Objective To compare the dosimetric differences between volumetric modulated arc therapy ( VMAT ) and static intensity modulated radiotherapy ( IMRT) for different tumor volume and different lesion length of esophageal cancer. Meth-ods The Elekta Oncentra4. 1 Planning System was adopted to design both VMAT and IMRT plans for fifty patients who were diag-nosed with thoracic esophageal cancer. All treatment plans of the 50 cases were evaluated using the dose-volume histogram parame-ters of PTV and the organs at risk. The monitor units (MUs) were Also examined. Results ①For GTV volume<15cm3 group, VMAT plan had superior homogeneity when compared with IMRT plan. For 15~55cm 3 group, PTV D98 for VMAT got closer to prescription dose. However, VMAT plan had lower V100 in the PTV, compared to IMRT plan for GTV volume>55cm 3 group. ②For lesion length<5cm group, VMAT plan led to a superior homogeneity, whereas conformity index was better in IMRT plan. For 5~9cm group, VMAT plan resulted in a slightly lower D90 and higher V95、V90,with superior conformity in the PTV. For lesion length>9cm group,VMAT plan achieved lower D90 and V100, whereas HI and CI were not significantly different.③For GTV vol-ume<15cm 3 group and lesion length<5cm group, lung V5 and MLD were slightly higher for VMAT plan. For 15~55cm 3 group and lesion length>9cm group, VMAT plan resulted in a slightly lower lung V10 and higher lung V30. For GTV volume>55cm 3 group,VMAT plan resulted in a lower lung V10. For 5~9cm group,sparing of lungs showed no statistically significant differences between the two techniques.④The Dmean of heart in VMAT plan was not statistically different in comparison with IMRT plan for GTV volume>55cm 3 group and lesion length>9cm group, but higher than in IMRT for GTV volume<55cm 3 group and lesion length<9cm group. It was no significant difference in VMAT over IMRT for Dmax to the spinal cord.⑤When compared with IM-RT plan, VMAT plan reduced the monitor units by an average of 11. 3% and 18. 1% in the 15 ~55cm 3 group and >55cm 3 group, and by an average of 16. 1% and 16. 0% in the 5~9cm group and >9cm group. However, VMAT plan provided an aver-age of 10. 4% and 14. 4% more monitor units than IMRT plan in the volume<15 cm 3 group and lesion length<5cm group. Con-clusion On the premise of similar sparing of OARs, VMAT plan, which provides superior target volume coverage in comparison with IMRT plan, is the first choice for a small thoracic esophageal target volume. In addition, VMAT plan provides equivalent conformal dose coverage and sparing of OARs for the medium thoracic esophageal target volume with less delivery time. So it is suggested that VMAT is the preferred one. However, IMRT is preferred for a large thoracic esophageal target volume, since VMAT plan has not superior target volume coverage and even increases the spring of OARs.%目的：比较容积旋转调强( VMAT)与静态调强( IMRT)在不同肿瘤体积及病变长度食管癌中的剂量学差异及优缺点,并评价各自应用的合理性。方法用医科达Oncentra4．1治疗计划系统,为50例胸段食管癌患者分别设计单弧VMAT、静态IMRT计划。根据肿瘤体积及病变长度进行分组,分别比较两种计划的靶区、危及器官受量及总机器跳数(MUs)。结果①GTV体积<15cm3组 VMAT HI更好。15~55cm3组,VMAT PTVD98更接近处方剂量。>55cm3组, VMAT PTVV100低于IMRT。②GTV长度<5cm组,VMAT HI优于IMRT,而CI IMRT更有优势。5~9cm组,VMAT D90低于IMRT,而V95、V90高于IMRT,CI更优。>9cm组,VMAT D90、V100低于IMRT,HI和CI无明显差异。③体积<15cm 3和长度<5cm组,VMAT肺V5、MLD高于IMRT；体积15~55cm 3和长度>9cm组,VMAT肺V10降低,V30增高；体积>55cm 3组,VMAT肺V10低于IMRT；长度5~9cm组,两种计划差异无统计学意义。④GTV体积<55cm 3和长度<9cm组VMAT心脏Dmean高于IMRT,而体积>55cm 3和长度>9cm组两种计划心脏受量相似。脊髓Dmax两种计划比较差异无统计学意义。⑤在机器调数( MU)方面GTV体积15~55cm 3和>55cm 3组及GTV长度5~9cm和>9cm组,VMAT的MU较IMRT分别减少11．3%、18．1%、16．1%和16．0%。而体积<15cm3和长度<5cm组,VMAT的MU则分别增加了10．4%和14．4%。结论在危及器官受量相似的前提下,对于小靶区食管癌VMAT的肿瘤靶区剂量分布优于IMRT,建议首选VMAT。而中等靶区其剂量分布及危及器官受量两种计划相似,但VMAT可明显缩短治疗时间,仍建议首选VMAT。对于大靶区食管癌在剂量学方面VMAT并不占优势,甚至会增加危及器官受量,建议首选IMRT计划。
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