MRI’s emerging role in breast cancer screening

The American Cancer Society now recommends MRI in addition to mammography for women at high risk for breast cancer.
MRI’s emerging role in breast cancer screening
By Harvard Medical International
Mon 05 Nov 2007 12:51 PM

We're constantly reminded that breast cancer is the second leading cause of death in women. But recently, we've also been getting some good news: The breast cancer death rate has been declining since 1990. More women are surviving - not only because of advances in treatment, but also because the disease is detected early, while it can still be cured. And regular screening with mammography is the main reason for that.

Generally, screening tests are designed to uncover evidence of a disease in people who have no symptoms. Public health organisations decide to recommend a screening test by balancing whether it can identify a disease early enough to permit successful treatment (the benefit) against its likelihood of missing the disease or suggesting that it's present when it's not, causing emotional upset and leading to unnecessary procedures (the risk).

A patient’s risk for breast cancer changes with age and family history.

Regular screening is detecting more tumours, but too many still turn up between tests. So researchers continue to look for better screening methods and health organisations tweak their guidelines as new information becomes available. Since the 1970s, the American Cancer Society (ACS) has revised its breast cancer screening recommendations six times. The National Cancer Institute and the US Preventive Services Task Force also regularly reconsider their screening advice.

In evaluating guidelines, the organisations ask expert panels to examine reams of evidence on methods of detecting asymptomatic breast cancer, including breast self-examination, clinical breast examination, mammography, ultrasound, magnetic resonance imaging, nipple aspiration, and ductal lavage. The panellists also pore through the research results for indications that some screening intervals and technologies might be more effective than others for certain groups of women. For example, health experts have long advised different breast screening schedules for women at different ages.

The ACS is recommending a separate protocol for women at very high risk for breast cancer. Their guidelines advise that these women receive magnetic resonance imaging (MRI) along with annual mammography.

Why MRI?

MRI uses magnetic fields rather than radiation to create an image of the breast. It helps in the search for malignancies because it provides exquisite soft tissue contrast. MRI was first widely used in the 1990s to find ruptured breast implants. It began to turn up tumours in dense breast tissue after gadolinium, an intravenous contrast agent, was used in conjunction with the MRI. Since then, MRI (with gadolinium) has been used as a follow-up test for suspicious mammograms. Only in the last few years have clinical trial results suggested that MRI may have a role as a first-line technology for detecting tumours. However, we don't know yet if tumour detection by way of MRI will result in the most important outcome of any screening test - increased survival.

MRI's value in detecting breast cancer is most evident in studies involving women at high risk for the disease. In a study published in the New England Journal of Medicine in 2004, Dutch researchers followed 1900 women with a risk of breast cancer greater than 15% due to genetic mutations or family history. (The average lifetime risk is 12.7%.) For nearly three years, the women received yearly MRIs as well as semi-annual clinical breast exams and yearly mammograms. Mammography detected 18 cancers but missed 22 that were found on MRI. MRI found 32 cancers and missed only eight that showed up on mammograms.

An Italian study published in the journal Radiology in March 2007 showed similar results for a comparable group of high-risk women. In that investigation, 278 women were given annual clinical breast exams, mammograms, MRIs, and ultrasounds. In the first two years, 18 cancers were found; 17 showed up on MRI, including six that all other methods had missed.

A study undertaken at six US medical centres and reported in Radiology in August 2007 compared MRI, mammography, and ultrasound in 171 women over age 25 whose lifetime risk of developing breast cancer was very high (20% or more). Sixteen biopsies were performed, and six cancers were diagnosed; MRI detected all six, mammography only two, and ultrasound only one. Neither mammography nor ultrasound found any tumours that MRI missed. Only MRI detected the four cancers found in women with dense breast tissue.

Why not use MRI for all women?

Given this impressive ability to detect tumours not found on mammograms, MRI might seem to be a logical choice for breast cancer screening across the board. Yet none of the nationally recognised advisory groups is recommending it for women at average risk. There are several important reasons for this:

• It leads to too many negative biopsies. Because MRI is so good at picking up any abnormal tissue, whether cancerous or not, it results in many biopsies of areas that are not cancer. In the studies cited above, many more biopsies yielding negative results were performed after MRI than after mammography.
• It can be stressful. The procedure requires a substantial investment of time and fortitude. During their hour in the MRI suite, some women feel claustrophobic, although confinement and the thumping noise of the machine have been made somewhat more tolerable than they once were.

• Mammography is getting better all the time. High-quality mammography can be found almost everywhere, from major medical centres to mobile vans. In particular, digital mammography, which improves detection in women with dense breast tissue, is increasingly available. Because it captures the image in a computer file rather than on film, it confers all the advantages of digital photos. By magnifying, manipulating, and changing the contrast on the image, radiologists can spot subtle but telling differences that might have been missed on film.

The breast cancer death rate has been declining since 1990.

• The radiation risk of mammography is very small. There's less than a one-in-a-million chance it will cause a radiation-induced cancer, and the risk declines with age and the postmenopausal decrease in ductal tissue. Women who have annual mammograms during their 40s may raise their risk of developing breast cancer by about 1% at age 50, but regular screening thereafter won't further increase the risk.

• MRI has limitations - and limited availability. High-quality breast MRI is still not universally available. Moreover, breast MRI can't be performed on women who have certain devices in place such as pacemakers or implantable cardioverter-defibrillators. The ability of MRI to detect tiny calcifications of early preinvasive breast cancer (ductal carcinoma in situ, or DCIS) is limited. Mammography readily sees these tiny calcifications. Mammographic detection of DCIS is a major reason for the observed decrease in breast cancer mortality.

• It's expensive. At more than $1,000, breast MRI costs about 10 times as much as mammography. Insurance may not cover the procedure, especially in patients deemed to be at low risk.

Who might benefit from breast MRI screening?

Although neither the National Cancer Institute nor the US Preventive Services Task Force have included MRI in their screening recommendations, the ACS recommends an annual MRI in addition to annual mammography for women who have a 20% or greater lifetime risk of breast cancer. Experts generally use the following standards to determine which women belong in that high-risk category:

• A known genetic risk determined by a positive test for mutations in the BRCA1 or BRCA2 genes or other strong breast cancer risk genes; or, if the patient has not been tested, a first-degree relative (parent, sibling, or child) who has tested positive for these mutations.

• A lifetime risk of 20% to 25% as calculated by tools developed by the NCI and other organisations. See the NCI risk calculator at www.cancer.gov/bcrisktool.

• Radiation therapy delivered to the chest between the ages of 10 and 30. Regular screening is particularly important for survivors of Hodgkin's disease, who have nearly a 50% risk of developing breast cancer.

• Li-Fraumeni familial cancer syndrome or any other rare genetic disorder that increases susceptibility to breast cancer by causing mutations in genes that suppress tumour growth or cause chromosomal damage.

The bottom line

Today, there are just two distinct screening protocols: ‘high risk' and ‘average risk.' A patient's risk for breast cancer changes with age and family history, so doctors should be aware if there are new cases of cancer in a patient's family. Clinicians may refer patients to a genetics counsellor for a detailed assessment. In the future, we may have a number of different protocols, each calibrated in light of the risks facing a specific group of women. Meanwhile, for most women over 40, an annual mammogram and clinical breast exam is still the best way to catch early-stage, highly treatable cancers. Patients should be reminded to make and keep that annual appointment.

This article is provided courtesy of Harvard Medical International. © 2007 President and Fellows of Harvard College

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