London ? The number of new cancer cases worldwide is expected to increase by 50 percent in the next 20 years, partly because poor nations are adopting unhealthy Western habits, the World Health Organization said Thursday.

The World Cancer Report is the first comprehensive examination of cancer around the globe, covering the current understanding of its causes, prevention and treatment.

“The overall message is that we can prevent a third of cancers,” said one of the report’s editors, Australian cancer specialist Bernard Stewart.

Worldwide, about 10 million people are diagnosed with cancer every year and 6 million people die from it. The report projects that the annual number of diagnoses will reach 15 million by 2020, based on current trends in smoking, diet and exercise.

Although one-third of the cases theoretically were preventable, that does not mean the coming increase realistically could be slashed by that amount, said WHO’s cancer chief, Dr. Paul Kleihues.

“I think what we can do is slow down the increase. Anything more is not realistic,” said Kleihues, director of WHO’s International Agency for Research on Cancer.

Rich nations have more cancer than poor ones, mostly because of tumors tied to bad habits such as smoking and drinking, eating too much or the wrong kinds of foods, and lack of exercise.

“If we want to go back to a lifestyle associated with a low incidence of cancer, small changes to our lifestyles would not be sufficient. We would really have to go down to a very restricted diet, no overfeeding, starting in childhood. I don’t think that’s realistic expectation,” Kleihues said.

From one angle, the task of stemming the impending rise in cancer is easier in poor countries, Kleihues said, because 23 percent of tumors there are due to infections that can be prevented now or soon.