The Applied Research of Phased Intensity-modulated Radiotherapy in the Treatment of Local-advanced Nasopharyngeal Carcinoma

Objective The aim of this study was to evaluate prospectively the volumetric and dosimetric changes of target volumes and organs at risk(OARs)during the course of intensity-modulated radiation therapy(IMRT)for local-advanced nasopharyngeal carcinoma(NPC)and the necessity and the best timing of phased intensity-modulated radiotherapy.Methods 20local-advanced nasopharyngeal carcinoma patients treated by concurrent chemotherapy and IMRT were included.CT and MR images were acquired for each patient before treatment and at weeks 2,3,4,5and 6during treatment.The target volumes and OARs were contoured based on the fused CT-MRI images and hybrid plans were generated.The volumetric changes and the dosimetric changes were measured by comparing the original plan and hybrid plans.Results Significant volumetric changes of target volumes and parotid gland were observed during the treatment course.The primary nasopharyngeal tumor(GTVnx),CTV1,involved lymph nodes(GTVnd)and left and right parotid glands,shrank at a mean rate of 14.7%,11.56%,11.40%,6.54%,6.78% per treatment week,respectively.There were no significant dosimetric differences in GTVnx,GTVnd,CTV1,the spinal cord and brain stem when comparing the initial CT imaging before treatment with the subsequent weekly CT imaging.On the contrary,the differences in the dosimitric parameters to the bilateral parotid glands were significant(F=6.73,P=0.007;F=7.43,P=0.007).Conclusion Remarkably volumetric changes were observed,however the dosimetric changes were inconspicuous except parotid.Phased intensity-modulated radiotherapy might bring benefits for protecting the parotid gland.