The Study Of Setup Errors For Postoperative Cervical Cancer In Image-guided Radiotherapy

Objective:Using KV-cone beam CT(CBCT) of Linear accelerator to study the setup errors in patients with postoperative cervical carcinoma in image-guided radiotherapy process.Methods:Treat13postoperative patients with cervical cancer by IGRT system of Varian Linear accelerator, to acquire their image data by means of on board imager before radiation. Collect all of the110times of CBCT images, and then compare them with planned CT images to obtain the translational errors of X-axis, Y-axis, Z-axis and rotational errors of R-axis. Finally, calculate the safety margins of PTV refer to CTV in three translational directions.Results:(1) The systemic errors on X, Y, Z, R directions were-0.861mm、0.287mm、0.102mm、0.282°, the random errors on X,Y, Z, R directions were3.543mm、7.809mm、2.717mm、1.096°.The times(%) of the setup errors on translational X, Y, Z directions being within5mm were92.647%、89.706%.97.059%, the times(%) of the setup errors on rotational R direction being within3°were94.118%.(2) According to the formula MPTV=2.5∑+0.7δ, the margins between the CTV and PTV should be5.0mm in X (left-right) direction,9.7mm in Y (cranio-caudal) direction and5.4mm in Z (anterior-posterior) direction, so that can include setup errors of90%. Considering the rotational variations, the margin should be expanded in cranio-caudal direction of the target.Conclusion:(1) Setup errors in the left-right direction(X)、cranial-caudal direction(Y)、anterior-posterior direction(Z) and rotational direction(R) are detected.(2) The systemic±random errors on X,Y,Z,R directions were-0.861±3.543mm;0.287±7.809mm;0.102±2.717mm;0.282±1.096°.(3) According our result, the margins between the CTV and PTV should be5.0mm in X (left-right) direction,9.7mm in Y (cranio-caudal) direction and5.4mm in Z (anterior-posterior) direction, so that can include setup errors of90%. Considering the rotational variations, the margin should be expanded in cranio-caudal direction of the target.