PO13: Development of a Risk-Based Workflow to Identify Dosimetric Relevance of Implanted Cardiac Devices (ICDs) for Brachytherapy Patients

Abstract

<h3>Objective</h3> To develop a risk-based workflow for assessing dosimetric relevance of ICDs (i.e pacemaker/defibrillators) in patients undergoing brachytherapy in our department. <h3>Materials and Methods</h3> In order to conservatively assess possible dose to ICD in clinical brachytherapy situations, worst-case scenarios were generated for brachytherapy procedures performed at our institution. Cases were organized by site and technique: Pelvic HDR [prostate HDR, gynecologic HDR, rectum HDR, pelvic intraoperative (IORT)], pelvic permanent implantations (prostate and other pelvic seed implantations with I-125 and Pd-103), upper abdomen/chestwall/H&N HDR (Esophagus HDR, upper body HDR IORT). Twenty patients were randomly selected among recently treated cases; prescription doses, treatment images (when available) and treatment plan dose distributions (TG43) were used to develop worst-case scenario geometries for each application. Patients did not have ICDs, and theoretical ICD locations were estimated assuming closest reasonable placement to the treatment site. We evaluated the data to set a policy to identify, in advance of treatment, situations in which ICD dose could exceed threshold of 2Gy, and we developed a risk-based workflow. <h3>Results</h3> For pelvis HDR cases, maximal total dose prescription was 28Gy and distance from treatment site to 2Gy threshold was 7cm. For pelvic permanent implantations, maximal total dose prescription was 144Gy using I-125, and distance from treatment site to 2Gy threshold was 5cm. Closest location of ICD to treatment site was estimated to be >>7cm in all pelvic cases, therefore ICD dose for pelvic cases was considered negligible. For upper abdomen/chest wall/H&N HDR cases, maximal total dose prescription was 15Gy and distance from treatment site to 2Gy threshold was 5cm. Worst-case scenario geometries for Esophagus HDR treatments, where applicator location was simulated to be close to the chest wall, predicts a distance to the ICD >5cm, with doses to ICD of 0.5-1Gy. For upper body IORT cases, due to clinical variability, applicator placement can be <5cm from the ICD, thus requiring physical measurements to confirm safety of treatment. Therefore, we proposed a risk-based workflow, taking into account (i) negligible radiation ICD risk for all pelvic cases, (ii) geometric/dosimetric assessment at time of planning for esophagus HDR cases and (iii) intra-operative assessment for IORT cases at or above upper abdomen. For cases (ii) and (iii) the physicist 2^nd checking the plan would review the electronic medical record prior to treatment to identify presence of ICD. For image-guided cases, the ICD should be contoured to evaluate distance from implant site, and dose calculated. For IORT cases, an intraoperative distance measurement from ICD to treatment area would be performed. If distance is ≤5cm, possibility of 2Gy to ICD must be acknowledged. The dose estimate should be recorded in specified form and signed by MD. Sterile lead shielding may be used, if feasible, internally to limit ICD dose. This risk-based workflow is represented in the figure. <h3>Conclusion</h3> We developed a risk-based workflow to evaluate ICD dose for brachytherapy patients. Development of this workflow could help identify patients at clinically relevant risk to ICD perturbation and conversely, reduce unnecessary interventions for negligible risk scenarios. This policy will need to be re-evaluated if new treatment modalities or fractionations are introduced. Dose rate effects were not considered (<<8Gy/min in all situations). Differences between TG43 and physical doses were not systematically explored.