Use of cone-beam imaging to correct for catheter displacement in high dose-rate prostate brachytherapy

Abstract

Purpose To determine the magnitude of catheter displacement between time of planning and time of treatment delivery for patients undergoing high dose-rate (HDR) brachytherapy, the dosimetric impact of catheter displacement, and the ability to improve dosimetry by catheter readjustment. Methods and Materials Twenty consecutive patients receiving single fraction HDR brachytherapy underwent kilovoltage cone-beam CT in the treatment room before treatment. If catheter displacement was apparent, catheters were adjusted and imaging repeated. Both sets of kilovoltage cone-beam CT image sets were coregistered off-line with the CT data set used for planning with rigid fusion of anatomy based on implanted fiducials. Catheter displacement was measured on both sets of images and dosimetry calculated. Results Mean internal displacement of catheters was 11 mm. This would have resulted in a decrease in mean volume receiving 100% of prescription dose ( V 100) from the planned 97.6% to 77.3% ( p < 0.001), a decrease of the mean dose to 90% of the prostate ( D 90 ) from 110.5% to 72.9% ( p < 0.001), and increase in dose to 10% of urethra (urethra D 10 ) from 118% to 125% ( p = 0.0094). Each 1 cm of catheter displacement resulted in a 20% decrease in V 100 and 36% decrease in D 90 . Catheter readjustment resulted in a final treated mean V 100 of 90.2% and D 90 of 97.4%, both less than planned. Mean urethra D 10 remained higher at126% ( p = 0.0324). Conclusions Significantly, internal displacement of HDR catheters commonly occurs between time of CT planning and treatment delivery, even when only a single fraction is used. The adverse effects on dosimetry can be partly corrected by readjustment of catheter position.