9 Management of Findings Initially Detected at MRI
When abnormal findings are detected on breast MRI studies, the next step is to differentiate between probably benign lesions that do not need tissue sampling and suspicious lesions that need to be biopsied. The morphologic characteristics, kinetic assessment, patient’s risk factors, and the indications for MRI studies are taken into account in deciding whether tissue sampling is needed. Probably benign lesions can be followed in short term. For suspicious lesions, the next step is to determine a method of percutaneous biopsy. Although the mainstay of the biopsy method is MRI-guided biopsy, an ultrasound-guided biopsy or a stereotactic biopsy may be performed depending on the lesion type. Selection of a biopsy method must be made with careful observation of relevant images. Any suspicious lesions that are not visualized on any other imaging modalities than MRI should be biopsied under MRI guidance. The difficulty of MRI-guided biopsy is variable depending on the lesion location. Once a biopsy is performed, the physician who performed the biopsy should make a decision if the pathologic result is concordant or discordant with imaging findings. The decision making process in this series of studies requires considerable experience of the breast imaging.
9 Management of Findings Initially Detected at MRI
Magnetic resonance imaging (MRI) is more sensitive in detecting breast lesions than conventional imaging by far, and is very useful for screening purposes. It is also true that MRI can be used for regional staging of a patient with known breast cancer, to look for satellites and determine the extent of a known malignancy. Due to its high sensitivity, however, MRI detects not only malignant lesions but also benign lesions. The challenge for MRI is to differentiate clearly between benign lesions that do not need tissue sampling and those suspicious lesions that need to be biopsied.
Table 9‑1 shows a summary of negative, benign, and probably benign lesions. In general, large mass lesions (>15 mm) with irregular shape, irregular or spiculated margins, heterogeneous or thick rim enhancement, and rapid enhancement tend to be malignant. Masses with homogeneous enhancement or dark internal septations and persistent kinetics tend to be benign. When evaluating any given lesion, it is important not only to evaluate the morphologic and kinetic characteristics, but also to take into account the patient’s risk factors, such as the presence of a genetic mutation or a strong family or personal history of breast cancer, before deciding whether tissue sampling is needed.
Table 9.1MRI features of normal, benign, and probable benign lesions
Nodular parenchymal enhancement
2. Benign lesions
Multiple similar-shaped and similar-sized lesions
Thin, regular rim enhancement
3. Probable benign lesions
Enhancing mass with dark internal septations
Slowly enhancing, circumscribed mass
Small linear NME (nonbranching)
Patient indications for MRI studies may also influence management. For example, an enhancing lesion initially seen on MRI tends to be malignant in patients who have metastatic axillary lymph nodes of unknown origin (CUP [carcinoma unknown primary] syndrome). Similarly, a finding in a patient referred with positive or close surgical margins following breast cancer surgery or with newly diagnosed breast cancer is more likely to require further investigation. A lesion identified in the same quadrant as that of a known cancer is likely to be a satellite lesion, and should be evaluated further. Availability of prior MRI examinations is very helpful for diagnosis, and when available, a search for interval change and new and developing lesions will aid in cancer detection.
There are no systematic methods for differentiating probably benign lesions from suspicious lesions. Published articles generally report that a BI-RADS 3 assessment made by experienced radiologists may result in a malignancy rate of 2 to 3%. Follow-up MRI is usually performed after 6 months when a BI-RADS 3 assessment is made, with short-term follow-up usually continuing at 6, 12, and 24 months, as long as the lesion does not change in appearance. Two-year stability of the lesion is usually considered to be sufficient for a benign diagnosis. If the lesion appears smaller or less enhancing, or if it disappears during the follow-up period, it can be downgraded as benign. A decision to downgrade a lesion must be made prudently, as the change in appearance of a lesion can be easily influenced by various factors: the injection rate/volume of contrast, differences in magnetic field strength, or even the patient’s positioning. On the other hand, if a lesion shows an increase in size or a change in its kinetic pattern from a benign to a suspicious pattern, a biopsy should be performed.
9.3 How to Work Up Suspicious Lesions Initially Detected on MRI
A percutaneous biopsy should be performed for any suspicious lesion. The work-up process and the biopsy method may vary, according to the lesion type (focus, mass or nonmass enhancement [NME]). The mainstay biopsy method for such lesions is MRI-guided biopsy. Conventional imaging can be used in some cases in order to avoid an MRI-guided biopsy, which is expensive, time-consuming, and uncomfortable for some patients.
First of all, one should review the most recent mammogram to search for any lesion that might correlate with the MR findings (subtle calcification or asymmetry or lymph nodes). If there is a finding seen that is amenable for biopsy, then a stereotactic biopsy should be considered. If the mammogram does not reveal a possible correlate, then MR-directed (second-look) ultrasound is the next option. There is variability in the rate of detection of MR-detected findings on ultrasound, depending on the MR lesion type (focus, mass, NME). In general, masses are most likely to be correlated with MRI, but a focus and NME are not. One study reports that the correlation rates of mass, focus, and NME are 67, 46, and 12%, respectively. The overall correlation rate was 57.5% (1,266/2,201) in one review series.
MRI-guided biopsy is the first choice for work-up, because it is not easy to correlate a focus with MR-directed ultrasound. There are, however, some exceptions. It is worth attempting an MR-directed ultrasound if a focus is in close proximity to a visible anatomic landmark lesion or structure. A landmark, for example, could be a known mass, a cyst, a surgical seroma, the nipple, etc. If there are such landmarks near the target, an ultrasound study could be performed, focused on a specific region for detection and biopsy.
It is worth trying to perform an MR-directed ultrasound for any kind of mass.