Changes in high-dose-rate tandem and ovoid applicator positions during treatment in an unfixed brachytherapy system.

Objective: To measure the change in applicator position during treatment in an unfixed high-dose-rate (HDR) brachytherapy system and to evaluate the effect of the shifts on dose calculations.

Methods: Posttreatment localization radiographs were obtained for 47 HDR treatments (26 tandem and ovoid applicators, 21 ovoids-only applicators). The authors measured the change in applicator position relative to the 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 posterior displacement. The average lateral and longitudinal shifts were less. Less displacement was observed with ovoids-only insertions. The largest displacement for ovoids-only applicators was 3 mm in the anteroposterior dimension. A high bladder dose difference (17.4%) for tandem and ovoid applications correlated with anterior shifts of the applicators.

Conclusions: 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 patient's hips and legs, as well as stabilization of applicators, would reduce these shifts.