Imaging-detected extranodal extension in head and neck cancer: clinical implications, diagnostic criteria, and the potential of photon-counting detector CT.

Imaging-detected extranodal extension (iENE) is increasingly recognized as an objective adverse prognostic factor in head and neck cancer. The newly published 9th edition of the UICC/AJCC TNM Classification now incorporates iENE into the nodal (N) categories for nasopharyngeal carcinoma, HPV-associated oropharyngeal carcinoma, and salivary gland carcinoma, typically upstaging patients to higher N categories, and designates imaging findings as an accepted standard for the clinical ENE (cENE) as the N3b category in all other head and neck subsites. To establish diagnostic criteria for iENE, the Head and Neck Cancer International Group (HNCIG) has proposed a four-tier grading system (2024): Grade 0 (iENE-negative), Grade 1 (perinodal fat extension), Grade 2 (coalescent lymph nodes), and Grade 3 (invasion of adjacent structures). Each grade carries distinct clinical implications, ranging from treatment de-escalation for Grade 0 to consideration of unresectable disease for Grade 3. Recent advances in ultra-high-resolution imaging, particularly photon-counting detector CT (PCD-CT) with 1024 × 1024 matrix in super-high-resolution (SHR) mode, may offer the spatial details necessary to more reliably detect these subtle findings compared with conventional imaging. Through direct comparisons with histopathological correlations, we demonstrate the potential of high-resolution imaging in detecting iENE. PCD-CT, with its simultaneous spectral imaging capabilities-including virtual monochromatic imaging-enhances soft tissue contrast for detecting subtle ENEs. In addition, material decomposition maps aid in the precise evaluation of tumor-tissue interfaces. This comprehensive review is an educational resource for radiologists applying the HNCIG consensus criteria including the newly adopted TNM staging requirements. While advanced imaging technologies such as PCD-CT show promising potential for enhanced tissue characterization, further validation studies are needed to establish their clinical utility in routine iENE assessment.