Combining Magnetic Resonance Imaging (MRI) with mammography is the best way to detect breast cancer in women who are genetically predisposed to develop the disease, according to a new study.

The findings are being published online Monday by The Lancet medical journal, to coincide with a report at the annual meeting of the American Society of Clinical Oncology here.

Dr. Ellen Warner, said the author of a commentary in The Lancet that this study shows that “MRI picks up twice as many breast cancers as mammography in very high risk women based on genetic factors and family history.”

However, she stressed, the procedure is not intended for general use. “It is much too expensive and there are far too many false positives. In a lower risk population the cost and morbidity per cancer detected would be unacceptably high,” said Warner, from Toronto-Sunnybrook Regional Cancer Center in Ontario.

Women with mutations in BRCA1 and BRCA2 genes are at high risk for developing breast cancer when they’re relatively young, exactly the age when dense breasts may cloud the sensitivity of mammography.

In a study of 649 women aged 35 to 49 at high risk for breast cancer based on genetic testing or family history, annual MRI and mammography screening detected a total of 35 breast cancers. Nineteen were detected by MRI only, 6 by mammography only, and 10 by both MRI and mammography.

The benefits of MRI were most pronounced in women with BRCA1 mutations. In this subgroup, MRI detected 92 percent of tumors whereas mammography only detected 23 percent.

In The Lancet paper, Dr. Martin O. Leach from the Institute of Cancer Research in London and UK colleagues, conclude that annual screening combining MRI and mammography would detect “most tumors” in this high-risk group.

Leach said it’s also important to note that the surgical biopsy rate was “no higher per cancer detected than in our national mammography screening program (in women above 50).”

He explained that the investigators had been concerned that MRI might produce a high number of false-positive results, leading to unnecessary biopsies. “This has turned out not to be the case,” Leach said.

Dr. Warner emphasized that it’s not known yet whether this earlier detection will mean these women are more likely to be cured. “It will almost certainly take several more years to answer this question.”

Lancet 2005;365.