Setup Errors Of Imrt For Head And Neck Cancers Measured And Analyzed By Kilo-volt Cone-beam Computed Tomography

Objective To measure the setup error of the intensity modulated radiation therapy for head and neck cancers by cone-beam computed tomography, and to analyze the effect of the setup error on the physics dose of targets and peripheral organs at risk of the intensity modulated radiation therapy.Methods 15 head and neck cancer patients were treated with the Varian 21EX IGRT system, all the patients had received kilovoltage cone-beam computed tomography (KV-CBCT) weekly before correct the setup error and after radiotherapy separately for a total of 204 times.The acquired two groups of X-ray volume image(XVI) were compared to planned CT images with calculated setup errors of X-axis, Y-axis and Z-axis both of translational and Z-axis rotational errors. Then analyze distribution of setup error and the changes of setup errors before correction and after radiotherapy. Ten nasopharyngeal carcinom patients were simulated three-dimensional errors in therapy on the treatment planning system,and recalculated their dose distribution again, and obtained the impact of the setup erors on the targets and organs at risk physics dose.Results All the 15 head and neck cancer patients received a total of 204 CBCT scans, the acquired CBCT images had a high quality clearly showing soft tissue organs, especially the tumor location, size and the surrounding structures. In less than 3mm were 514 scans(83.99%), and in less than 5.00 mm were 597 scans(94.61%) in all of setup errors. There was no greater than 3°rotational errors. The setup errors on translational X, Y, Z directions befoer the correction were (-0.21±1.8)mm,(-0.24±2.10)mm, (0.36±1.92)mm; after radiotherapy were (-0.09±0.87)mm, (0.18±0.95)mm, (0.05±0.91)mm. The Z-axis rotational errors befoer the correction and after radiotherapy were (0.37±1.35)°, ( 0.18±0.29)°. Most of setup errors after radiotherapy (including translational and rotational errors) less than befoer the correction obviously,having statistics signification. The CTV to PTV margins of X-axis, Y-axis, Z-axis and PORV were 3.80 mm, 4.20 mm, 3.50mm, 0.3~3.5mm, respectively. The translational errors less than 3mm effected the targets and organs at risk dose unobviously.Conclusion Setup errors for head and neck cancer patients were good repeatability, and minor.Non-everyday CBCT scans can reduce the systematic error and random error, improve the accuracy in a certain extent and reduce the additional dose as possible.