The Discussion About KV-CBCT Monitoring The Set-up Error And Realizing The Feasibility Of Adaptive Radiotherapy In Esophageal Cancer Patients

PurposeThe set-up error and the residual set-up error are monitored on esophageal patientswith three-dimensional conformal radiotherapy through the weekly KV-CBCT, forproviding the basis for reasonable esophageal CTV-PTV margin in our department. Thetreatment plan is updated through the KV-CBCT image to evaluate the feasibility andadvantage of off-line adaptive radiotherapy, and establish an esophageal on-line adaptiveradiotherapy model.Materials and methodsSection oneWe select19cases of esophageal cancer patients. Each patient takes CBCT imageevery week, the images in the corresponding3D match box are got to be automatic greyregistration with the positioning CT images, if error of any direction more than3mmthreshold, then accept a online placement reset. After that, we do again KV-CBCTscanning, and follow the same way for images registration, the set-up error we collected isthe error after correction in3D direction. Then analysis data offline and calculatetheoretical CTV-PTV margin according to van Herk formula.Section twoPretreatment: Copy the planning CT images together with target, use the method of bulkdensity assignment, give the Lung tissue CT value is:-800Hu, the rest are defined as theorganization of the density of water, that is, Body-Lung CT value:0Hu,Transplantate CTplan (plan1) to the density distribution CT2images, named plan2. According to the targetand normal tissue dose and DVH charts, we compare the dose distribution of differences ofplan1and plan2. Main evaluating indexes: the average dose of target (GTV, CTV, PTV) and95%volume dose coverage (D95); average dose of spinal cord and1%volumequantity (D1); Lung average dose (MLD) and V20.Adaptive: On the basis of the first part conclusion and preprocessing conclusion, wedelineate the target and the organs at risk in the first week of CBCT image of the19patients, creat a new CTV-PTV margin, called adaptive plan: planA, and compare with theclinical plan: planC in dose distribution. Evaluation parameters are: PTV volume,95%volume PTV coverage (PTVD95),99%volume GTV coverage (GTVD99), spinal cordD1cc, total lung V20, total lung V30. When analyzing the off-line KV-CBCT images, wecalculate the required time of online updating treatment plan with Varian OBI system.ResultsSection oneBefore correction, the set-up error of107CBCT images in Lat, Vrt, Lng direction is0.39±0.31cm,0.24±0.23cm,0.28±0.22cm respectively, altogether79times (74%) needto accept a online correction. According to van Herk formula, the no correction CTV-PTVmargin is:1.00cm,0.91cm,0.96cm respectively. After correction, the set-up error of107CBCT images is:0.17±0.13cm,0.16±0.12cm,0.14±0.14cm respectively. According tovan Herk formula, the no correction CTV-PTV margin is:0.50cm,0.46cm,0.49cmrespectively.Section twoThe plan with bulk density assignment and the conventional plan show very goodconsistency. The dose difference of target about plan1and plan2is within3%, of normaltissue is less than2%. Compare with clinical treatment plan, adaptive plan have noeffection on target coverage, the difference in95%PTV coverage,99%GTV coverage wasnot statistically significant. Total lung V20and total lung V30are obviously shrinks,respectively V20:18.26%±2.98%vs.12.14%±2.89%, V30:9.19%±3.45%vs.5.71%±2.63%, there is a significant statistical difference(P=0.0001, P=0.001). In1cc dose ofspinal cord, adaptive plan is cutted by228cGy±127.75cGy, the difference is statisticallysignificant (P=0.04). A model of online updating treatment plan is builded up, frompositioning to transporting the updated plan, Varian OBI system totally need to take about 15minutes.ConclusionsThe set-up error can be reduced and the CTV-PTV margin can be narrowed throughon-line KV-CBCT guided radiotherapy. After the correction of set-up error, we can use thebulk density assignment method, making the KV-CBCT images to participate in replanning.In the premise of not affect the target coverage, the first week of offline adaptive plan canavailably protect the organs at risk. Varian OBI system totally need to take about15minutesfor online updating the plan, theoretically up to the online ART standard, however,implementing online ART is far away to be reached.