Intramammary lymph nodes (IMLNs) are small, normal structures sometimes identified during routine breast imaging, such as a mammogram or ultrasound. The unexpected finding of a node in or near the breast can understandably cause concern about the possibility of cancer. While IMLNs are part of the normal anatomy and are most often benign, their role in the lymphatic system means they are occasionally involved in disease processes. Understanding their characteristics and cancer risk is helpful.
What Intramammary Lymph Nodes Are
Intramammary lymph nodes are small, encapsulated organs found within the breast tissue, distinguishing them from the larger collection of nodes located in the axilla (armpit). These nodes are a component of the lymphatic system, a network that manages fluid balance and immune defense. Their function involves filtering lymph fluid draining from the breast tissue, trapping foreign particles, and housing immune cells.
The majority of the breast’s lymphatic drainage flows to the axillary nodes, but IMLNs serve as regional collection points. They most commonly occur in the upper outer quadrant of the breast, but they can be found anywhere within the glandular tissue.
Frequency of Detection and Benign Characteristics
IMLNs are detected on screening mammography in approximately 5% of studies, though prevalence can reach 48% depending on the imaging method used, such as MRI or ultrasound. When identified, an IMLN is usually classified as benign and requires no further investigation if it displays typical characteristics.
A normal, benign IMLN typically appears as a small, well-defined, oval or kidney-bean shaped structure, generally under one centimeter (10 mm) in its longest diameter. A distinguishing feature is the presence of a fatty hilum, which is fat visible in the center of the node that appears lucent on a mammogram or echogenic on an ultrasound. This fatty center, along with a smooth margin and oval shape, strongly indicates the node is functioning normally and is not a concern for malignancy.
Specific Statistics on Cancer Involvement
Cancer involvement in an IMLN is uncommon, occurring primarily through metastasis from a nearby breast tumor. Primary malignancy originating within the node, such as lymphoma, is exceptionally rare. Metastasis is the most common form of involvement, where cancer cells spread from a primary breast tumor through the lymphatic vessels.
In patients with a known breast cancer diagnosis, the incidence of metastasis to an IMLN ranges between 1% and 34% of cases. One study found that IMLN metastases were present in about 32% of confirmed invasive cancer cases. However, the vast majority of these patients also had metastasis to the main axillary lymph nodes. Isolated IMLN metastasis, where the IMLN is involved but the axillary nodes are clear, is a rare event, documented in a small percentage of cases. Spread to an IMLN is considered a significant factor that can change the cancer’s staging and prognosis.
The vast majority of IMLNs that appear abnormal are reactive, meaning they are enlarged due to benign causes like infection, inflammation, or an immune response. Suspicion for malignancy arises when the node loses its typical benign features. This includes becoming larger than one centimeter, developing a rounded shape instead of an oval one, or losing the appearance of the fatty hilum.
Diagnostic Evaluation and Next Steps
When an IMLN does not exhibit classic benign features, or if a patient has a concurrent breast malignancy, further diagnostic evaluation is necessary. Imaging characteristics that raise suspicion include a rounded shape, loss of the central fatty hilum, or an eccentric cortical thickness of 3 millimeters or greater. If these atypical features are identified on a screening mammogram, a targeted ultrasound is often the next step to examine the node’s structure and size in detail.
If the ultrasound confirms suspicious morphology, the management protocol may involve a short-term interval follow-up, typically in six months, to monitor for changes. If the node is highly suspicious—for example, significantly enlarged, completely rounded, and lacking a fatty hilum—a biopsy may be recommended for definitive diagnosis. This is often performed using an ultrasound-guided fine-needle aspiration or core biopsy. The resulting pathology determines whether the node is benign and reactive or contains metastatic cancer cells.