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

Abstract

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). 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. 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. 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.