Room: Exhibit Hall | Forum 5
Purpose: MR-Linacs are becoming a rapidly utilized modality due to their ability to provide excellent soft tissue contrast for image guidance. The constant magnetic field provides challenges for performing patient-specific IMRT QA due to the effects of the field on the measurement tools. In this study, we investigated the Sun Nuclear ArcCHECK-MR (AC-MR) in absolute dose mode on a 0.35-T MR-Linac.
Methods: Dose linearity data and output factors were acquired with the AC-MR to verify its response in a 0.35T magnetic field. Additionally, simple open field geometries were delivered to the AC-MR to verify its ability to measure absolute dose compared to the TPS. Dose distributions for 95 patient plans for various treatment sites (such as pancreas and breast) were delivered to the AC-MR in absolute dose mode at various gamma passing rates. Additionally, off-axis dependencies were investigated for breast patient plans as the 70-cm MR-linac bore width limits large lateral shifts.
Results: For dose linearity, the ArcCHECK-MR has a maximum percent difference from the mean at 2MU of -1.61%. The output factors were within 2% of the TPS for field sizes down to 1.66cmx1.66cm. For a 9.98cmx9.98cm AP field, the CAX diode measures 0.04% higher than the TPS expected dose. For all 95 patients, the average gamma passing score for 3%/3mm, 3%/2mm, and 2%/2mm is 97.53%Â±1.97%, 93.34%Â±3.86%, and 89.27%Â±5.41%, respectively. For 15 breast patients, there is an r=0.849 correlation between gamma passing rate and target center to isocenter distance. When looking at 2%/2mm gamma passing rates with and without the inclinometer turned on, the passing rates are 86.43%Â±7.15% and 90.18%Â±7.38%, respectively.
Conclusion: This is the first study to look at absolute dose mode on the ArcCheck-MR in an MR-field across a wide range of patients and provides considerations that are needed when taking ArcCHECK-MR measurements in the 0.35-T MR-linac.