Volumetric Modulated Arc Therapy And Fixed Gantry Dynamic Intensity Modulated Radiation Therapy For NPC: Dosimetric Comparison

Targets in Nasopharyngeal carcinoma (NPC) are very complex，meanwhile thereare many important Organs at risk spring targets. In order to get good dose distributionfor targets and avoid dose exceed constrains for OARs, fixed gantry IntensityModulated Radiation Therapy (IMRT) technique is preferential for NPC case. IMRTtechnique brought many clinical benefits for NPC patients. IMRT reduced the dose forOARs, especially for Parotid and Oral Cavity. But there was significant disadvantage inIMRT techniques. The IMRT treatment plan is usually quite complex and contained7to9treatment fields. This increased the treatment plan delivery time. Anotherdisadvantage is that Monitor Units (MU) is usually two or three times more than threedimensional conformal radiation therapy (3DCRT). Recently, Volumetric ModulatedArc Therapy (VMAT) technique has come to use in clinical. We could get cut thetreatment delivery time, reduce the MUs and improve the comfort of patient by VMAT.The plan quality could also be improved or at least keep the same as IMRT plan.The VMAT plans in this study were all base on RapidArc technique. RapidArcoptimization algorithm has gone through the first and second generation PRO1andPRO2. Now it produced the third generation PRO3.The object of this work is to studythe dosimetric characters of VMAT-RapidArc plan and its clinical application in NPCradiation therapy by comparing the VMAT-RapidArc plan base on PRO3optimizationalgorithm with IMRT plan.10NPC cases were selected in this study. The RapidArc plans and IMRT planswere designed for each case. Dose distribution and DVH for Targets and OARs werecompared for RapidArc and IMRT plans. Meanwhile, the Target CI and HI along withMUs and treatment delivery time were studied for the two groups. According to theresults, we found that targets dose distribution, conformal index (CI), target coverage(TC) and quality of coverage (QOC) were almost the same between plans in the twogroups. There is no statistical significance. Dmaxand D2%of targets for RapidArc planwere less than IMRT plan，Homogeneity Index (HI) of targets of RapidArc plan wasbetter than IMRT plan. P<0.05, there was statistical significance. Meanwhile, the Dmeanfor Parotid, Dmaxfor Lens, Dmaxand D1%for Spinal cord were all improved in RapidArcplans compared with IMRT plans. The treatment delivery time and MUs were reduced by74.7%and62.5%comparing RapidArc with IMRT plan.According to the above results, we could obtain opinion that both RapidArc planand IMRT plan reached the dose distribution constrains for NPC radiation therapy. TheTarget HI and OARs sparing were improved in RapidArc plans. Also the treatmentdelivery efficiency was much improved in RapidArc plans.