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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 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 is being called the first to combine national cancer costs for all types 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, a health policy researcher at Emory University who has focused on the cost of health care.

"It seems like we're buying increases in survival," Thorpe said.

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

The 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.

Those last two findings surprised some experts.

Recent government reports have found that the percentage of Americans with private health insurance has been shrinking and recently hit its lowest mark in 50 years. Yet the study found that the proportion of cancer treatment costs paid by private insurance rose.

And companies have been tightening or cutting employee benefits, causing out-of-pocket costs to go up for many patients. Yet the study found that the proportion of bills paid by patients declined.

That last finding in particular was striking, said Lichtenfeld, the cancer society's deputy chief medical officer.

He alluded to widely reported increases in personal bankruptcies prompted by medical bills. "There's no question that the out-of-pocket costs for some patients have risen dramatically," Lichtenfeld said.

The rising price of certain treatments also should be acknowledged, he said.

The challenge of rising prices was recognized by American Society of Clinical Oncology (ASCO), which last year released its first guidelines counseling cancer doctors on how to talk to patients about deciding between less expensive chemotherapy drugs made more sense than newer, more expensive products.

The study did not add in the cost of diagnostic tests and scans, which are cost drivers. And the data does not include the last five years, which saw some extremely pricey cancer drugs come on the market.

The picture may have changed since the study's data was collected and the U.S. economy deteriorated, said Dr. Neal Meropol, a Case Western Reserve University cancer expert who worked on the ASCO guidelines.

Newer treatments along with wider testing are driving up the overall cost of cancer care, Meropol said.

"My concern is that costs are getting shifted to patients and there is a potential for increasing disparities" in cancer care, he added.