A QUALITY-CONTROL STUDY OF THE ACCURACY OF PATIENT POSITIONING IN IRRADIATION OF PELVIC FIELDS

Purpose: Determining and improving the accuracy of patient positioning in pelvic fields. Methods and Materials: Small pelvic fields were stu died in 16 patients treated for urological cancers using a three-field isocentric technique. Large pelvic fields were studied in 17 gynecolo gical cancer patients treated with anterior and posterior (AP-PA) para llel opposed fields. Quantitative analysis of 645 megavolt images and comparison to 82 simulation images were carried out. Results: Small pe lvic fields: for the position of the patient in the field, standard de viations of the difference between simulation (SIM) and treatment (MV) images were 3.4 mm in the lateral direction, 5.3 mm in the cranio-cau dal direction, and 4.8 mm in the ventro-dorsal direction. 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 [1 standard deviation (SD)] in the lateral direction and 6.5 mm in the cranio-caudal direction. A sys tematic shift of the treatment field in the cranial direction had occu rred in the majority of patients. A positioning technique using laser lines and marking of the caudal field border was shown to be more accu rate. Conclusion: Studies of positioning accuracy in routine irradiati on 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 minimum by monitoring and improving positioning technique s. Treatment verification by megavolt imaging or film should be used t o detect and correct systematic variations early in the treatment seri es.