Smaller kidney tumours are being detected in the U.S. and more surgeries are performed, but death rates continue to rise, researchers have found.

Renal cell carcinoma is the most common form of kidney cancer. In 2006, it will affect an estimated 5,000 Canadians, and 1,550 will die from the disease, according to the Canadian Cancer Society.

"With increased early detection and treatment of small tumours, we would expect to see a decrease in mortality associated with kidney cancer," said study author Dr. Brent Hollenbeck, a professor of urology at the University of Michigan. "Surprisingly, that’s not what we found."

Between 1983 and 2002, early detection and surgical treatment increased, but mortality rates from kidney cancer over the study period rose dramatically from 1.2 to 3.2 deaths per 100,000 people in the U.S., the researchers report in Wednesday's issue of the Journal of the National Cancer Institute.

The seemingly contradictory findings can be explained in part by the higher incidence of larger, more lethal tumours, said the study's lead author, Dr. John Hollingsworth, a fifth-year surgical resident at the school.

While more small kidney tumours less than four centimetres in size are now being detected with MRI and CAT scans, the number of people with larger tumours has not decreased. It's the masses larger than seven centimetres that seem to be deadlier, Hollingsworth said.

Since increased treatment has not reduced kidney cancer mortality, it could be that the strategy of surgically removing smaller kidney tumours may not always be warranted, the researchers suggest.

"We’re not saying that surgery for patients with small renal masses is inappropriate," Hollingsworth said. "Perhaps there are some patients with small kidney tumours for whom surgery is not the best option."

The researchers looked at cancer registry data from more than 34,000 patients with kidney cancer.

Fish prevention strategy?

A second study appearing in the Journal of the American Medical Association concluded women who said they dined on fatty fish such as salmon and mackerel at least once a week showed a 74 per cent lower risk of getting kidney cancer compared to non-fish eaters, after 10 years.

Fatty fish contain more omega-3 fatty acids and vitamin D compared with lean fish such as cod and tuna.

Previous research suggests the fatty acids may help slow the growth of cancer, but the findings have not been consistent. People with kidney cancer often have low levels of vitamin D.

Alicja Wolk of the Karolinska Institute in Stockholm and her team did not look at whether fatty fish could help prevent other types of cancer. The kidney cancer results also need to be confirmed since this was the first such population study, the researchers said.

Wolk's team tracked more than 61,000 women aged 40 to 76 for about 15 years. Participants filled in questionnaires about their diet when the study began in 1987 and then again in 1990.