CHANGES IN HIGH-DOSE-RATE TANDEM AND OVOID APPLICATOR POSITIONS DURING TREATMENT IN AN UNFIXED BRACHYTHERAPY SYSTEM

PURPOSE: To measure the change in applicator position during treatment in an unfixed high-dose-rate (HDR) brachytherapy system and to evalua te the effect of the shifts on dose calculations. MATERIALS AND METHOD S: Posttreatment localization radiographs were obtained for 47 HDR tre atments (26 tandem and ovoid applicators, 21 ovoids-only applicators). The authors measured the change in applicator position relative to th e patient's bone anatomy. Doses to the target and critical structures were calculated for posttreatment applicator positions for comparison. RESULTS: Average displacements of the tandem and ovoid applicators in anteroposterior dimension were 5 and 4 mm for the tandem and ovoids, respectively. Anterior displacement occurred twice as often as posteri or displacement. The average lateral and longitudinal shifts were less . Less displacement was observed with ovoids-only insertions. The larg est displacement for ovoids-only applicators was 3 mm in the anteropos terior dimension. A high bladder dose difference (17.4%) for tandem an d ovoid applications correlated with anterior shifts of the applicator s. CONCLUSION: Patient movement in an unfixed HDR brachytherapy system can displace the applicators, especially the tandem. Anterior shifts correlate with high bladder dose differences. Immobilization of the pa tient's hips and legs, as well as stabilization of applicators, would reduce these shifts.