Appraisal of Radiologic Size Criteria for Malignant Lymph Nodes in Parotid Region in Nasopharyngeal Carcinoma: Implication for Parotid‐Sparing IMRT

ABSTRACT Purpose We evaluate the appropriateness of current radiology size criteria (short axial diameter, SAD ≥ 10 mm) for malignant nodes in the parotid region (PLN), and appreciate the clinical implications of the Node‐Reporting‐and‐Data‐System (Node‐RADS) in nasopharyngeal carcinoma (NPC). Methods and Material NPC patients who received curative IMRT were included. Pre‐treatment MRI was reviewed by two radiologists to record the presence/absence, size, and configuration of PLN, and calculated Node‐RADS score. Malignant PLN was defined as either fine‐needle aspiration (FNA) positive or the occurrence of subsequent out‐field in situ PLN failure following parotid‐sparing IMRT. Results Pre‐treatment PLN was identified in 74 (11.8%) of 627 consecutive patients. SAD PLNs were 5–6 mm ( n = 53, 72%), 6–8 mm ( n = 15, 20%), 8–10 mm ( n = 4, 5%), and ≥ 10 mm (2, 3%), respectively. FNAs were positive in 6 patients (2 each in SAD 6–8 mm, 8–10 mm, and ≥ 10 mm). Out‐field in situ PLN failure occurred in 6 patients: 1 with SAD 5–6 mm, 3 with SAD 6–8 mm, and 2 with SAD 8–10 mm. The risk of PLN being malignant was 1.9% (1/53), 33.3% (5/15), 100% (4/4), and 100% (2/2) in SAD 5–6 mm, 6–8 mm, 8–10 mm, and ≥ 10 mm subsets, respectively. Malignant PLN with Node‐RADS score ≥ 4 were 10% (1/10), 45.5% (5/11), and 100% (4/4) in SAD 5–6 mm, 6–8 mm, 8–10 mm subsets, respectively. Conclusion Current radiologic size criteria of ≥ 10 mm for positive PLN should be reduced to ≥ 8 mm. Node‐RADS score of 4–5, which includes adverse morphological nodal features, could improve the specificity of identifying malignant PLNs in the 6–8 mm subset.