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

The study confounds coventional wisdom in several respects. The soaring price of new cancer treatments has received widespread attention, but the researchers conclude that rising costs were driven mainly 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’d 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.

The study compared medical cost data from the late 1980s to that of the early 2000s.

The study is being called the first to combine national cancer costs for all kinds of payers and see how they’ve changed over time. The figures are reported in 2007 dollars.

It found that 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 over 20 years 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 researchers used data from national telephone surveys done in 1987 and from 2001 through 2005, which gathered information on medical conditions as well as who paid the bills. More than 164,000 people were surveyed.

The study did not offer precise estimates of how the number of people treated for cancer changed from the late 1980s to the early 2000s.

But it showed dramatic increases in the number of cancer cases covered by the government’s Medicare and Medicaid programs. Medicare, which covers the elderly and disabled, has consistently covered about a third of the nation’s cancer costs. Medicaid accounts for only 3 percent.

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, an Emory University health policy researcher who has focused on the cost of health care.

Researchers also found:

• The percentage of cancer costs from inpatient hospital care fell from 64 percent to about 27 percent. A shift to less expensive outpatient care, along with cost containment efforts by large health insurers, helped keep down increases in the costs per patient, the authors said.

• The proportion of cancer costs paid by private insurance rose from 42 to 50 percent.

• The proportion of costs paid out of pocket by patients — including copayments and deductibles — dropped from 17 percent to 8 percent.

The study is being published in Cancer, a medical journal of the American Cancer Society.