REPRODUCIBILITY OF CONFORMAL RADIATION-THERAPY IN LOCALIZED CARCINOMAOF THE PROSTATE WITHOUT RIGID IMMOBILIZATION

The reproducibility of 3-dimensional (3D) conformal therapy in localiz ed prostate cancer was studied in 14 patients, based on retrospective analysis of 196 anteroposterior-posteroanterior (AP/PA) and lateral po rtal images. The patients were treated supine without rigid immobiliza tion using six isocentric coplanar conformal fields. Three different o bservers independently compared the portal and simulation images, dete rmining the deviation of each portal film from the corresponding simul ation film. No significant deviations were observed in the cephalo-cau dal or lateral axes (0 mm median values). However, a systematic median shift of 5 mm (0 to +10, range) was observed in the anteroposterior d irection, presumably as a consequence of a sagging in the treatment co uch under the patient's weight. After modification of the treatment co uch, no further systematic anteroposterior shifts have been observed. These results demonstrate that the daily setup of conformal prostate i rradiation fields can be performed with acceptable reproducibility wit hout the use of special immobilization devices.