Italian researchers say the findings may eventually help women and their doctors avoid complete removal of the breast (mastectomy) and opt for more breast-conserving treatment approaches if MRI (magnetic resonance imaging) tests show the breast cancer is confined to one area of the breast.
Previous studies have shown that up to 59% of women with breast cancer may have another tumor in the same breast.
In the study, 99 women who had been diagnosed with breast cancer received both a mammogram and MRI exam to determine if there were additional cancers.
All of the women then had mastectomies, and their breast tissue was examined under a microscope.
Overall, the pathology tests found 188 additional cancers. Of those, MRI was significantly better at detecting the additional tumors. Mammography detected 124 tumors (66%) while MRI detected 152 (81%).
Researchers found the tumors that were missed by mammography were significantly larger and more aggressive overall than the cancerous areas that were missed by MRI, says researcher Francesco Sardanelli, MD, of the University of Milan, in a news release.
The findings appear in the October issue of the American Journal of Roentgenology.
MRI was even more effective for women with dense breasts, says Sardanelli. Studies have shown that increased breast density -- meaning more breast and less fatty tissue -- can make it harder for mammography to detect small breast cancers.
Researchers say it's the first study to compare the effectiveness of MRI and mammography with the whole breast pathology results.
Because the entire breast was examined under the microscope, researchers say they were able to detect even the smallest tumors, which made the comparison of MRI and mammography very reliable.