What imaging features may be useful to distinguish benign from malignant lymph nodes?
Nodal size is one of the criteria used to differentiate reactive from malignant lymph nodes. Although larger nodes tend to have a higher incidence of malignancy, reactive nodes can be as large as malignant nodes, and therefore the use of a size threshold alone will lead to many false positive and false negative results. Most inflammatory diseases, except for granulomatous infections such as by tuberculosis, involve lymph nodes diffusely and homogeneously, generally preserving their oval shape.
Malignant lymph nodes tend to be round in shape with a short-to-long axis ratio (S/L ratio) >0.5, whereas reactive or benign lymph nodes are more likely to have an S/L ratio <0.5. Although a round shape helps to identify a malignant lymph node, it should not be used as the sole criterion and is more specific when disruption of the normal nodal architecture is also evident. In the normal neck, about 90% of nodes with a maximum transverse diameter greater than 5 mm will have echogenic hilum visible on high-resolution US. Malignant lymph nodes usually do not show an echogenic hilum, which may be an important clue to detect disease involvement of nodes that are normal in size and shape. However, identification of a normal-appearing hilum cannot reliably exclude the presence of a small burden of metastatic disease, especially when occurring in the periphery of the node.
Visualization of an internal reticular echo pattern of lymph nodes may also be encountered and is relatively specific for lymphoma