The Target Volume Dosimetric Comparison Between Two Different Split IMRT Planning Methods (Synchronous Vs.Composite) In Nasopharyngeal Carcinoma

Objective:To compare the dosimetric difference of intensity-modulated radiotherapy (IMRT) treatment plan for nasopharyngeal carcinoma (NPC) with two different design methods in the second stage.Methods:Twelve patients with NPC were selected. The Intensity-modulated radiotherapy plans were designed for every one, Two methods were used in the optimization process of second stage, Mixed with the result of the first stage for the first method (synchronous Plan) and Without the influence of the first stage for the second one (composite Plan). With the same prescription dose requirement for target volumes and limit value for organs at risk (OAR) in the whole plan for the two plans, dosimetric parameters of targets and OARs were evaluated.Results:There were almost the same DVH curve in the whole plan with two design methods. But for the second stage plan, The percentage of prescription dose delivered to the PTVnx、PTVnd and PTV1was significantly decrease (95.49%;56.38%, t=11.94, P=0.000;95.35%:46.33%, t=10.64, P=0.000;94.90%:66.70%, t=11.78, P=0.000). The homogeneity indexes of PTVnx、 PTVnd�'�PTV’(defined as the region of PTV1subtracted by PTVnx and PTVnd) in the composite plan were much better than in the synchronous plan (0.07:0.21, t=-12.63, P=0.000;0.06:0.23, t=-8.33, P=0.000;0.13:0.38, t=-12.70, P=0.000). The conformity indexes of PTV1and PTVnx+nd (defined as the region of PTVnx add PTVnd) were also much better in the composite plan (0.78:0.44, t=12.60, P=0.000;0.78:0.27,t=14.28, P=0.000), The monitor unites were much more in the synchronous Plan (852.50:1564.08, t=-11.40, P=0.000).Conclusions:Comparing with the synchronous plan there were more sufficient coverage of prescription dose and with better homogeneity indexes and conformity indexes for PTV in the second stage for the composite plan. The latter required less monitor unite.