent of cancer patients who continue to smoke will get another tumor in their lifetime. It?s like dodging one bullet and getting hit by another," Timmerman, a professor of radiation oncology and neurosurgery at the University of Texas Southwestern Medical Center, said.

"Our therapy may cure one tumor but that?s just a small but meaningful battle, not winning the war on cancer."

The lung cancer rate in the United States has fallen sharply since 1990, the year cancer hit a peak here.

Nearly 40 per cent of the decrease in the overall cancer death rate in US men between 1990 and 2006 was due to fewer men dying of lung cancer, which many experts believe is due to a fall in the number of Americans who smoke, said Gapstur.

But Americans have another lifestyle issue to worry about in the war on cancer: obesity and being overweight.

"Current estimates say around two-thirds of US adults are overweight or obese, and obesity and (being) overweight are now known to cause many types of cancer," said Gapstur.

"Avoiding overweight and obesity is going to be critical for keeping our progress going forward in this battle against cancer," she said.

Every year, more than 100,000 incidents of cancer in the United States are caused by being overweight or obese, a study by the American Cancer Society has shown.

Catherine DeAngelis, editor-in-chief of JAMA and a practicing physician, worried that one medical advance could unwittingly help cancer to win the war being waged against it.

"I worry that people are mistaking human papillomavirus vaccine for the cure for cancer," DeAngelis told AFP.

"I?m worried that women will get vaccinated and will stop having pap smears," which have been one of the most successful early detection tools for uterine cancer, she said.

Meanwhile, an article published Tuesday in the Archives of Internal Medicine said the US media focused too much on battles won in the war against cancer and not enough on failures.

Half the men and women who are diagnosed with cancer in their lifetime will die of the disease or related complications, it said.

And yet, only 7.6 per cent of more than 400 cancer stories published between 2005 and 2007 in the United States were about people who were dying or had died of cancer. Four times more stories were upbeat accounts of cancer survivors.