Recommendation for Cranial Upper Border of Level Ⅱb in Clinical Target Volumes (CTV) for Nasopharyngeal Carcinoma

Purpose: To recommend the cranial side of level Ⅱb in clinical target volumes (CTV) for nasopharyngeal carcinoma (NPC) patients receiving intensity-modulated radiotherapy, to help reach a consensus on contouring level Ⅱb in CTV.Methods: From 2012 to 2016, 331 non-metastatic NPC patients treated with IMRT were retrospectively enrolled. According to the AJCC 8th staging system of NPC, there were 15 patients with stage Ⅰ, 76 stage Ⅱ, 103 stage Ⅲ, and 137 stage Ⅳ. Distribution of cervical lymph nodes in NPC was assessed based on imaging results. Comparison of the safety and the parotid dose parameters between patients with or without a reduction in the range ofⅡb was taken by SPSS 25.0 and R 2.14.2 software.Results: Metastasis rates of the most common diseased lymph nodes, lateral retropharyngeal lymph node andⅡb lymph node, were 82.8% and 64.0%, respectively. Among patients with levelⅡb involvement, the upper borders of metastatic nodes in 13.7% of cases were beyond the caudal edge of C1. The D50 and V26 values for parotid glands were significantly reduced after modified the upper bound of level Ⅱb in CTV (P = 0. 000).Conclusions: The upper bound of level IIb，in principle，should reach the lateral skull base during the delineation of the cervical CTV for NPC. To protect the parotid glands, however, individualized reduction in the upper bound of level IIb is recommended for patients who meet the formulated standard.