Role of MRI in breast cancer management
ABSTRACTIn breast cancer, different situations call for different imaging tests. Mammography is the test of choice for screening women with no signs or symptoms of breast cancer. For diagnosis, tailored mammographic views and ultrasonography are the norm. Magnetic resonance imaging (MRI) is highly sensitive for cancer staging, problem-solving, posttreatment surveillance, and other indications. It can detect primary breast cancers and additional foci of cancer that are occult to standard imaging. Continued improvements in technology and studies to assess outcomes will help to better define MRI’s role in breast cancer.
KEY POINTS
- Whether rates of death and local recurrence are reduced when additional breast tumors found by MRI are treated remains to be seen.
- MRI contrast enhancement occurs in many cancers, but it may occur for benign reasons; thus, the finding of contrast enhancement does not establish the diagnosis of breast cancer.
- The National Comprehensive Cancer Network currently recommends screening with both mammography and MRI starting at age 20 to 25 for women at high risk of hereditary breast cancer and ovarian cancer.
- A breast MRI evaluation costs about 10 times more than screening mammography and may not be covered by health insurance, but coverage for this indication appears to be improving gradually.
DIAGNOSTIC MAMOGRAPHY FOR FURTHER WORKUP
Screening mammography yields findings of uncertain significance or frank concern in roughly 1 out of 10 examinations. In these cases, the examination is considered to be incomplete. If the finding is of unclear significance, previous mammograms, if available, may reveal whether the finding has remained the same or changed over time. If a worrisome change has occurred or if no prior films can be obtained, a diagnostic study with additional imaging must be carried out so that the radiologist can decide if a lesion is actually present.
Diagnostic mammography is a tailored examination that may include special projections to better visualize a specific region of concern, spot-compression views to disperse dense breast tissue, or magnification views to characterize microcalcifications. In cases of known breast cancer, diagnostic mammography helps detect additional foci of cancer in the same or in the contralateral breast.8
DIRECTED ULTRASONOGRAPHY TO EVALUATE REGIONS OF CONCERN
While ultrasonography is not part of the standard breast cancer screening protocol,9 directed or “targeted” breast ultrasonography is routinely used in the diagnostic workup to evaluate particular regions of concern. Ultrasonography is used in combination with diagnostic mammography to evaluate mammographic masses, palpable lumps, asymmetric tissue, and architectural distortions.
Breast ultrasonography can usually distinguish cystic lesions from solid lesions, and it is used to guide core biopsy or fine needle aspiration of suspicious breast lesions. It is relatively inexpensive, widely available, and reliable when performed by a skilled and knowledgeable operator.
HOW MRI WORKS
Magnetic resonance imaging takes advantage of the magnetic properties of hydrogen nuclei (protons) in breast tissue. A small fraction of the protons in the patient are brought into alignment with a strong magnetic field within the MRI scanner. Then, the protons are exposed to a brief pulse of radiofrequency energy, which displaces their magnetic vectors. As the protons “relax” and realign along the applied magnetic field, energy is released. This energy, the electromagnetic magnetic resonance signal, is detected and electronically processed to construct an image, exploiting the different “relaxation times” of the different tissues in the breast to generate image contrast.
A standard breast MRI examination requires an intravenous paramagnetic contrast agent, usually a gadolinium chelate, to increase the sensitivity of the study. Gadolinium-based contrast material causes shortening of the T1 relaxation time of tissues in which the contrast agent accumulates, thereby increasing signal intensity (or “enhancement”) in those tissues.
Contrast enhancement may occur in malignant tissues with defective or “leaky” capillaries, but it also can occur in benign tissues, such as normal lymph nodes or benign proliferative processes. Thus, the finding of contrast enhancement does not by itself establish the diagnosis of breast cancer.
The patient must remain still
The patient is positioned prone for about 30 to 40 minutes inside the MRI scanner with the breasts encompassed by specially designed imaging coils, which maximize the signal strength and achieve high spatial resolution. The prone position also minimizes motion of breast tissue and transmitted physiologic motions, further ensuring good image quality.
Contrast enhancement over time
To display how contrast enhancement resolves over time, a series of scans must be obtained. First, a baseline scan is recorded. Then, the contrast material is given, and multiple postcontrast scans are obtained at equally spaced time intervals, typically 1 to 1.5 minutes apart. Usually five to seven postcontrast scans are recorded. During this time, the patient must continue to lie still without moving.
For each individual volume element (voxel) of breast tissue, which may measure 1 mm3 or less, a curve representing contrast enhancement vs time can be constructed. Such curves tend to show one of three typical trajectories or patterns, known as “washout,” “plateau,” and “progressive.” With additional postprocessing, these three contrast enhancement patterns are reduced to color coding and are mapped onto the gray-scale MRI image in the form of a color overlay, so that overall enhancement patterns in both breasts can be discerned at a glance by the radiologist.
These enhancement patterns initially were believed to be reliable indicators of malignant and benign conditions, but further experience has shown considerable overlap of the enhancement patterns between benign and malignant tissues. Thus, the diagnostic value of enhancement patterns is limited. As a rule of thumb, the washout pattern of enhancement (rapid uptake of contrast material followed by rapid washout) is thought to indicate malignancy in 60% to 70% of lesions that are suspicious in other respects.
Abnormal contrast enhancement of the suspicious region must be considered along with morphologic features, the degree of enhancement in adjacent normal-appearing tissue, and the correlation with mammographic or ultrasonographic findings.
Better for invasive ductal carcinoma than invasive lobular carcinoma or ductal carcinoma in situ
At present, we have no foolproof method of diagnosing cancer by MRI alone, though in many cases invasive ductal carcinoma can be predicted with a high degree of confidence. The accuracy of breast MRI is lower for “non-mass-like” enhancement, as is often seen in invasive lobular carcinoma and ductal carcinoma in situ.
Contraindications, problems
MRI for breast cancer evaluation is contraindicated in women with cardiac pacemakers, implanted neurostimulators, and certain older models of aneurysm clips and cardiac prosthetic valves. However, this is becoming less of a problem as MRI-compatible devices of recent design become more prevalent. To ensure safety, patients should complete a screening questionnaire for ferromagnetic devices before they are allowed to undergo breast MRI.
Claustrophobia may preclude an MRI study, but this is less of an issue now, as the newer “short-bore” magnet designs reduce the sensation of confinement.
A problem of increasing importance today is patient obesity: obese patients may not fit into the MRI scanner. So-called “open” MRI scanners are not a good alternative, as they cannot provide the high-resolution images of uniform quality required for breast MRI.
Another factor affecting the use of MRI for breast cancer diagnosis is the limited number of facilities that offer it. Head, spine, and orthopedic MRI services are more widely available.
Cost may be an issue. A breast MRI evaluation costs about 10 times more than screening mammography and may not be covered by health insurance, although coverage for this indication appears to be improving gradually.
