Retrospective Study Of Setup Errors On CBCT-based Image Guided Radiation Therapy For Retcal Cancer

This study mainly focuses on evaluating setup errors of image guided radiation therapy(IGRT) for rectal cancer with belly-board device using kilovolt cone beam CT( KVCBCT). In the study, cancer patients were positioned with thermoplastic body membrane. The movement of clinical target volume(CTV) and set-up deviation were assessed. Using the data of movement and set-up deviation, we can calculate the radiotherapy treatment target margin to provide some basis for clinical doctors.A retrospective study has been carried out for 66 patients of rectal cancer were randomly selected. All patients were emptying bladder, and then drinking 1000 ml water.One hour later, CT simulation scan was performed for patients immobilized with belly-board device and thermoplastic body membrane in prone position. The CT images were transferred to a pinnacle workstation for exacting of target region and treatment planning. Before irradiation, a KVCBCT scan was performed and image registration was done via ELEKTA synergy. Each set of setup errors in right-left(RL),superior-inferior(SI), and anterior-posterior(AP) directions were gathered. An independent-samples t-test and paired-samples t-test statistical analysis was conducted with the 330 sets of data using SPSS 16.0.The statistical analysis showed that setup errors from the 758 datasets were depicted a Gaussian distribution. The system errors ± random errors in RL, SI and AP were(-0.148±0.2441) mm,(0.1196±0.4228) mm and(-0.137 ±0.276) mm, respectively.Referring to the formula for planning target volume margin calculation, M =2.5? + 0.7?,the margins were calculated as 0.7813 mm,1.335 mm,0.8085 mm, respectively.The margins derived from this retrospective study have confirmed the premise that the treatment plans were executed in patients with high reliability, thereby created a high sense of confidence for the clinicians.