A 32-year-old woman presented for high-risk screening with an International Breast Cancer Intervention Study (IBIS) tool lifetime risk of 23.1%. A new enhancing focus was noted on MRI at 11 o’clock, 4 cm from the nipple, measuring 4 mm (Figure 1). This focus stands out despite marked background parenchymal enhancement. The focus demonstrates rapid uptake with plateau and persistent kinetics and is not hyperintense on T2-weighted imaging (T2WI) (Figure 2). MR-guided biopsy was performed.
A small nonspace-occupying distinct enhancing dot on MRI with no describable morphologic features
Focal nonmass enhancement
In the American College of Radiology Breast Imaging Reporting and Data System (ACR BI-RADS) Atlas, the breast MRI lexicon defines a focus as a unique enhancing dot that is too small to characterize further morphologically as a mass or nonmass enhancement. It is not a space-occupying lesion. In the fourth edition, mass and foci were differentiated by size. Findings smaller than 5 mm were defined as a focus, while findings larger than 5 mm were defined as a mass. The fifth edition no longer uses size criteria but instead uses morphology.1 The presence of margins and shape defines a mass, and a focus is a specific, isolated, enhancing dot that is too small to be assigned morphologic descriptors.Because of its small size, a focus in breast MRI must be evaluated based on characteristics other than morphologic features. Patient-related factors in conjunction with lesion features should be used for decision-making.
A focus is usually too small to apply enhancement quantitative analysis with kinetic curves. The enhancement intensity and presence of washout must be visually analyzed. Mammography and ultrasound are not usually helpful to further evaluate MRI-detected foci as correlation is challenging due to their small size. The strategy must be based on the patient’s risk factors and other MRI findings.
The first step when evaluating a small enhancing lesion in the breast is to decide whether an enhancing area represents a true focus, focal nonmass enhancement (NME), mass, or background parenchymal enhancement. A small mass may be misinterpreted as a focus. Masses are space-occupying lesions and are 3-dimensional. Masses can be described in terms of morphology, margins, and internal enhancement characteristics. The pathologic differential considerations for masses and foci may have significant overlap; however, management of these entities is different and based on their respective imaging features. If an enhancing finding can be defined by morphology, margins, or internal enhancement characteristics, it should be described as a mass. These imaging characteristics of masses help guide management and determine predictive value of malignancy.1 Foci are not able to be managed based on imaging characteristics alone.
NME is defined as a discrete site of enhancement that cannot be defined as a mass or focus. It is characterized in terms of distribution and internal enhancement pattern. The types of distribution include focal, linear, segmental, regional, multiple regions, and diffuse. While most types of NME are not confused with foci, focal NME, which entails enhancement of less than a quadrant of breast tissue at a discrete site, may be difficult to differentiate from a single focus since the size cut-off of 5 mm is no longer used as a defining criterion for a focus.1 Focal NME represents an area of enhancement distinct from the surrounding parenchyma but is not a space-occupying mass and is typically interspersed with nonenhancing fatty or glandular tissue. These areas appear normal on nonenhanced T1-weighted images and usually have no correlate on T2-weighted images. As with masses, the descriptive features of NME assist in management, whereas the assessment of foci cannot solely rely on imaging features.
It is important to differentiate a focus from background parenchymal enhancement (BPE), which is defined as the normal enhancement of a patient’s fibroglandular breast tissue, according to the BI-RADS Atlas.1 BPE is divided into four categories based on the amount of glandular contrast uptake, and the category assessment is usually determined on the first postcontrast dynamic image (about 90 seconds after contrast administration). Normal background parenchymal enhancement may fluctuate in pattern and degree of enhancement, depending on various physiologic factors. The management of a focus depends on the degree of BPE. If a focus does not stand out from other background foci, it should be considered as normal BPE. Conversely, if there is no significant BPE and the enhancing finding is small, a focus should be considered suspicious.
A focus commonly represents a benign process. The benign differential diagnoses include papilloma, lymph node, fibroadenoma, stromal fibrosis, adenosis or fibrocystic change. Malignancy rate of an enhancing focus is widely variable at 1% to 23%.
