An Analysis Of Setup Error With On Board Imager For Body Tumors Treated With Intensity Modulated Radiation Therapy

Objective:To analyze the setup errors by on board imager in body tumors treated with Intensity Modulated Radiotherapy for calculating safety margins of PTV refer to CTV.Methods: 47 patients with chest tumor and 13 patients with abdomino-pelvic tumor were treated with Varian-21EX Linear accelerator. All patients had received cone beam computed tomography (CBCT) before radiotherapy was delivered once a week.The acquired images of cone beam computed tomography were co-registered with planning CT and errors of isocenter positions on x,y,z axes and rotational deviations were obtained and analyzed.Results : The systemic±random errors on left-right(x) ,cranio-caudal(y) and anterior-posterior(z) directions were (-0.3194±3.6943) mm,(0.4851±6.2636) mm,(0.1701±4.4391) mm for the chest;(-0.4721±2.5821) mm,(0.8689±5.4938) mm,(0.1016±3.3285) mm for the abdominopelvis,respectively.The systemic±random errors on the rotational deviations(r) were (0.2080±1.4925)°for the chest and (-0.2398±1.6766)°for the abdominopelvis,respectively. Conclusion:For the patients with chest tumors undergoing intensity modulated radiation therapy,the margins between the CTV and PTV should be 7 mm in left-right direction,11 mm in cranio-caudal direction and 8 mm in anterior-posterior direction,that can include setup errors of 90%.For the patients with abdomino-pelvic tumors undergoing intensity modulated radiation therapy,the margins between the CTV and PTV should be 5 mm in left-right direction,10 mm in cranio-caudal direction and 6 mm in anterior-posterior direction,that can include setup errors of 90%.Considering the rotational variations,the margin should be expanded at the end of the target.