Receiving a breast Magnetic Resonance Imaging (MRI) report that mentions Non-Mass Enhancement (NME) can be a source of significant anxiety. This term describes an area of abnormal contrast uptake in the breast that does not form a distinct, three-dimensional lump or mass. NME is a common finding on breast MRI. While it requires careful follow-up and evaluation, it is benign in the majority of cases.
Understanding Non-Mass Enhancement
Non-Mass Enhancement is distinguished from a typical breast mass, which has clearly defined borders and occupies a distinct space. NME, by contrast, is a region of enhancement that tends to spread out, appear hazy, or intersperse with normal tissue, lacking the sharp outline of a mass. It can be a small area or a large region that does not fit the criteria for a mass or a tiny, dot-like focus less than five millimeters in size.
The appearance of NME is based on the use of a contrast agent, a gadolinium-based dye, injected during the MRI scan. This dye highlights areas of increased blood flow, a process called “enhancement.” Since cancerous tumors often create their own new, leaky blood vessels to fuel their growth, they tend to enhance brightly. However, increased blood flow and enhancement can also occur due to non-cancerous conditions like inflammation, hormonal changes, or normal background tissue enhancement, which makes NME challenging to interpret initially.
Categorizing NME Patterns and Risk
Radiologists use standardized descriptors from the Breast Imaging Reporting and Data System (BI-RADS) to analyze NME and estimate the likelihood of malignancy. This classification focuses on two main visual criteria: the shape of the enhancement (distribution) and the texture within the area (internal enhancement characteristics).
A focal distribution is the least concerning, as it is limited to a small area less than one-quarter of a breast quadrant. Regional or diffuse distributions, spanning a larger area or scattered randomly throughout the breast, are often associated with benign changes like hormonal fluctuations. However, distributions that follow the path of the breast ducts are considered more suspicious.
Linear NME appears as a straight, curved, or branching line, suggesting involvement along a single duct. A segmental distribution is concerning; it forms a triangular or conical shape with the point directed toward the nipple, suggesting that the enhancement is filling a branching ductal system. This pattern is highly associated with cancer, specifically Ductal Carcinoma In Situ (DCIS).
The internal pattern describes the texture of the enhancement within the distributed area. A homogeneous pattern, which is uniform and confluent, is the most favorable and is often associated with benign outcomes. Patterns described as heterogeneous (non-uniform) or clumped (small, aggregated enhancement) carry a higher risk. The most suspicious internal characteristic is clustered ring enhancement, where small rings of enhancement are grouped together, which has a high positive predictive value for malignancy.
Malignant Versus Benign Causes
Only a minority of non-mass enhancement findings ultimately prove to be cancer. The overall malignancy rate for biopsied NME typically ranges between 15% and 30%, meaning that 70% to 85% of NME findings are benign. The specific probability is heavily influenced by the patterns described in the imaging report.
The majority of NME cases are caused by common, non-cancerous changes in the breast tissue. These benign causes include:
- Fibrocystic changes, which involve hormonal fluctuations that cause tissue to swell and enhance.
- Hormonal enhancement, particularly in pre-menopausal women scanned at a sub-optimal time in their menstrual cycle.
- Pseudoangiomatous stromal hyperplasia (PASH), which is an overgrowth of supportive tissue.
- Inflammation (such as mastitis) or the effects of prior radiation therapy.
When NME is malignant, it most frequently represents Ductal Carcinoma In Situ (DCIS), which is a non-invasive cancer confined to the milk ducts. Less often, it can represent invasive ductal carcinoma or invasive lobular carcinoma (ILC). The most suspicious patterns, such as segmental distribution paired with clumped or clustered ring enhancement, are the ones most likely to correspond to DCIS in the pathology report.
Next Steps in Evaluation and Diagnosis
Once NME is identified on an MRI, the next step is determining if the finding is visible on other imaging modalities, such as mammography or ultrasound. If the enhancement has a corresponding finding on these other modalities, a biopsy can be performed using the less invasive and more widely available guidance of ultrasound or mammography.
When NME is only visible on the MRI, the definitive diagnostic procedure is an MRI-guided core needle biopsy. This procedure is necessary because the enhancement cannot be reliably targeted without the real-time imaging of the MRI machine. During this procedure, a small sample of the enhancing tissue is removed and sent to a pathologist for a definitive diagnosis.
For NME findings deemed to be very low-risk, such as a focal, homogeneous pattern, the radiologist may recommend an alternative to an immediate biopsy. This alternative is a short-term imaging follow-up, typically with another MRI in six months, to confirm that the finding is stable or resolving. This surveillance approach is used when the features of the NME suggest it is likely a transient or physiological change rather than a developing malignancy.