Comparison and evaluation of VMAT and IMRT for the treatment of initial treated nasopharyngeal carcinoma

Objective To compare planning quality,treatment efficiency and delivery accuracy for initial treated nasopharyngeal carcinoma (NPC) with step & shoot intensity modulated radiation therapy (ssIMRT) and volumetric modulated arc therapy (VMAT).These results will help make a clinic choice of the therapeutical technique.Methods Twenty-six NPC cases were planned with the same dose prescription and objective constrains by means of 9-field ss-IMRT and VMAT respectively.Compare:( 1 ) plan dosimetric distribution,conformity index and homogeneity index of the targets,the average dose,maximum dose and interested dose-volume histograms of organs at risk (OAR) et al;(2) delivery times of the therapy plans ;(3) the accuracy of treatment plans dose verification.Results Both therapeutical plans can achieve the clinic dosimetric demands.Compared to ss-IMRT,VMAT had less inferior target coverage.The CI and HI of the PGTV was 0.57 and 0.08 ( ss-IMRT),0.48 and 0.12 (VMAT) respectively ( t =-4.52,- 8.33,P =0.000,0.000).Except of brain stem,VMAT had higher mean dose and maximum dose of OARs than ss-IMRT (t=-9.57 - -3.71,P=0.000 -0.001).The spinal cord D1cc and parotids D50％ were increased by 11.9％ and 6.5％ averagely.The treatment times of ss-IMRT and VMAT were 803.7 s and 389.3 s respectively (t =24.12,P =0.000),while V MAT decreasing by 51.6％.The pass ratios of γ (3mm,3％ ) from the dose verification were 99.4％ (ss-IMRT) and 98.0％ (VMAT) respectively ( t =5.19,P =0.000).Conclusions The dose distribution of VMAT for initial treated nasopharyngeal carcinoma can achieve the clinic demands,but slightly worse than 9-field ss-IMRT.VMAT has the advantage of high efficiency and dosimetric accuracy. Key words: Nasopharyngeal neoplasms/radiotherapy; Radiotherapy,step and shoot intensity modulated; Radiotherapy,volumetric modulated arc; Plan evaluation