On ultrasound, a cancerous lymph node typically appears rounded, dark, and missing its normal bright center. That bright center, called the fatty hilum, is one of the most reliable landmarks radiologists look for, and its absence is the single most specific sign of malignancy, with a positive predictive value between 58% and 97%. Understanding what normal and abnormal nodes look like on ultrasound can help you make sense of imaging results while you wait for a definitive answer.
What a Normal Lymph Node Looks Like
A healthy lymph node has a predictable appearance on ultrasound. It is oval or bean-shaped, darker than the surrounding muscle, and contains a bright white line running through its center. That bright line is the fatty hilum, a strip of fat and blood vessels that appears continuous with the fat outside the node. Think of it like the white crease inside a kidney bean. When this structure is clearly visible, it’s a strong indicator the node is behaving normally.
Signs That Suggest Cancer
Several features raise concern when a radiologist evaluates a lymph node. No single finding confirms cancer on its own, but the more of these changes that appear together, the higher the suspicion.
Loss of the Fatty Hilum
This is the most telling change. As cancer cells infiltrate a lymph node, they gradually replace the normal fatty center. On ultrasound, the bright line thins, becomes eccentric, and eventually disappears entirely. A node with a completely absent or replaced hilum is the most specific ultrasound sign of malignancy. The process is progressive: early involvement may only push the hilum to one side, while advanced disease wipes it out completely, leaving a uniformly dark node.
Round Shape
Normal nodes are elongated, like an oval. Cancer tends to make them rounder. Radiologists measure this by comparing the short axis to the long axis. When that ratio exceeds 0.5 (meaning the node is more than half as wide as it is long), it’s considered suspicious. A perfectly round node, where both measurements are nearly equal, is more concerning than one that retains its natural oval contour.
Thickened Cortex
The cortex is the outer rim of the lymph node, the dark shell visible on ultrasound. In a healthy node, it’s thin and even. Cancer causes this cortex to thicken, either uniformly or in a focal bulge on one side. As the cortex grows, it compresses the hilum from the outside in. This thickening is often the earliest detectable change, appearing before the hilum disappears entirely.
Internal Necrosis or Cystic Changes
Some cancers cause the inside of a lymph node to break down, creating pockets of fluid or dead tissue. On ultrasound, these appear as dark areas within the node. Necrosis tends to have irregular, jagged edges, while cystic changes have smoother, well-defined borders. Cystic changes within a metastatic node are particularly associated with certain cancers: throat cancers linked to HPV show cystic foci in roughly 33% to 50% of cases. Thyroid cancer metastases can also produce cystic-appearing nodes.
Metastatic Nodes vs. Lymphoma
Cancer reaches lymph nodes in two main ways. It can spread there from a tumor elsewhere in the body (metastasis), or the cancer can start in the lymph node itself (lymphoma). These look slightly different on ultrasound, though there is overlap.
Metastatic nodes are typically round, dark, and missing their hilum. They may show internal necrosis, especially with squamous cell cancers of the head and neck. Focal cortical bulging, where one side of the node appears swollen, is a common early pattern because cancer cells initially deposit in one area of the node.
Lymphoma nodes also tend to be round and dark with a lost hilum, but they have a few distinguishing features. They more often show a fine net-like internal pattern called reticulation, giving the node a slightly textured appearance rather than the uniformly dark look of a metastatic deposit. Lymphoma nodes also tend to have very sharp, well-defined outer edges.
What Ultrasound Can and Cannot Tell You
Ultrasound is good at identifying nodes that look suspicious, but it has real limitations. In studies of breast cancer patients, ultrasound correctly identified cancerous axillary nodes only about 33% of the time (sensitivity), meaning it misses a significant number of involved nodes. Its strength is specificity: when ultrasound says a node looks normal, it’s right about 97% of the time. In practical terms, a normal-looking node on ultrasound is reassuring, but a node that looks fine isn’t a guarantee.
Because of these limitations, ultrasound is rarely the final word. When a node looks suspicious, the next step is usually a tissue sample. Fine-needle aspiration, where a thin needle is guided into the node under ultrasound, is the most common approach. Core needle biopsy, which takes a slightly larger sample, may be used when more tissue is needed for diagnosis.
What Happens After a Suspicious Finding
If your ultrasound report describes a node with concerning features but your doctor isn’t immediately recommending a biopsy, a short-term follow-up scan (often at about three months) may be the plan. This is common for incidentally discovered nodes, ones found during imaging done for another reason. A node that shrinks or stays stable over 12 months is generally reclassified as benign. However, stability at three months alone doesn’t rule out cancer; some malignant nodes remain unchanged for weeks before growing. Your doctor will factor in your overall clinical picture, including any known cancer history, symptoms, and the specific appearance of the node, to decide whether watching or sampling makes more sense.
Size matters in this decision, but not as much as internal features. A small node with a lost hilum and round shape is more concerning than a larger node that retains its normal oval shape and bright center. Radiologists weigh the combination of findings rather than relying on any single measurement.