Evaluation of Needle Displacement during Prostate HDR Brachytherapy Implant and Treatment
Abstract
Purpose
Prostate and perineum edema are known issues for prostate brachytherapy. It could significantly impact the dosimetry for prostate HDR implants with needle displacement since there is a 2-3 hour interval between implant and treatment. The purpose of this project is to determine the patterns of needle displacement and determine practical clinical workflows to minimize this effect and deliver the most accurate treatment plan.
Methods
13 patients were proactively evaluated during their HDR prostate treatment. Each patient received three CT images: CT1, CT2 and CT3. CT1 was taken right after the implant. CT2 was taken after the plan approval, prior to treatment. Evaluation of needle displacements was performed by rigidly registering the prostate fiducial markers on CT1 and CT2 and measuring the difference in location of the first dwell positions. If the displacements between CT1 and CT2 were greater than 5mm, we replanned using CT2 as new reference image. CT3 was taken immediately after the patient received treatment.
Results
Five out of 13 patients required replanning after CT2 evaluation. Average time difference between CT1 and CT2 were 79±3min; CT3 and CT2 were 72±5min (non-replan), and 130±13min (replan). The average first dwell displacements between CT2 and CT1 were 4.3mm (non-replan), and 9.0mm (replan); between CT3 and CT2 were 3.8mm. A saturating hyperbola model was used to estimate peak displacement rate. Since our typical sim-to-treat time was less than 2 hours, we looked for a time where the rate of displacement is less than 2mm/hour for time interval of 2-3hours, which was predicted to be 100mins.
Conclusion
Our data showed that post-implant needle displacement is an unavoidable issue. However, since the displacement rate is not linear and largest in the first 100min after implant, delaying planning scan by at least an hour may be an optimal workflow to increase delivered dose accuracy.