| Objective:To assess the feasibility and potential advantages ofvolumetric modulated arc therapy (VMAT) for Esophageal Carcinoma bycomparing the dosimetric differences between VMAT and static intensitymodulated radiotherapy (IMRT).Methods:Thirty patients with potentially radical radiotherapy andbiopsy-proven esophageal cancer were selected in this study from September2011to December2012, including5cases located in the cervical,5cases thelower thorax,10cases the upper thorax and10cases the middle thorax. Fixedby a heat plastic device, each patient was recruited to conduct with CTsimulation in the same treatment position. All images were transmitted to thetreatment planning system by local area network. VMAT plans with a singlearc (starting angle of179°, ending angle of181°) and IMRT plans with fivefields were designed for each of30patients by using the Elekta Oncentra4.1three-dimensional treatment planning system. PTVs were prescribed to60Gyin30fractions. Dose volume histograms were used to compare the dosedistribution on target volumes and organs at risk. The monitor units (MU) andtreatment time (TT) were also evaluated to measure the treatment efficiency.Elekta Synergy linear accelerator was used to verify and deliver the treatmentplans. With three-dimensional dose verification system Delta4verified thedosimetric of treatment plans and analyzed the results using the3mm/3%Gamma evaluation criteria. To evaluate the inter-fraction and intra-fractionsetup errors during the treatment with accelerator airborne kilovoltage conebeam computer tomography (CBCT), nine cases with VMAT technology weretaken CBCT images which were registered to the planning CT images beforeand after radiotherapy. In any direction if the setup error have exceeded athreshold of3mm, and then corrected the translation setup error online, this improved the accuracy of positioning. It provided theoretical basis forplanning target volume by analyzing the setup errors distribution law.Statistical analysis was performed using an SPSS statistical package (Version13.0).Results:（1）All the VMAT and IMRT plans can satisfy the clinicaldosimetry requirements. With the similar target coveragement, VMAT planshad better conformal index (CI) for PTV than IMRT plans (P=0.008). For thehomogeneity index (HI) of PTV, VMAT plans were slightly better than IMRTplans (0.768vs.0.742, P>0.05). Comparing with IMRT plans, the low-doseregion(V5, V10) of VMAT plans for the normal tissue outside the targetarea(BODY-PTV) were significantly higher, while the V20was significantlylower（P<0.05）. There were no significant dosimetric differences betweenVMAT and IMRT about the lung. VMAT plans were better than IMRT plansby reducing the Dmax of the spinal cord（4229.07cGy vs.4317.2cGy,P=0.032）.(2)For the cervical esophageal cancer, there were significantdifferences in terms of D98（5877.05cGy vs.5860.54cGy） and D50（6222.46cGy vs.6270.19cGy）of PTV between VMAT and IMRT plans(P<0.05). VMAT plans increased the values of V5~V30and MLD of lung.The V5, V10, V15and MLD of the lung were significant higher in VMATplans (P<0.05).(3) For the upper thorax esophageal cancer, there were nosignificant dosimetric differences between IMRT and VMAT in PTV and lung.The values of heart V30and Dmean were significantly larger in VMAT thanthose in IMRT（P<0.05）. However, comparing with IMRT the Dmax of spinalcord was lower in VMAT（4228.40cGy vs.4361.10cGy, P=0.006）.(4) For themiddle thorax esophageal cancer, the distribution of the dose between the twotechniques was similar. VMAT reduced V10, V15, V20of the lung (P<0.05),while elevated D2, V40of the spinal cord (P<0.05) and V30, V40, Dmean ofthe heart (P>0.05).(5) For the lower thorax esophageal cancer, there were nosignificant dosimetric differences between IMRT and IMAT in PTV and OARs.VMAT plans had the tend to reduce lung V20and V30（P>0.05）.(6) The dosedistribution results for normal tissue outside the target area (BODY-PTV) of different lesion locations：For cervical and upper thorax esophageal cancer,comparing with IMRT, the low-dose region V5, V10of VMAT were increasedby5.05%to5.11%and3.92%to6.41%resbectively（P≤0.016）. V20ofVMAT for upper thorax esophageal cancer was decreased by0.73%（z=-2.90,P=0.037）. For middle and lower thorax esophageal cancer, VMAT increasedlow-dose region V5, V10, but decreased V20. The differences were notstatistically significant（P>0.05）.(7) The3mm/3%average Gamma pass ratewas92.75%for VMAT and92.98%for IMRT（P=0.826）.(8)The average MUof VMAT （460.66MU） was reduced by11.8%compared with IMRT（522.55MU）,（t=-3.87, P=0.001）. The effective treatment delivery time ofVMAT（139.6s）compared with IMRT（298.73s）was reduced by an averageof53.27%（t=-16.943, P=0.000）.(9) Before correction, the setup errors inX-axis, Y-axis, Z-axis were (0.387±0.357) cm,(0.568±0.418) cm,(0.360±0.250) cm respectively, in which the error of the head and footdirection (Y-axis) greater than5mm was up to47.78%. Setup errors aftertreatment: X-axis (0.09±0.09) cm, Y-axis (0.12±0.1) cm, Z-axis (0.11±0.1) cm,in all directions more than96.5%of the setup errors were≤0.3cm.Conclusions: Compared with IMRT, VMAT slightly improved the OARsdose sparing and the target CI, HI with similar dose distribution to the target.VMAT can be used as an effective and safe radiotherapy technology in thetreatment of esophageal carcinoma.VMAT and IMRT can be applied todifferent locations esophageal cancer. VMAT required fewer MU, shorten thetreatment time significantly, improved the efficiency of the implementation ofthe plan, reduced the uncertain factors during the treatment and thediscomfortableness among patients, had the potential to improve the curativeeffect. VMAT and IMRT plans had the similar Gamma pass rates,3mm/3%ofthe average Gamma pass rates>90%. CBCT can improve the accuracy ofpositioning, the precision of radiation therapy and providea basis for delineation of PTV in esophageal cancer. VMAT can be used as an effectiveand safe radiotherapy technology in the treatment of esophageal carcinoma. |
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