ATLANTA — The cost of treating cancer in the United States nearly doubled over the past two decades, but expensive cancer drugs might not be the main reason why, according to a surprising new study.

The study confounds conventional wisdom in several respects. The soaring price of new cancer treatments has received widespread attention, but the researchers conclude that rising costs were mainly driven by the growing number of cancer patients.

The study also finds cancer accounts for only 5 percent of total U.S. medical costs, and that has not changed in the last few decades.

"I will say I'm a bit surprised," said Dr. Len Lichtenfeld of the American Cancer Society, who said he would have expected the proportion of cancer costs to rise.

The researchers also found that private insurers now cover a greater share of cancer treatment costs — about 50 percent — while patients' out-of-pocket costs have fallen over the past two decades.

Though taken aback by some of the findings, Lichtenfeld and other experts did not dispute the study, which compared medical cost data from the late 1980s to that of the early 2000s. But they said the picture surely has changed in the last several years.

The study found cancer treatment costs rose from nearly $25 billion in 1987 to more than $48 billion by the end of 2005.

The rise in costs is mainly due to an increase in how many cancer patients there are, said the study's lead author, Florence Tangka of the U.S. Centers for Disease Control and Prevention.

The U.S. population is aging, and older people tend to get cancer at higher rates, Tangka noted.

Better and more advanced treatments mean more people with cancer are remaining alive, so the spending increases represent money well spent, said Kenneth Thorpe, a health policy researcher at Emory University.