177 The accuracy of patient positioning in irradiation of rectum cancer

Abstract

Purpose Evaluation of correct positioning using an ionisation chamber on-line portal imaging system in routine clinical radiotherapy of rectum cancer. Methods and Materials In 13 patients with pelvic irradiation, portal images were obtained using a fast electronic megavoltage radiotherapy imaging system. A total of 208 portal images and 13 simulator films were used to determine the values of setup deviations in the X-Y-directions and the rotation of fields in a fixed coordinate system, and the accuracy of the manually positioned blocks. Results Mean standard deviations of the difference between simulation and treatment images were in mm: 7.33 and 7. 13 for X and Y, 3.55 for the rotation fault, and 4.55 for the position of blocks. Maximal variations of standard deviations were respectively 2.29–11.91; 2.39–7.83; 1.84–7.07; 1.17–10.09. Conclusion The errors of field positions summed up to a mean of up to 11.91 mm in one patient. Maximal errors counted in single fields up to 30 mm. The mean error of manual block positioning was not acceptable. Thus, mechanically fixed blocks are now used after obtaining the results of this study. A daily control should be considered for difficult patients.