Evaluating the Rationality of the International Guideline About Selective Coverage of Level Ib in CTV With NPC

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.