Object To analysis the results replacement repeatability using real time portal imaging system

     Method From March 2000 to December 2004, 395 patients for at least 5 fractions per patients in 2 orthogonal beams with the aid of electronic portal imaging device (EPID), 790 portal films treated patients were compare with 742 orientation films both longitudinally and transversely and then were analyzed about its replacement repeatability respectively. Setup corrections were applied based on off-line decision protocol, with parameters derived from knowledge of random setup errors in the studied patients group.

    Result Random setup errors in head and neck replacement repeatability were within 4mm in every directions. For patients of breast cancer, chest tangential field irradiation with both arms uplift were within 4mm, but only arm uplift set-up error were over 6mm. For patiens of lung cancer ,In thorax irradiation, set-up error were within 8mm, 16mm and 11mm for lateral, the anterior-posterior, and superior-inferior directions, respectively. Random setup errors in pelvis irradiation in the lateral anterior-posterior, and longitudinal were within 6, 8, 11mm respectively. Conclusion The daily use of portal imaging system can help us not only avoid the random mistakes in positioning, it also helps us detect and correct them. The replacement repeatability is thus improved by using the fixation equipment.

* correspondence author