In 2006, Liberman and colleagues studied 666 consecutive nonpalpable, mammographic occult lesions detected by MRI. The malignancy rate of lesions 4 mm or less was 2.7%.2 In 2009, Eby and colleagues showed the overall cancer yield for foci was low at 0.6%.3 Weinstein and colleagues in 2010 found the overall malignancy rate of foci was 2.1%.4 Higher malignancy rates were observed in more recent studies from 15% to 21%.5-7 The overall malignancy rate of an enhancing focus was 8.4% combining the results of all these studies (Table 1).
Kinetic analysis was reviewed for a potential role in evaluating enhancing foci in five studies by Han et al (2008), Eby et al (2009), Abe et al (2010), Jansen et al (2011), and Raza et al (2012).3,5-8 These studies showed no statistical difference between groups of foci showing persistent enhancement and foci with washout enhancement. Foci with washout enhancement and plateau enhancement also showed no statistical difference. In a study evaluating probable benign breast MRI lesions, no malignancy was shown in the follow-up of foci with persistent enhancement. The presence of a washout pattern and older age were found to be significant predictors of malignancy for an enhancing focus in a study by Youichi et al (2017).9 The main decisive node on the decision tree was the presence of a washout pattern, followed by whether the patient’s age was > 63 years. Small malignant lesions do not always show an expected enhancement pattern. Kinetic analysis is not specific for malignancy and should not be used alone to guide management.
In 1999, Kuhl and colleagues studied the role of using T2 signal characteristics to improve positive predictive value of breast MRI. They found that breast cancers were T2 isointense or T2 hypointense in relation to breast parenchyma in 87% of cases and fibroadenomas were T2 hyperintense in 71% of the cases.10 However, there are breast cancers that show high T2 signal including triple negative cancers, mucinous tumors, and papillary carcinomas. More studies are needed to determine if an enhancing focus with a corresponding T2-hyperintensity can be managed as a benign finding; therefore, this should be managed with a short interval follow-up MRI. These prior studies suggest that an enhancing focus with T2-hypointensity may warrant a biopsy or at minimum a 6-month follow-up.
Interval change is an important characteristic to consider in evaluating an enhancing focus. An enhancing focus that is new or enlarging has a significant malignant potential. Ha and colleagues found the malignancy rate was 27.2% when combined with the T2 hypointensity of the lesion. They also found that malignancy was detected only when 1 or 2 foci were followed. When 3 or more foci were present, no malignancy was present.11
The presence of a genetic mutation is a crucial consideration for management of enhancing foci on MRI. BRCA1-associated breast cancers showed more benign morphologic features, but exhibited aggressive pathologic features, such as the triple-negative phenotype. Compared with sporadic breast cancers, BRCA-associated breast cancers also exhibit different morphologic features at imaging. Kuhl et al reported that 23% to 38% of genetic breast cancers exhibited benign morphologic features, particularly BRCA1-associated breast cancers that appear similar to fibroadenoma or cysts.12 Since BRCA-associated breast cancers have been shown to have more benign findings on MRI, patients with a known mutation should be referred for biopsy rather than short interval follow-up.
Focus: Adenosis and pseudoangiomatous stromal hyperplasia
An enhancing focus is a common finding on breast MRI, and a management strategy is important. When there are numerous foci and/or bilateral foci, these should be considered BI-RADS 2, representing background parenchymal enhancement. An isolated focus with T2 hyperintensity without washout is considered BI-RADS 3 in patients not BRCA positive. If a patient has a known BRCA mutation, a benign-appearing focus should be considered suspicious as foci in these patients may represent aggressive cancers. A new or enlarging isolated focus with T2 hypointensity should be considered BI-RADS 4, for which biopsy should be performed under MRI guidance (Table 2).
Choe A, Sivarajah R. Enhancing Foci on Breast MRI. J Am Osteopath Coll Radiol. 2021;10(1):37-39.