Volumetric Modulated Arc Therapy for Nasopharyngeal Carcinoma: A Dosimetric Comparison with Step-and-shoot IMRT

【Objective】 To compare volume modulated arc therapy(VMAT) with step-and-shoot intensity-modulated radiation therapy(IMRT) for nasopharyngeal carcinoma(NPC) patients with regard to the sparing effect on dosimetric quality and efficiency of delivery.【Methods】 Fifty-two patients who were diagnosed with NPC and were treated with VMAT as their primary treatment,and had no distant metastasis were included in this study.The target area received three dose levels(70,60,and 56 Gy) in 33 fractions using simultaneous integrated boosts technique.For a fair comparison,The Elekta Monaco Planning System was adopted to design both VMAT and IMRT plans.Dosimetric comparisons between VMAT and IMRT plans were analyzed to evaluate(1) coverage,homogeneity,and conformity of PTV,(2) sparing of OARs,(3) delivery time,and(4) monitor units(MUs).【Results】 The VMAT and IMRT plans had similar PTV coverage.IMRT plans provided a better conformity index(CI)(VMAT vs.s-IMRT,1.09 ± 0.02 vs.1.07 ± 0.02;P 0.001)and homogeneity index(HI)(VMAT vs.s-IMRT,0.48 ± 0.15 vs.0.54 ± 0.13;P 0.001) than VMAT.When compared with IMRT,VMAT a had a better sparing effect on brain stem(VMAT vs.s-IMRT,57.99 ± 6.43 vs.58.22 ± 5.78;P 0.001),and worse on spinal cord,parotid and larynx(P 0.05).But the effect of other sparing was similar between VMAT and IMRT.The delivery time per fraction for VMAT(7.0 min) was much lower than for IMRT(13 min,P 0.01).【Conclusions】 Our results indicate that VMAT provides similar dose coverage of PTVs and sparing of normal tissue to IMRT,and shorter delivery time than IMRT.