Three‐dimensional source position verification in image‐guided high‐dose‐rate brachytherapy using an XCT‐based gel dosimeter

Background Comprehensive quality assurance (QA) for a seamless workflow of high‐dose‐rate brachytherapy, from imaging to planning and irradiation, is uncommon, and QA of the source dwell position is performed in one‐ or two‐dimensions. Gel dosimetry using magnetic resonance imaging (MRI) is effective in verifying the three‐dimensional distribution of doses for image‐guided brachytherapy (IGBT). However, MRI scanners are not readily accessible, and MRI scanning is time‐consuming. Nevertheless, X‐ray computed tomography (XCT) is available for IGBT planning. Purpose In this study, we designed and developed an efficient method for QA for a seamless workflow of IGBT with a new commercially available XCT‐based polymer gel dosimeter. Methods To enable direct insertion of brachytherapy applicators, the gelatinizing agent of the dosimeter was modified. A cylindrical polyvinyl chloride jar was filled with the modified gel dosimeter, which was subsequently used to determine the reproducibility of source dwell positions, detectability of source positional errors from intentionally introduced catheter length offsets, effect of looped source transfer tubes on the average displacement, extent of inter‐observer variation, and gel robustness following multiple needle‐insertions. Three ProGuide sharp needles were inserted into the jar. The dwell time at each point was determined to identify the irradiated volume with a diameter of approximately 10 mm on XCT images. All the times were the same. The plan was delivered using an afterloader with an Ir‐192 radioactive source, and the irradiated gel dosimeter was scanned using an XCT scanner. The subtracted images were generated from pre‐ and post‐irradiated images. Volumes with incremented Hounsfield units were manually identified and contoured. The centroid of the volume was defined as the measured source dwell position. Subsequently, planned source dwell positions were extracted from the DICOM file of the plan. Finally, the source dwell positions in plan and irradiated gel were compared in three axes. Results The hardness of the dosimeter was 1250% greater than that of the previously reported gel dosimeter. Source dwell positions were visually identified in the XCT image. Testing of CT acquisition, planning, irradiation, and analysis was completed in approximately 1 h. In the reproducibility test of source dwell positions, created by inserting three needles (each with three source dwell positions), the average displacements of the source positions from the first source dwell position were within 0.5 mm in all three directions. In the detectability test, displacements were less than 1 mm in the x – y plane but greater than 1 mm in the z ‐axis, which was the source path direction. When errors of 1–3 mm were intentionally introduced, the measured displacement was within 0.7 mm of the median (range: 0.21–1.65 mm) of intentional errors. When the transfer tube was looped, the source dwell position displaced by approximately 1 mm. After 20 needle‐insertions, the source dwell position displacement was within 1 mm. The maximum inter‐observer variation of contouring was 0.57 mm. Conclusions The XCT‐based gel dosimeter enabled verification of three‐dimensional source dwell positions for a seamless workflow of IGBT with high precision and efficiency.