Objective To assess the effect of optimizing the neck node level Ⅳ in IMRT clinical target volume( CTV) for nasopharyngeal carcinoma( NPC) on the treatment outcome and thyroid dose,in order to provide the basis for optimization of IMRT target area in NPC. Methods For selected 64 cases of NPC patients treated with IMRT,including 32 cases of neck node level Ⅳ region which optimized,and a control group of 32 cases which Ⅳ region is still within the CTV; Radiation dose parameters of thyroid were measured using ...