Impact of magnetic resonance imaging-related geometric distortion of dose distribution in fractionated stereotactic radiotherapy in patients with brain metastases

Purpose The geometric distortion related to magnetic resonance (MR) imaging in a diagnostic radiology (MR DR ) and radiotherapy (MR RT ) setup is evaluated, and the dosimetric impact of MR distortion on fractionated stereotactic radiotherapy (FSRT) in patients with brain metastases is simulated. Materials and methods An anthropomorphic skull phantom was scanned using a 1.5‑T MR scanner, and the magnitude of MR distortion was calculated with (MR DR -DC and MR RT -DC) and without (MR DR -nDC and MR RT -nDC) distortion-correction algorithms. Automated noncoplanar volumetric modulated arc therapy (HyperArc, HA; Varian Medical Systems, Palo Alto, CA, USA) plans were generated for 53 patients with 186 brain metastases. The MR distortion at each gross tumor volume (GTV) was calculated using the distance between the center of the GTV and the MR image isocenter (MIC) and the quadratic regression curve derived from the phantom study (MR RT -DC and MR RT -nDC). Subsequently, the radiation isocenter of the HA plans was shifted according to the MR distortion at each GTV (HA DC and HA nDC ). Results The median MR distortions were approximately 0.1 mm when the distance from the MIC was 60 mm (0.23, 0.47, 0.37, and 0.57 mm in MR DR -DC, MR DR -nDC, MR RT -DC, and MR RT -nDC, respectively). The dose to the 98% of the GTV volume (D 98% ) decreased as the distance from the MIC increased. In the HA DC plans, the relative dose difference of D 98% was less than 5% when the GTV was located within 70 mm from the MIC, whereas the underdose of GTV exceeded 5% when it was 48 mm (−26.5% at maximum) away from the MIC in the HA nDC plans. Conclusion Use of a distortion-correction algorithm in the studied MR diagnoses is essential, and the dosimetric impact of MR distortion is not negligible, particularly for tumors located far away from the MIC.