Studying The Timing Of Adaptive Radiation Therapy For Nasopharyngeal Carcinoma During Intensity-Modulated Radiation Therapy

Purpose:This study aimed to find when and how to perform an adaptive radiation therapy replanning by evaluating the anatomic and dosimetric variations of target volumes and organs at risk(OARs)during intensity-modulated radiation therapy(IMRT) for nasopharyngeal carcinoma (NPC).Materials and Methods:Nineteen undifferentiated non-keratinizing NPC patients treated with IMRT were recruited. Each patient had repeated kilovoltage computed tomography (KV-CT) scans similar to the initial simulation CT after each five fractions. Automatically re-contouring the target volumes and OARs by using deformable registration method to conduct CT-CT fusion in the MIM maestro image software. The assessment of anatomic changes, such as gross target volume (GTVnx) and involved neck lymph nodes volume (GTVnd), parotid glands volume and the shift of center of mass, by comparing the initial CT and repeated CT. Hybrid plans with recontouring were generated. The dose-volume histograms (DVH) of the hybrid plan and the original plan were compared to evaluate the domestic changes during the course of IMRT.Results:At treatment completion, the mean percentage volume loss of GTVnx, GTVnd, left and right parotid glands were65.6±13.3%,72.7±13.3%,39.8±14.4%and37.4±15.5%, respectively. The mean percentage loss of weight was8.3±5.1%.The mean center of mass of the left parotid gland shifted progressively towards the medial and superior aspects by0.27±0.25cm and0.14±0.20cm, respectively. The mean center of mass of the right parotid gland shifted medially by0.31±0.19cm. Lost weight significantly correlated with reduction in parotid glands and shift toward to the middle. There was no correlation between the weight loss and the volumetric changes of GTVnx and GTVnd.Comparing with the original plan, there was no significant difference between homogeneity index (HI) in all target volumes. The mean dose(Dmean), the doses to95%(D95),the volume to95%prescribed dose (V95) and conformity index (CI) of PTVnx,PTVnd and PTV1were improved during the course of radiation. The number of high dose regions remained the same delivered to PTVnx and PTVnd. The mean dose of PTV2showed no difference during IMRT but the D95,V95and CI were decreased significantly. Dose to OARs such as brain stem, spinal cord, parotid glands, glottis, right eyeball and left lens increased significantly as treatment progressed. The decrease of PTV2dose conformity or coverage and increase of OARs dose correlated with various anatomic changes.Conclusions:There were significant progressive anatomic and dosimetric variations of target volumes and OARs during IMRT for NPC. Underdose of target volume or overdose of normal avoidance structures was affected by various anatomic changes. According to the dosimetric analysis, D95/V95/CI of PTV2, the Dmax to brain stem, spinal cord, right eyeball and left lens,and the Dmean/V30to parotid glands and glottis were taken into account for predicting the need for ART. Under the statistical evaluation and practical consideration, two replans after1week and3weeks radiation were suggested for performing ART in this study.