A study suggests overweight breast cancer patients often do not get enough chemotherapy -- and that might help explain why they tend to fare worse than thinner women.

Doctors typically use weight, height and body-surface measurements to determine the proper doses of the powerful cancer-fighting drugs. But many doctors avoid that approach with overweight breast cancer patients because they worry about the toxic side effects of very large doses, said Jennifer J. Griggs, the study's lead author and a breast cancer specialist at the University of Rochester in New York.

The phenomenon has been reported in treating other kinds of cancer.

In her study of 9,672 women who received standard drugs for breast cancer, overweight women were 20 percent more likely than thin women to receive lower doses than a size-based formula would indicate they need; obese women were more than twice as likely; and very obese women were almost six times more likely.

Previous studies have suggested that overweight patients are more likely than lean ones to suffer a recurrence of their breast cancer, partly because excess fat increases the amount of circulating estrogen, which fuels the growth of some tumors.

Griggs said insufficient chemotherapy is probably also to blame. "It certainly seems like a plausible explanation," she said. Her study appears in Monday's Archives of Internal Medicine.