ELSEVIER 0360~3016(95)02034-9 l Technical Innovation and Notes A QUALITY CONTROL STUDY OF THE ACCURACY OF PATIENT POSITIONING IN IRRADIATION OF PELVIC FIELDS

: Determining and improving the accuracy of patient positioning in pelvic fields. ttiisz%, and Materials: Small pelvic fields were studied in 16 patients treated for urological cancers using a three-field isocentric technique. Large pelvic fields were studied in 17 gynecological cancer patients treated with anterior and posterior (AP-PA) parallel opposed fields. Quantitative analysis of 645 megavolt images and comparison to 82 simulation images were carried out. Results: Small pelvic fields: for the position of the patient in the field, standard deviations of the difference between simulation (SIM) and treatment (MV) images were 3.4 mm in the lateral direction, 5.3 mm in the cranio-caudal direction, and 4.8 mm in the ventro-dorsal diction. Alterations in the positioning technique were made and tested. Large pelvic fields: differences between simulation and treatment images for the position of the patient in the field were 4 mm [l standard deviation (SD)] in the lateral direction and 6.5 mm in the crania-caudal direction. A systematic shift of the treatment field in the cranial direction had occurred in the majority of patients. A positioning technique using laser lines and marking of the caudal field border was shown to be more accurate. Conclusion: Studies of positioning accuracy in routine irradiation techniques are needed to obtain data for definition of the margins for each treatment site at each institution. Random variations should be kept at a minimmn by monitoring and improving positioning techniques. Treatment verification by megavolt im