THE ROLE OF ELECTRONIC PORTAL IMAGING IN TANGENTIAL BREAST IRRADIATION - A PROSPECTIVE-STUDY

Side effects due to irradiation of normal tissues and local failure ca n be associated with deviations in the patient positioning in radiatio n therapy. In particular, tangential breast irradiation may include no rmal lung tissue or even a small portion of the heart in the field. A prospective study was performed to assess the precision and the reprod ucibility of the tangential breast irradiation technique with the help of on-line electronic portal imaging devices (EPID). The influence of respiration on the treatment set-up was evaluated. Also, a comparison was made with simulation films to study the degree of concordance wit h the intended treatment. Twenty patients with early breast cancer rec eiving post-operative radiotherapy were entered in the study. Geometri cal parameters were measured from daily on-line portal images taken fo r approximately 17 fractions of each tangential fields. Multiple image s were also acquired (six per field) for six fractions for all patient s, yielding a total of 2120 images including the simulator films. Rand om and systematic errors were obtained. Variations of the parameters b etween various fractions and within the same fraction were about 3 mm (1 SD) or less. Variation between simulation and treatment set-up was 4.3 mm or less. Large maximum deviations, reaching 22.9 mm, were obser ved in rare cases. This confirms the need to implement daily verificat ion procedures and to correct deviations in the treatment set-up. The study has shown that EPID can help reaching a high accuracy in patient treatment.