The role of electronic portal imaging in tangential breast irradiation: a prospective study.

Side effects due to irradiation of normal tissues and local failure can be associated with deviations in the patient positioning in radiation therapy. In particular, tangential breast irradiation may include normal lung tissue or even a small portion of the heart in the field. A prospective study was performed to assess the precision and the reproducibility 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 with the intended treatment. Twenty patients with early breast cancer receiving post-operative radiotherapy were entered in the study. Geometrical parameters were measured from daily on-line portal images taken for approximately 17 fractions of each tangential fields. Multiple images were also acquired (six per field) for six fractions for all patients, yielding a total of 2120 images including the simulator films. Random and systematic errors were obtained. Variations of the parameters between 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 observed in rare cases. This confirms the need to implement daily verification 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.