Imaging of extranodal extension: why is it important in head and neck cancer?

The assessment of extranodal extension by imaging (iENE) is important in the management of patients with metastatic nodes from head and neck cancer (HNC). Over many decades iENE has guided surgical and radiotherapy planning using both early iENE, to detect small metastatic nodes that otherwise appear normal on imaging, and late iENE, to detect invasion into adjacent structures. Currently, new prognostic indicators of iENE are under a spotlight for treatment planning, prognosis and staging. This is aided by ongoing international interdisciplinary consensus building on iENE grading for prognosis and by the advantages of imaging over pathological assessment (pENE), which include coverage of all nodal groups in all patients irrespective of age, cancer stage or treatment. These advantages are especially relevant to those cancers treated primarily by (chemo)radiotherapy, such as viral-related human papillomavirus (HPV)-positive oropharyngeal squamous cell carcinomas and Epstein-Barr virus-positive nasopharyngeal carcinomas. Evidence for the role of iENE as an indicator of poor prognosis is accumulating. Inclusion of iENE in the N category of the upcoming American Joint Committee on Cancer (AJCC) and the International Union Against Cancer (UICC) HNC staging guidelines is under consideration and has already been included in the latest AJCC ninth version of cancer staging guideline for nasopharyngeal carcinoma. New indications for iENE in treatment planning are under evaluation in areas such as the selection of HPV-positive oropharyngeal squamous cell carcinoma for treatment de-escalation. However, there are many gaps in our knowledge of iENE, which are a concern in multidisciplinary meetings because of their potential effect on decisions regarding disease management. New indications for iENE in the management of HNC patients are an exciting advance but more research is needed for it to reach its full potential.