| It is the time of precision radiotherapy,and the accuracy and repeatability of patient set-up could significantly influence treatment outcome.The following factors are related with set-up errors: changes in patient's body shape,changes in tumor shape and volume,changes in size,shape and position of organ at risk,the experience of radiotherapy technicians,and et al.We used cone-beam computed tomography(CBCT)scanner to obtain patients CBCT images before radiotherapy,and then the CBCT images were registered with planned CT images to analyze and calibrate set-up errors.This method could improve tumor local control rate and reduce radiotherapy toxicities.Furthermore,we analyzed and calculated external margins from clinical tumor volume(CTV)to planning tumor volume(PTV),which provided evidence for target contouring.This study used CBCT scanner to obtain CBCT images before radiotherapy by Elekta Synergy linear accelerator.100 patients with head and neck cancer and 50 patients with thoracic and abdominal tumors were included in our study.We recorded translational errors on left-right(X axis),superior-inferior(Y axis)and anterior-posterior(Z axis)directions.Then CTV to PTV margins on above three directions were calculated based on the following formula: M(PTV)=2.5?+0.7?.The set-up errors on X,Y,Z axis directions were 0.379±1.255 mm,-0.185±1.835 mm,0.691±1.680 mm in head and neck cancer patients and were 0.526±1.958 mm,0.742±3.726 mm,0.448±2.470 mm in patients with thoracic and abdominal tumors,respectively.Based on following formula: M(PTV)=2.5?+0.7?,the CTV to PTV margins on X,Y,Z axis directions were 1.826 mm,0.822 mm,2.903 mm in patients with head and neck cancer,and were 2.686 mm,4.463 mm,2.849 mm in patients with thoracic and abdominal tumors,respectively.Based on analyzing set-up errors data from Hunan Cancer Hospital,our results provided a reference for CTV to PTV margins and maybe useful in reducing set-up errors and improve radiotherapy outcome. |
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