Breast MRI gains support as important diagnostic tool while computer-aided detection improves efficiency and quality of exams

Kirkland 20 September 2004More than 8,5 million women in the United States are at high risk for breast cancer due to their family history or genetic predisposition. Magnetic resonance imaging (MRI) is an important and highly sensitive breast imaging modality. Breast MRI studies are increasing by 40 percent per year in the United States. A recent major study published in the New England Journal of Medicine provided strong evidence that breast MRI exams can be effective in finding tumours in women at high risk for breast cancer that are sometimes missed by screening mammography.

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"MR imaging helps us see the breast in a new way and maps out for us how extensively a patient's disease has progressed", stated Constance Lehman, M.D., Ph.D., associate professor of radiology and director of breast imaging at the University of Washington in Seattle. Dr. Lehman is the author of a study presented at the Radiological Society of North America Meeting that suggested computer-aided-detection evaluation of breast MRI could reduce unnecessary biopsies of MR-detected lesions.

Clinical indications for breast MRI exams continue to expand as physicians increasingly recognize the value of this highly useful technology. Indications now include determining extent of disease (staging), therapeutic monitoring, determining breast implant integrity, and screening for high risk patients. There is increasing scientific literature showing the effectiveness of MRI in screening women at high risk, including younger women whose greater breast density can reduce the effectiveness of other imaging modalities.

"MRI can pick up small cancers and invasive cancers at a higher sensitivity than mammography", explained Dr. Lehman. While mammograms use X-rays, MRI uses radio waves and magnetic fields, which make the contrast between breast tissues 10 to 100 times greater. Magnetic resonance imaging assesses blood flow in the breast. If there is cancer in the breast, there is increased blood flow and the formation of abnormal vessels. Breast MRI is very encouraging for women who are at high risk because it gives them an additional mode for breast cancer to be detected early and treated early", stated Dr. Lehman.

"Specialists counseling women at genetically increased risk for developing breast cancer should help them consider the pros and cons of breast MRI as a screening option for their individual circumstances", recommended Elizabeth Morris, M.D., a researcher at Memorial Sloan-Kettering Cancer Center in New York City. "Women at high risk for breast cancer should not forego their annual mammograms, but should consider adding MRI screening which many insurance companies now cover." Dr. Morris is the lead author of a study published by the American Roentgen Ray Society (ARRS) that showed MRI is an accurate way to look for any disease remaining in the breast after a lumpectomy.

At the annual meeting of the American Roentgen Ray Society, Dr. Morris presented results of a study of 367 women who underwent MRI screening. All of the women were at high risk for breast cancer due to a personal or family history, had pre-malignant breast lesions or were carriers of BRCA1/2 mutations that confer breast cancer risk.

Biopsies were advised in 64 women and carried out in 59. Cancer undetectable by mammography or physical examination was found in 17 women, or 24 percent, of the group. High-risk lesions were detected in another 13 women and benign disease was found in the remainder. Dr. Morris stressed that MRI is not designed to replace mammography. "Magnetic resonance imaging is very sensitive, but it does pick up a significant number of false positives", she warned. The high false-positive rate with MR could cause anxiety or unnecessary surgery.

The efficiency of breast MRI may be improved by new computer-aided-detection (CAD) technologies that automate the processing of the hundreds of images produced by an MRI. CAD applies a colour-coding system that visualizes tissues according to their similarity to known tumour properties: red for areas most predictive for malignancy, blue for possibly benign regions, and green for questionable areas. Using CAD, physicians are able to view detailed three-dimensional, colour-coded images of areas in the breast with abnormal characteristics.

In the study led by Dr. Lehman and presented at the Radiological Society of North America Meeting, investigators evaluated 33 breast lesions (9 malignant and 24 benign) seen only on MRI. Dr. Lehman used CAD to analyse abnormal tissue and how it "enhances" - or absorbs contrast agent - at different rates than normal breast tissue. CAD analyses the various rates of enhancement exhibited by breast tissue and marks precise tissue areas where curves are characteristic of abnormal cells. Analysis of the tissue enhancement in CAD-processed images was performed at three different thresholds (50 percent, 80 percent and 100 percent); each threshold stands for a predictive value based on the extent to which tissue characteristics are associated with abnormality.

At all three enhancement thresholds, CAD had a sensitivity of 100 percent, meaning all malignant lesions were identified. For benign lesions, the false-positive rate was reduced by 25 percent at the 50 percent threshold (non-significant); by 33 percent at the 80 percent threshold (borderline-significant) and by 50 percent at the 100 percent threshold (significant). "If these results are validated by a larger study, the number of unnecessary biopsies of MR-detected lesions could be reduced in half without a concomitant decrease in cancer detection", Dr. Lehman concluded.

If a suspicious lesion is seen on a breast MRI, a biopsy is often called for. In a related study, Dr. Lehman reviewed a new technique for performing a vacuum-assisted breast biopsy guided by MRI, without having to send the patient to surgery. She reviewed 38 MRI-guided biopsies in 28 patients. All of the biopsies were technically successful, and there were no complications. The average time of the procedures ranged from 38 minutes for single biopsies in a single breast to 64 minutes if performed on both breasts.

"The short time to do the biopsies is important both for patient comfort and so we can sample the tissue while the contrast-enhanced lesion is still visible", Dr. Lehman explained. "This new method for MRI-guided breast biopsy allows us to accurately and successfully sample tissues seen only on MRI."


Leslie Versweyveld

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