For the first time, a substance has been shown to reduce a cancer risk in a controlled clinical study.

Isotretinoin, a drug used to treat severe acne, helped prevent second tumors from forming in cancer of the head and neck.A three-year study followed patients who had cancer of the head and neck, which carries a high risk of recurring. Some patients were given high doses of isotretinoin (marketed as Accutane), a derivative of vitamin A, while others received a placebo.

Patients who received isotretinoin had a six-fold reduction in the formation of second tumors.

According to Dr. Ronald Blum, clinical director of New York University Medical Center's Kaplan Cancer Center, these results bode extremely well for cancer treatment.

Blum cautioned that the drug is not without its limitations. While isotretinoin lowered the incidence of second tumors, it did not prevent the original tumor from recurring.

The drug also has adverse side effects, including raising the level of fat in the blood and drying the skin, which prevented some patients from completing the study. Since isotretinoin can also cause birth defects, it may not be used by women who are or may become pregnant.

Follow-up studies are currently being pursued at NYU Medical Center to evaluate applications of retinoid therapy on other cancers.

"Recent developments in molecular biology have provided scientists with an improved understanding of how cancers develop, which enables us to design better ways to treat them," Blum added.

The hope is that physicians will be able to identify people who are at high risk for cancer, because of genetic tendency or other risk factors, and treat these people before the cancer develops.

Blum emphasized that chemoprevention - the use of drug therapy to prevent an illness - should not take the place of eliminating risk factors which can be controlled.

In the case of cancer of the head and neck, these risk factors have been clearly identified: smoking and excessive alcohol consumption.