| dc.description.abstract | Quality assurance in radiotherapy involves a process of verification to ensure that each patient receives treatment as specifically prescribed by their radiation oncologist. Currently, various 3D quality assurance systems that are compatible with MRI technology are in use today, including ArcCHECK-MR and Delta4-MR. Additionally, PTW is introducing the OCTAVIUS 4D MR system. This research presents a basic characterization of the OCTAVIUS 4D MR system and a comparative analysis of three MR-compatible 3D QA devices for IMRT Plan QA: ArcCHECK-MR, Delta4-MR, and OCTAVIUS 4D MR with OCTAVIUS Detector (OD) 1500 MR. Goal. The primary goal of this research was to evaluate the performance of various 3D arrays for quality assurance in radiotherapy within a 1.5 T MR-linac and to identify any limitations these devices might have for patient-specific quality assurance. Before performing the patient quality assurance, a basic characterization of the OCTAVIUS 4D MR with OD 1500 MR and OD 1600 MR detector plates in a 1.5 T MR-linac was performed. 25 patients’ treatment plans across different target sizes and locations were measured and compared using ArcCHECK-MR, Delta4+ MR, and OCTAVIUS 4D MR with OD 1500 MR. A comparison of the standard clinical plans and plans containing monitor units (MU) and position er- rors was performed. The impact of device geometry on the results was explored by analyzing the gamma percentage difference, mean gamma index, and other metrics provided by the software.
Method. For the evaluation, gamma passrate and mean gamma index were obtained by 3D global gamma comparison analysis with 3%/3mm and 2%/2mm criteria. Nominal thresholds of 95% and 90%, respectively, were applied. For intentionally erroneous plans additional metrics were collected such as dose difference (median, mean) and distance to agreement where possible.
Results. The results of the basic characterization of the OD 1500 MR and OD 1600 MR demonstrated no significant differences in short-term reproducibility (< 0.2%), dose linearity (< 1%), field size dependency (< 0.7% for field sizes larger than 5 cm × 5 cm), dose rate dependency (< 0.8%), dose-per-pulse dependency (< 0.4%) and angular dependency (standard deviation < 0.5%).
In the analysis of plans with deliberately introduced errors, the Delta4+ MR showed higher variation in results, the ArcCHECK-MR showed greater error sensitivity. All systems were able to identify position errors that were out of tolerance level, however, bigger targets were less sensitive to misplacements. The gamma test was not particularly effective at detecting dose errors in any of the systems in which it was used, especially underdose in small target regions or highly modulated beams. The gamma test should be used in combination with a dose assessment to identify these kinds of inconsistencies.
Conclusions. The new MR compatible OCTAVIUS detectors and the OCTAVIUS 4D Phantom MR are suitable for QA of patient treatment plans in a 1.5T MRI-linac and for measurements with the offset.
All devices demonstrated their capability to produce good results for standard clinical plans. However, minor misalignment during the setup procedure could affect the accuracy of the analysis. The limited resolution and density of the detectors can impact the precision of results in plans with small high-dose regions. When investigating plans with introduced errors, gamma comparison analysis alone was revealed to be insufficient for MU error detection and should be combined with a dose assessment. Nevertheless, all devices showed a worsening in results for plans with deliberately introduced errors. The responses of the ArcCHECK-MR and Delta4+ MR were more sensitive but also had larger uncertainty. | |
