A Comparison Of Patient Position Displacements From Optical Surface Imaging And Cone Beam CT Registrations For Radiotherapy

Background and purpose:Optical surface image has many attractive features as a mean used in radiotherapy for patient positioning tasks such as set-up,monitoring and gating.But the application of it is limited by the relativity of the movement of surface and the deep tumor.In our study,we compare set-up displacements derived from optical surface imaging system to displacements derived from cone beam computed tomography(CBCT)system,and then we quantify the accuracy and applicability of optical surface imaging system based on the stand of CBCT.Material and methods:The ten groups of known displacements are moved by the solid model and the displacement is detected by the optical surface imaging system,and then calculated system error of the system in detecting displacements.Displacements from optical surface imaging system and CBCT system registrations were compared for 8 patients treated in the head and neck region and 9 patients in abdomen fixed in vacuum pad and 16 patients in abdomen fixed in thermoplastic film for a total of 524 set-ups.Surface data captured by optical surface imaging system at the first treatment fraction was used as main reference for the optical surface imaging system,while bony structures from the planning CT were used as a reference for the CBCT method.In addition to the first treatment,the two groups of images were registered with the reference image to get the positioning error of the two system,and then analyze the accuracy of the optical surface imaging system quantitatively based on the results of CBCT registration.Results:The system error of the optical surface imaging system is less than 0.5mm in three directions.The mean differences(1SD)between the optical surface imaging system and CBCT displacements were-0.29(± 1.12)mm?-0.34(±1.05)mm and 0.39(± 1.06mm)for head and neck region,0.35(±3.16)mm?0.19(±4.41)mm and 0.10(±3.14)mm for abdomen fixed by vacuum pad,0.17(±2.20)mm?1.05(±5.15)mm and 0.27(±2.57)mm for abdomen fixed by thermoplastic film in the lateral,longitudinal and vertical directions,respectively.Conclusions:Compared with CBCT the surface of optical imaging system for radiotherapy guided placement in the head and neck region is accurate,but in the region of abdomen were less accurate.So we can use the optical surface imaging system together with CBCT to reduce the additional radiation brought by CBCT and can carry out real-time monitoring of the treatment process.