Effect of setup error on dose distribution in stereotactic radiotherapy for lung cancer
Objective To analyze the effect of cone beam CT on setup error and radiation dose distribution in stereotactic body radiotherapy(SBRT)of lung cancer.Methods Twenty-eight patients with SBRT pulmonary malignant tumor underwent CBCT scanning before each radiotherapy.The CBCT images were matched with the planning CT images to obtain the setup errors in the left-right(X axis),superior-inferior(Y axis),and anterior-posterior(Z axis).The margins of clinical target volume(CTV)expanded to planning target volume(PTV)was calculated.The actual dosimetric distribution in the target areas and in the organs at risk(OARs)were estimated by inputting the errors into Eclipes treatment planning system for simulating the actual radiation if the isocenter was replaced by the actual location which obtained from the CBCT scan.The effects of the errors in radi-ation dose distribution were assessed by comparing with the errors in the simulation and original plan.Results A total of 222 CBCT scans were obtained in 28 patients.The setup errors of the left-right(X axis),superior-inferior(Y axis),and anterior-posterior(Z axis)were(2.00±0.90),(2.05±1.00)and(1.70±0.70)mm,respectively.The margin of CTV of lung cancer in three directions(X,Y,Z)were 7.60 mm,9.14 mm,5.41 mm,respectively.In the simulation test without shifting the couch,D98％and D2％of the target area plan were(84.6±77.0)％and(100.9±100.0)％,and the difference was statis-tically significant(P＜0.05).The dose Homogeneity index(HI)and Conformal index(CI)of the original plan were signifi cantly better than the plans without couch moving(P＜0.05).There was no significant difference between the simulated un-moved couch plan and the original plan(P＞0.05).In the simulation plan without shifting the couch,the cardiac Dmean and spi-nal DMax were(99.0±95.0)％and(122.4±98.0)％respectively,and the differences were statistically significant(P＜0.05).Conclusion CBCT can effectively reduce the setup errors of SBRT in lung cancer to avoid exposure to organs at risk and prevent insufficient exposure to target area.
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