If changes in prostate brachytherapy HDR catheter position occur— and are not detected and corrected—it will compromise treatment.

Purpose/Objective Safe high-quality HDR brachytherapy relies on the treatment source dwell-position accurately matching the intended anatomic position. Movement of implanted catheters relative to the anatomy might affect the relative source dwell positions, in turn compromising dose to the target, and increasing dose to surrounding tissue. We developed an image-guided treatment-verification system to assure treated source location. The system uses a flat panel detector embedded in the treatment couch for source tracking, and a ceiling-mounted x-ray system for imaging and registration. Patients are normally rescanned if catheter displacement values exceed our clinical threshold. By taking movements measured immediately prior to intended treatment—and calculating the dosimetry of what would have been delivered should the displacement be uncorrected—we might model the consequences of the counterfactual. That is: what would have happened if not corrected, allowing evaluation of the potential impact on patients. Material/methods Ten sequential patients were selected for dosimetric evaluation of implanted catheter displacement. Patients were implanted with plastic catheters and planned using a CT scan. Immediately before treatment, we x-rayed the patients and measured the absolute displacement of the catheters (relative to the prostate) in relation to the catheter positions identified at CT. We applied the average catheter displacement measured to the CT based treatment plan for each patient and re-calculated the changes for prostate, urethra and rectum metrics currently used in the clinic. Statistically significant differences were assessed using non-parametric Gardner-Altman estimation plots of the calculated differences, with statistically significant differences defined as the probability of a 2-sided Kruskal test result less than 0.05. Results The range of catheter displacement measured was +2.6 - -15.4mm (negative displacement represents an caudal catheter shift relative to the prostate). A summary of dosimetric changes are shown in Figure 1. The unpaired median difference between planned prostate and shifted D90% is -10.2 [95.0%CI -29.7, -0.435] (2P=0.0343), and for the V100 was -8.65 [95.0%CI -21.0, -1.0] (2P=0.00406). For the planned and shifted urethra D10%, planned urethra and shifted D10% the median difference was 7.22 [95.0%CI -0.03, 11.5], 2P= 0.0343, and for the rectal D2cc% the median difference was 3.89 [95.0%CI -3.86, 13.8], 2P= 0.29. Conclusion Undetected catheter displacement could result in the delivery of a poor-quality treatment, especially for urethral doses and target doses. Deviations from planned dosimetry may be associated with poorer patient outcomes. We believe pre-treatment verification imaging of catheter location is extremely important to ensure correct treatment delivery.

Sarah Graves

Alfred Health Radiation Oncology
Radiation Therapist

Sarah Graves is a Radiation Therapist at the Alfred Health Radiation Oncology department in Melbourne. She undertook her training at the centre in 2014 and has been an active member of the brachytherapy team for the past 3 years. Sarah enjoys the aspect of working within a strong multi-disciplinary team in Brachy and hopes to further expand her knowledge in the future.