The link between smoking and kidney cancer may be stronger than experts have appreciated.

Pulling together data from 24 studies conducted since the 1960s, an international research team found that the risk of kidney cancer was 38 percent higher among people who had ever smoked versus those who had never picked up the habit.

And the more smokers had puffed over a lifetime, the greater the risk to their kidneys - a so-called dose-response relationship that supports a direct link between smoking and kidney cancer.

Smoking is responsible for most cases of lung cancer, as well as cancers of the throat, mouth and bladder; it also contributes to a number of other cancers, including tumors of the pancreas and cervix. It’s only in recent years, however, that the habit has been acknowledged as a risk factor for kidney cancer, and the extent of the risk has not been clear.

“This study actually quantifies the risk,” said lead author Dr. Jay D. Hunt, a researcher at the Louisiana State University Health Sciences Center in New Orleans. “And we found that the risk is larger than we’d thought.”

In fact, Hunt told Reuters Health, before conducting this study, he had been skeptical that the relationship between smoking and kidney cancer was real - in part because past studies have yielded conflicting results. Even when an association has been found in individual studies, he noted, it has generally been “modest.”

The new findings, according to Hunt, indicate that, compared with certain other cancers - particularly lung cancer - the kidney cancer risk associated with smoking is indeed modest.

But it may be a bigger risk than generally appreciated, he said, with even light to moderate smoking boosting a person’s chances of developing the disease.

The study, published in the March 10th issue of the International Journal of Cancer, is what’s known as a meta-analysis. Hunt and his colleagues culled data from 24 previous studies conducted in North America, Europe and Australia, and used a mathematical model to estimate the overall risk of kidney cancer associated with various levels of smoking.

The researchers found that among men, those with a history smoking were more than 50 percent more likely to develop kidney cancer than those who had never smoked. Among women, smokers had a 22 percent greater risk of developing the disease.

Hunt cautioned against reading too much into the gender difference. While men started taking up the cigarette habit in large numbers after World War I, he noted, smoking did not take off among women until after World War II. At least some of the studies in this meta-analysis may reflect that time lag.

“The risk may be every bit as great in women,” said Hunt.

He and his colleagues also found that even light smokers - those who averaged fewer than 10 cigarettes a day - had an elevated risk of kidney cancer; they were 60 percent more likely than non-smokers to develop the disease. The odds were still higher with moderate smoking, defined as 10 to 20 cigarettes per day.

These findings diverge from previous estimates, according to Hunt, who said that modest levels of smoking have typically been thought to carry little to no risk of kidney cancer.

There was, however, a bright spot in the findings - namely, that the risk of kidney cancer appears to keep dropping in the years after a person kicks the smoking habit. The risk of the disease was substantially lower among former smokers who had abstained for 10 years or more, compared with those who had quit more recently.

This trend, Hunt said, can definitely be seen as “good news” for former smokers.

SOURCE: International Journal of Cancer, March 10, 2005.