CHICAGO, June 2 (Reuters) - An MRI scan can find extra tumors that mammograms missed in women who have been diagnosed with breast cancer, researchers reported on Saturday.

Mammograms had detected the first tumor in the 390 women they studied, but MRI detected extra tumors in 16 percent of them, the researchers said.

Magnetic resonance imaging or MRI can see structures missed by mammograms, which are a type of X-ray.

"These results prove that MRI can detect tumors missed by traditional exams, and can be vital in helping women choose the right course of treatment for their breast cancer," said Dr. Laura Vallow of the Mayo Clinic in Jacksonville, Florida, who led the study.

Vallow and colleagues told a meeting of the American Society of Clinical Oncology in Chicago that the women with extra tumors in the same breast tended to be younger, and they tended to have larger primary tumors -- the first tumors found.

"This is an important finding because breast cancer tends to be more aggressive when diagnosed in younger women," Vallow said in a statement.

"This suggests that younger women who want breast-conserving surgery to treat newly diagnosed cancer may benefit from an MRI scan of the entire breast."

Breast cancer, which will be diagnosed in more than 180,000 U.S. men and women in 2007, is usually found in one of two ways -- mammogram, or when a patient or doctors feels a lump in a breast.

Mammograms can only detect some tumors, however, and surgeons often only know how big a tumor is when they physically remove it. Ultrasound can also be used to help doctors, but sometimes other tumors go undetected.

Vallow's team also found that MRI can also help to better detect tumors in a woman's other breast after she is diagnosed with cancer in one breast. They said MRI detected tumors in the second breast missed by mammograms in 3.2 percent of 401 women.

In some cases, MRI found tumors in both breasts that were not found by mammograms, the researchers said.

In March another team reported similar findings in the New England Journal of Medicine, and the American Cancer Society recommended that women with cancer in one breast should get an MRI scan of the other breast to make sure the cancer is not there, too.

"The combined results of these studies validate what we have already seen in our clinic, and that is that MRI breast cancer screening is quickly becoming an indispensable tool," Vallow said.

REUTERS