Difference in normal tissue complication probability of lower cranial nerves between target volumes recommended by Radiation Therapy Oncology Group and China in intensity-modulated radiotherapy for T1-2 nasopharyngeal carcinoma

Objective To investigate the difference in normal tissue complication probability (NTCP) of lower cranial nerves (LCNs) between target volumes recommended by Radiation Therapy Oncology Group (RTOG) and China in intensity-modulated radiotherapy (IMRT) for T1-2 nasopharyngeal carcinoma (NPC), and to provide the evidence of dose-volume effect for the protection of LCNs in IMRT for NPC. Methods A total of 20 patients with T1-2 NPC who were treated from 2013 to 2015 were enrolled, and LCNs were delineated on CT images. Target volume delineation and treatment plan designing were performed according to the method recommended by RTOG0225(RTOG target volume delineation method) or the Chinese Working Committee for Clinical Staging of NPC in 2010(Chinese target volume delineation method), and the differences in the dose to LCNs and NTCP were calculated. Results In the RTOG and Chinese methods for target volume delineation, Dmax to the left and right LCNs was 7450±273 cGy/7294±309 cGy and 7361±160 cGy/7190±395 cGy, respectively (P=0.018 and 0.042), Dmean was 6735±285 cGy/6660±333 cGy and 6446±429 cGy/6299±467 cGy, respectively (both P=0.000), and the NTCP was 60%±10%/57%±13% and 51%±15%/45%±17%, respectively (both P=0.000). Conclusions It is feasible to precisely delineate target volume with the LCNs as a routine OAR and predict NTCP in IMRT for T1-2 NPC. The NTCP of the LCNs is closely associated with target volume dose and irradiated volume. The dose to the LCNs and NTCP determined by the Chinese target volume delineation method are significantly lower than those determined by the RTOG method. Key words: Nasopharynx neoplasms/intensity modulated radiotherapy; Lower cranial nerves; Normal tissue complication probability