Assessment Criteria and Clinical Implications of Extranodal Extension in Head and Neck Cancer

Tumor breaching the capsule of a lymph node is termed extranodal extension (ENE). It reflects aggressiveness of a tumor, creates anatomic challenges for disease clearance, and increases the risk of distant metastasis. Extranodal extension can be assessed on a pathology specimen, by radiology studies, and by clinical examination. Presence of ENE in a pathology specimen has long been considered a high-risk feature of disease progression and would ordinarily benefit from the addition of chemotherapy to adjuvant radiotherapy. Although the eighth edition of the Union for International Cancer Control/American Joint Committee on Cancer stage classification dichotomizes pathologic ENE according to its presence or absence, emerging evidence suggests that the extent of a pathologic ENE may provide additional value for risk stratification to guide adjuvant therapy. Recent data suggest that the prognostic importance of pathologic ENE is also applicable for HPV-associated head and neck squamous cell carcinoma. In addition, compelling data demonstrate that indisputable radiologic ENE is a powerful risk stratification tool to identify patients at high risk for treatment failure, especially distant metastasis, applicable for both HPV-positive and HPV-negative head and neck squamous cell carcinoma. However, the definition and taxonomy of radiologic ENE requires standardization. The goal of this review is to clarify the contemporary understanding of the prognostic implications of ENE in head and neck squamous cell carcinoma, present the nuances of what is presently known and unknown, and elucidate how to classify ENE pathologically and radiologically with an understanding of the strengths and weaknesses of each approach. Finally, with the development of several risk stratification methods, the relative role of ENE and other prognostic schema will be explored.