(The New York Times News Service) -- A fever of unknown origin is sometimes a sign of cancer, including lymphoma, kidney and liver cancer, according to a new Danish study. But the risk is apparently not nearly as high as reported in some previous studies.

Such a fever is one that lasts more than three weeks with temperatures above 38.3 degrees Celsius -- about 101 degrees Fahrenheit -- with an unidentifiable cause. Patients with these fevers appear to be at a slightly higher risk of cancer.

"We found an association between fever and cancer," says study author Dr. Henrik Toft Sorensen, a professor of medicine in the Department of Clinical Epidemiology at Aarhus University Hospital in Denmark.

However, he adds, "the absolute risk is very low -- much lower than reported in other studies."

Previous studies had shown an association of fever and cancer of 20 percent to 30 percent, Sorensen says.

"But we found very few cases of cancer related to fever," he says, "compared with the incidence of cancer in the general population."

In their study, reported in the Sept. 28 online issue of The Lancet Oncology, Sorensen and his colleagues collected data on 43,205 patients who had been treated in Danish hospitals for fever of unknown origin from 1977 through 1998. During more than six years of follow-up, the researchers compared the incidence of cancer among these patients with the general population.

They found that patients with fever were at a 2.3-percent-increased risk of developing cancer. After one year the risk was highest for cancers of the blood, liver, brain or kidney.

In addition, more of the people diagnosed with cancer had cancer that had spread to other organs, compared with patients who didn't have a fever. The increased risk continued after one year, the researchers say, but at a lower level.

Some cancer patients with fever also had worse outcomes, including a slight increase in mortality compared with other cancer patients.

Because the increased risk of cancer associated with fever is slight, Sorensen doesn't think extensive cancer workups for patients with fever are needed.

"You probably do not need to look for cancer and do a lot of tests in a patient coming into a hospital with fever of unknown origin," he says, "because your risk of cancer is very low."

One expert thinks that the finding concerning fever-related cancers may reflect better diagnosis of fever and better cancer diagnoses.

"We know some cancers are associated with fever," says Dr. Yelena Novik, an oncologist at New York University Cancer Institute and an assistant professor of oncology at the university's School of Medicine.

There are still some cases of fever of unknown origin that may be a sign of cancer, Novik says, "but we are probably better at diagnosing fever and cancer."

Novik advises that any fever of unknown origin be checked, including a screening for cancer.

"Don't let the fever go on," she says. "Make sure all the possible causes for the fever have been explored."

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