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.
MRI IS SENSITIVE, BUT NOT SO SPECIFIC
The role of MRI in evaluating breast disease has been studied and debated since contrast-enhanced breast MRI was introduced in 1985.10 Interest has grown steadily as evidence of its usefulness has accumulated. Improvements in MRI scanners have included better image resolution, dedicated breast coils, and rapid dynamic contrast-enhanced imaging.
The overall sensitivity of MRI for breast cancer is relatively high, with estimates ranging from 85% to 100%.11 In invasive ductal carcinoma, its sensitivity approaches 100%.12 Sensitivities for invasive lobular carcinoma and ductal carcinoma in situ are lower and not yet well defined.
In contrast, MRI’s specificity for breast cancer is much more variable, ranging from 37% to 100%. The discrepancies among estimates of specificity are attributed to multiple confounding methodologic factors in the studies to date, such as differences in imaging protocols, patient selection criteria, patient ages, interpretation criteria, and the level of experience of the interpreting radiologist.12
False-positive results may be caused by benign conditions such as fibroadenomas, intramammary lymph nodes, proliferative and nonproliferative fibrocystic changes, and mastitis, as well as by radial scars, atypical ductal hyperplasia, and lobular carcinoma in situ.13–15 In premenopausal women, the menstrual cycle may bring about regional physiologic variation in enhancement of the normal breast parenchyma, which may either simulate the appearance of a lesion or obscure a true lesion.16
Thus, breast MRI may detect cancer that is occult to mammography, but it also carries the risk of worrisome incidental findings that may only be resolved by biopsy. Such uncertain findings are troubling for both the radiologist and the patient when mammography, ultrasonography, and the physical examination are all normal. Clearly, breast MRI cannot be counted on to reassure the “worried well” patient.
MRI is not for screening in the general population
While its high sensitivity for invasive ductal carcinoma17 would seem to make breast MRI attractive for breast cancer screening, it has the disadvantages of lower sensitivity for invasive lobular carcinoma and ductal carcinoma in situ,17,18 as well as the potential to raise suspicions of breast cancer that may be difficult to resolve. For these reasons, MRI is not suitable for routine breast cancer screening in asymptomatic women, although it is recommended for patents in some high-risk groups, as we discuss later.
Data from the Memorial Sloan-Kettering Cancer Center suggest that MRI can detect mammographically occult breast cancer in high-risk populations.19 This study evaluated 367 women at high risk (ie, with a personal history of breast cancer, lobular carcinoma in situ, or atypia, or with a family history of breast cancer). Biopsy was recommended in 64 (17%) of the women on the basis of MRI findings. Biopsy revealed cancer in 14 (24%) of 59 women who underwent biopsy. Subgroup analysis further suggested a 50% positive predictive value of biopsy based on MRI findings in women with both a positive family history and a personal history of breast cancer.
Further studies of breast MRI for screening high-risk populations are under way in North America and Europe.
CLINICAL APPLICATIONS OF MRI OF THE BREAST
MRI has been shown to be useful in:
- Staging biopsy-proven primary breast carcinoma
- Detecting an occult primary breast cancer in a patient with proven axillary node involvement but negative results on mammography and ultrasonography
- Ascertaining the extent of disease after lumpectomy with positive margins or close margins
- Investigating suspected pectoralis muscle invasion
- Assessing response to chemotherapy, including preoperative chemotherapy
- Looking for suspected recurrent disease, such as in a postsurgical scar
- A compelling clinical presentation with negative or equivocal imaging results
- Problem solving, ie, workup of uncertain imaging findings that could not be resolved even after special mammographic and ultrasonographic techniques were used
- Needle localization and guided biopsy
- Known or suspected rupture of breast implants
- Screening patients with certain well-defined risk factors for breast cancer.
The current standard of practice does not support the use of MRI to replace problem-solving mammography and ultrasonography. A negative MRI study does not preclude biopsy of a suspicious lesion found with mammography or directed ultrasonography.
Lesion characterization and staging
Surgical options for treating breast cancer are breast-conserving surgery and mastectomy, taking into account the tumor size, multifocality or multicentricity, local extent vs distant spread, nodal status, and patient preference. Studies have shown that MRI is more accurate than mammography and ultrasonography in defining the extent of tumor burden as characterized by tumor size and multifocality or multicentricity.20,21
Preoperative MRI also has been shown to change therapeutic decisions when additional disease was detected and then proven by image-guided biopsy.19 In a study by Fischer et al22 in 336 women with breast cancer, MRI led to a change in therapy in 19.6% of patients by demonstrating unsuspected multifocal or multicentric ipsilateral lesions or contralateral carcinomas. In all cases, a confirming tissue diagnosis, either before or after MRI, was obtained before surgery. Given the potential for false-positive findings on breast MRI, biopsy of newly detected suspicious lesions is generally necessary before mastectomy is contemplated.
MRI in the follow-up assessment
After excisional biopsy, MRI may help determine the presence or absence of residual tumor if there are positive or close margins shown by surgical pathology, or if residual microcalcifications persist on the postbiopsy mammogram.23 The time between surgical biopsy and follow-up MRI affects the sensitivity of MRI for residual tumor. Frei et al24 reported a sensitivity of 89% to 94% when imaging was done at least 28 days after excision.21
MRI also is useful in identifying and differentiating tumor recurrence from postsurgical or postradiation scar when conventional imaging is indeterminate.25 In a study of 45 women with suspected tumor recurrence after lumpectomy, with or without radiotherapy and chemotherapy, Lewis-Jones et al26 reported a sensitivity of 100% and a specificity of 94% for MRI in detecting new tumor vs posttreatment fibrosis.26
Inflammatory changes in the breast tissue after surgery and radiation therapy limit the accuracy of MRI. Tissue enhancement can be seen in the operative bed for up to 6 months after surgery and for up to 24 months after radiation therapy. In general, local tumor recurrence appears after this interval. Therefore, the postsurgical timing of the MRI examination is important.
