The purpose of this study was to evaluate its rationality in real-world data and provide clinical evidence for the refinement of nodal CTV delineation in nasopharyngeal carcinoma(NPC).

Intensity-modulated radiation therapy (IMRT) has gradually replaced two-dimensional radiation therapy as it offers improved target conformity. Xerostomia is still the most common side effect of radiotherapy in patients with NPC treated with IMRT. Most stimulated saliva is secreted by the parotid glands (PGs), while the submandibular glands (SMGs) produce most of the unstimulated saliva and mucins, which may influence the degree of a dry mouth sensation.There are still differences in the understanding of nasopharyngeal cancer experts at home and abroad on the delineation of clinical target volumes., suitable criteria for elective irradiation of neck level Ib need to be re-evaluated.Therefore, we conducted a retrospective study to assess the feasibility of recommendation to level Ib in the International Guideline for Delineation of Clinical Target Volumes.

Inclusion Criteria:

1. histologically confirmed NPC by biopsy (clearly recorded in the pathology report or medical history) ;
2. no evidence of distant metastasis at initial diagnosis and receiving radical IMRT in our center
3. the following conditions: ① level IB metastasis, ② involvement of the submandibular gland, ③ involvement of structures that drain to level Ib as the first echelon site ( the oral cavity, anterior half of nasal cavity involvement) ④ involvement of level II LNs with extracapsular extension（because the judgment of extracapsular invasion is greatly influenced by subjective factors, this study only include patients who had high grade ENE(G2/G3 as demonstrated in our previous study: Oral Oncol. 2019 Dec;99:104438.)⑤ level II nodal involvement with maximum nodal axial diameter greater than 2 cm.
4. all patients who were treated at Lin Shaojun's attending group from June 2005 to December 2012.
5. presence of complete baseline magnetic resonance imaging data of the nasopharyngeal skull base and neck and treated in our center at initial diagnosis .

Exclusion Criteria:

1. disease progression during IMRT;
2. fail to obtain tumor efficacy evaluation information in the medical records of the research center;
3. previous malignancy or other concomitant malignant disease;
4. receiving blind treatment in clinical research.

Patients who met the inclusion criteria and did not meet the exclusion criteria of our center.