The axilla, or armpit, is an anatomical space between the upper arm and chest wall containing nerves, blood vessels, and numerous lymph nodes embedded in fatty tissue. Lymph nodes are small, bean-shaped filters that capture waste, foreign particles, and immune cells as fluid drains from the adjacent breast and upper limb. An axillary ultrasound is a non-invasive imaging technique used to examine these nodes, primarily when cancer spread from a nearby primary tumor is suspected.
Why and How the Axillary Ultrasound is Performed
The primary indication for an axillary ultrasound is to assess the regional spread of cancer, most frequently in patients diagnosed with breast cancer. Determining the status of the axillary lymph nodes before surgery is important, as physical examination alone often misses positive cases. Ultrasound provides a quick, non-invasive method to screen all nodes in the axilla for subtle changes.
The procedure is straightforward, typically performed using a high-frequency linear-array transducer that transmits and receives sound waves. The patient is usually positioned supine oblique, with the arm on the affected side raised and rotated behind the head. This positioning stretches the axillary soft tissues, making the lymph nodes more accessible and easier to visualize.
The radiologist systematically scans the three levels of axillary lymph nodes, focusing on those that drain the area of the primary tumor. In addition to standard grayscale imaging, Color Doppler ultrasound is often used to assess blood flow patterns within the nodes.
Recognizing Malignancy on Ultrasound Images
A normal lymph node appears on ultrasound as an oval or bean-shaped structure with a distinct, bright center called the fatty hilum. The outer layer, or cortex, is thin and uniformly dark, reflecting its cellular composition. A cancerous axillary lymph node loses these benign features as metastatic cells infiltrate and replace the normal tissue architecture.
One noticeable change is the loss of the normal oval morphology, causing the node to become more rounded. Radiologists measure the short-to-long axis ratio, where a ratio approaching or exceeding one suggests a higher probability of malignancy. This rounding occurs as cancer cells proliferate and expand within the node.
The appearance of the cortex and the hilum is another significant visual criterion. In a cancerous node, the fatty hilum is often compressed, distorted, or completely replaced by tumor, appearing as a diminished or absent central bright area. Concurrently, the cortex thickens, often exceeding three millimeters, and this thickening may be irregular or focal.
Assessing blood flow using Color Doppler provides functional information about the node. Normal nodes exhibit central blood flow through the hilum. Malignant nodes often display abnormal vascular patterns, such as increased flow along the periphery of the cortex, known as peripheral vascularity. The combination of a rounded shape, loss of the fatty hilum, and peripheral blood flow strongly suggests metastatic disease.
The Next Step: Ultrasound-Guided Biopsy
While ultrasound characteristics are highly suggestive, they are not definitive for confirming a cancer diagnosis, as benign conditions like infection can mimic malignant features. If the ultrasound reveals a lymph node with suspicious morphological or vascular characteristics, the next step is an ultrasound-guided biopsy. This procedure obtains a tissue sample for pathological examination, which establishes the definitive diagnosis.
The most common technique is a core needle biopsy (CNB), which provides a small cylinder of tissue and offers a higher diagnostic yield than fine needle aspiration (FNA). The procedure is minimally invasive, performed on an outpatient basis using local anesthesia. The patient is positioned similarly to the initial scan, allowing the radiologist clear visualization of the target node.
Using the real-time ultrasound image as a map, a specialized biopsy needle is precisely guided into the suspicious area of the lymph node. Several small tissue samples are collected and sent to a pathology laboratory for analysis. The pathologist examines the tissue under a microscope to confirm the presence of malignant cells and characterize the type of cancer.
Impact on Cancer Staging and Treatment Decisions
A confirmed positive axillary lymph node finding (N+ status) carries significant weight in the overall staging of a patient’s cancer, particularly breast cancer. The TNM (Tumor, Node, Metastasis) staging system uses this information to classify the extent of the disease. The presence of cancer cells in the axilla indicates regional spread, suggesting a different disease trajectory than cancer confined solely to the primary site.
This information influences the treatment strategy by requiring a more comprehensive approach to managing the disease. A positive node finding may prompt the medical team to recommend systemic therapy, such as chemotherapy, often administered before surgery as neoadjuvant therapy. The purpose of this pre-operative treatment is to shrink the tumor and potentially eliminate cancer cells in the lymph nodes.
A confirmed positive node typically dictates a more extensive surgical plan for the axilla, such as an axillary lymph node dissection, rather than a less invasive sentinel lymph node biopsy alone. The finding also often necessitates post-operative radiation therapy directed at the axilla and surrounding areas. This ensures the patient receives the most appropriate and tailored sequence of treatments.