In short, there is no definite set of features that define BI-RADS 3 lesions. While the literature reports on malignancy rates in different types (e.g. mass, non-mass, foci) of BI-RADS 3 lesions, no definite data on diagnostic criteria defining the BI-RADS 3 category are given. BI-RADS 3 category should be assigned to lesions presenting benign appearing imaging features in case the radiologist feels the need for further confirmation. Presence of suspicious morphologic features that are unlikely associated with a benign diagnosis should always be called BI-RADS 4 and not BI-RADS 3. Specific features will be discussed in the respective lesion type sections.

Care should be taken in transferring conventional mammography and ultrasound criteria directly to breast MRI. For instance, a newly diagnosed lesion showing only benign features does not necessarily need to be followed-up. This holds true especially for mass lesions with circumscribed margins and persistent or plateau enhancement curves. These findings are generally benign, especially when additional T2w and DWI features are considered (Fig. 14.1).

Fibroadenomata, the most common benign lesions in the breast, usually show a circumscribed T2w correlate and high diffusivity on Apparent Diffusion Coefficient (ADC) maps [22]. The latter constitutes the juvenile myxoid or fluid-rich fibroadenoma type. These lesions can even show wash-out curve types, but the combination of high ADC and circumscribed margins excludes the only malignant lesion with high ADC values: invasive mucinous cancer. Fibroadenomata do mature, leading to a loss of water content and an increased hypovascularized stroma component over time. This loss of water may even cause low ADC values that are due to the low T2- signal rather than a real diffusion restriction. Although Schrading et al. have coined the term of fibroadenoma-like appearing cancers in high-risk patients [23], others have not confirmed this finding, and the authors’ conclusions are likely due to the reading method applied at that time (alternator views on printed films, visual assessment of signal intensity time curves). In our own clinical experience, we have never encountered a cancer lacking all three MRI hallmarks of malignancy: non-circumscribed or spiculated margins, plateau or wash-out curve types and restricted diffusivity. Moreover, basic consideration of tumor biology implies that dangerous, fast growing tumors may appear with circumscribed margins but their fast growth requires strong and typical hypervascularization and restricted diffusivity due to high cellularity. Again, the combination of circumscribed margins with low and persistent contrast medium uptake excludes any malignant diagnosis: invasive cancer is either not circumscribed or, if circumscribed, presents a highly proliferative lesion that will always show strong contrast uptake followed by wash-out or plateau curves.

The MRI BI-RADS lexicon is characterized by the lack of a clinical decision rule—a precise description of which diagnostic criteria constitute a specific diagnosis, e.g. BI-RADS 3. Although there are several classification systems in breast MRI, such as the Göttingen score [24] or the Jena Tree [25, 26], these systems do not provide rules to differentiate between benign and probably benign lesions. However, they assign levels of suspicion to specific feature combinations, allowing the user to assess whether a lesion is benign or whether the lesion is still benign but may need further follow-up. Still, the decision to differentiate between benign and probably benign lesions is largely a decision based on the clinical background, including patient age, individual breast cancer risk and prior imaging findings. That said, we can conclude the following: first, a lesion that is already known and does not show any imaging progression over time should generally not be assigned as BI-RADS 3 on MRI. Second, a newly diagnosed lesion should not be called BI-RADS 3 if unambiguous benign imaging features are present. This does also hold true for the high-risk screening situation. Here, many authors and colleagues prefer immediate biopsy of newly diagnosed lesions. However, considering the variety of MR imaging protocols and their sensitivity for contrast media, new or stronger enhancing lesions may show such characteristics either due to protocol differences or the cyclical physiologic enhancement in premenopausal women.

The clinical indication for the breast MRI should also be considered in evaluation of BIRADS 3 lesions. If a patient is referred to MRI, e.g., due to an asymmetric density in mammography without remarkable findings on ultrasound, the pretest probability for breast cancer is very low and the indication for the examination questionable. If an incidental lesion, that is a lesion not corresponding to the mammographic asymmetry, shows only benign characteristics, the likelihood of malignancy is negligible, and the lesion should be termed benign and not probably benign. The high sensitivity of MRI implies that many lesions detected by MRI may have been already present but were not seen on conventional imaging.