Search for abnormal nodes, particularly in expected drainage sites. Submandibular and jugulodigastric nodes are more likely to be affected by benign hyperplasia than other nodal groups. Lymph nodes can normally be larger and more numerous in younger patients.
Nodes can be measured in either the short or long axis, and they can be measured only on axial images or include diameters in the craniocaudal dimension on reformatted images. Nodes ≥ 15 mm are pathologically enlarged. Necrotic nodes are most commonly from HNSCC but may uncommonly occur with lymphoma. In the setting of head and neck cancer, necrotic or cystic change, even in small nodes, is a specific marker of malignancy.
Primary sites to consider with cystic morphology are papillary thyroid cancer and oropharyngeal HNSCC.
Thyroid cystic metastases may have hyperintense signal on T1-weighted MRI due to thyroid protein or blood products. HPV-associated HNSCC and thyroid malignancy may have purely cystic metastases, and both can occur in young adults.
Cystic or necrotic nodes can be large with a small or occult primary tumor. In particular, small tumors of the base of tongue and tonsil are frequently not symptomatic and can be overlooked on imaging.
Calcified lymph nodes are most commonly found with thyroid carcinoma due to the presence of psammomatous calcifications with papillary and medullary carcinoma. The calcifications in papillary carcinoma may have a speckled appearance.
Other causes of nodal calcification are mucinous adenocarcinoma; treated lymphoma; treated or untreated HNSCC; and, less commonly, tuberculosis.
Although mediastinal nodal calcifications are usually a marker of benign disease, cervical nodal calcifications are more frequently seen with malignancy.
Metastatic disease can change the shape of the node by infiltrating nodal tissue and expanding the nodal capsule. Thus, rounded rather than oval nodes are suspicious. As disease progresses, the capsule no longer contains the node; ill-defined irregular margins in a lymph node are a sign of malignancy and may represent extracapsular spread of tumor.
Site Classification and Check Sites
Midline tumors, nasopharyngeal carcinoma (NPC), and epiglottic and oral cavity tumors frequently drain bilaterally.
Retropharyngeal nodes and parotid nodes are not classified in levels I–VII and can be forgotten. Retropharyngeal and parotid nodes may be overlooked when they drain primary tumors that are remote in location. Thyroid and nasopharyngeal cancers can drain to the retropharyngeal nodes and NPC can drain to the parotid.
Skin cancer metastasizes to level V and superficial nodes, such as parotid, posterior auricular, facial, and occipital nodes.
The Virchow node is a left supraclavicular lymph node near the junction of the thoracic duct and the left subclavian vein where the lymph from much of the body drains into the systemic circulation. When this is an isolated finding on neck CT or MRI, the main differential diagnoses are thyroid cancer and thoracic and abdominal malignancy.
Features Important for Staging
The radiologist’s role is to describe three characteristics pertinent to the N staging: size, single versus multiple nodes, and side of nodes.
The supraclavicular nodes in NPC and retropharyngeal nodes in thyroid carcinoma are lower-level nodes and represent more advanced nodal disease. The presence of these nodal groups should lead to a careful search of systemic metastases, most commonly in the lung.
Features Important for Management
Local invasion of a lymph node first occurs beyond the nodal capsule and then into adjacent structures. The characteristics of extracapsular spread are poor prognostic indicators and associated with reduced survival (further 50%). Imaging findings of extracapsular spread are irregular margins, fat stranding, and loss of fat planes with adjacent structures. Extracapsular spread is more likely in larger nodes. Nodal metastases can invade adjacent muscle, bone, neural, and vascular structures.
Arterial invasion affects surgical options and leads to devastating vascular complications, such as occlusion, pseudoaneurysm, and carotid blowout. The most sensitive sign of arterial invasion is loss of fascial planes, and the most specific sign is narrowing or irregularity of the artery. Another important sign of arterial invasion is circumferential arterial encasement by > 180–270°, which is associated with increased likelihood of adventitial invasion and portends a grave prognosis.
Small or cystic nodes can be false negative on FDG PET. FDG uptake is not expected in well differentiated thyroid carcinoma, although dedifferentiated tumors are typically FDG-avid. FDG uptake is variable for medullary thyroid carcinoma, and even large metastases can be FDG-negative.