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We assessed the effect of beam size on plan robustness for intensity-modulated proton therapy (IMPT) of head and neck cancer (HNC) and compared the plan quality including robustness with that of intensity-modulated radiation therapy (IMRT). IMPT plans were generated for six HNC patients using six beam sizes (air-sigma 3–17 mm at isocenter for a 70–230 MeV) and two optimization methods for planning target volume-based non-robust optimization (NRO) and clinical target volume (CTV)-based robust optimization (RO). Worst-case dosimetric parameters and plan robustness for CTV and organs-at-risk (OARs) were assessed under different scenarios, assuming a ± 1–5 mm setup error and a ± 3% range error. Statistical comparisons of NRO-IMPT, RO-IMPT and IMRT plans were performed. In regard to CTV-D99%, RO-IMPT with smaller beam size was more robust than RO-IMPT with larger beam sizes, whereas NRO-IMPT showed the opposite (P  < 0.05). There was no significant difference in the robustness of the CTV-D99% and CTV-D95% between RO-IMPT and IMRT. The worst-case CTV coverage of IMRT (±5 mm/3%) for all patients was 96.0% ± 1.4% (D99%) and 97.9% ± 0.3% (D95%). For four out of six patients, the worst-case CTV-D95% for RO-IMPT (±1–5 mm/3%) were higher than those for IMRT. Compared with IMRT, RO-IMPT with smaller beam sizes achieved lower worst-case doses to OARs. In HNC treatment, utilizing smaller beam sizes in RO-IMPT improves plan robustness compared to larger beam sizes, achieving comparable target robustness and lower worst-case OARs doses compared to IMRT. [ABSTRACT FROM AUTHOR]

Background Purpose Methods Results Conclusions 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.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.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.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.The XCT‐based gel dosimeter enabled verification of three‐dimensional source dwell positions for a seamless workflow of IGBT with high precision and efficiency. [ABSTRACT FROM AUTHOR]

Background: Coincidence of the treatment and imaging isocenter coordinates is required to safely perform small‐margin treatments, such as stereotactic radiosurgery of multiple brain metastases. A comprehensive and direct methodology for verifying concordance of kilovoltage cone‐beam computed tomography (kV‐CBCT) and treatment coordinates using an x‐ray CT‐based polymer gel dosimeter (dGEL) and onboard kV‐CBCT was previously reported. Using this methodology, we tested the ability of a new commercially available x‐ray CT‐based polymer dGEL with a rapid response to provide efficient quality assurance (QA). Purpose: The aim of this study was to evaluate the robustness of the three‐dimensional geometric QA methodology using dGEL. Methods: The dGEL were commercially manufactured. The prescribed dose for each field was determined by visually identifying the 5, 10, and 20 Gy isodose lines. A linear accelerator was used to irradiate the gels with seven non‐coplanar beams. An in‐house analysis program was used to identify the beam axes and treatment isocenter in kV‐CBCT coordinates by processing the pre‐ and post‐irradiation CBCT images. The impact of the radiation dose on the test reproducibility was examined, and the detectability of an intentional geometric error was assessed. Results: The treatment isocenter was within 0.4 mm of the imaging isocenter for all radiation doses. The residual error of the test with the intentional error was within 0.2 mm. The analysis and image quality variations for a single dGEL introduced displacement errors less than 0.3 mm. Conclusions: The test assessed the coincidence of treatment and kV‐CBCT isocenter coordinates and detected errors with high robustness. Even for a 10 Gy dose, the test yielded results comparable with those obtained using higher radiation doses owing to the rapid response of the dGEL dosimeter. [ABSTRACT FROM AUTHOR]