Assessment of MRI Geometric Distortion for Radiotherapy Planning
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Background
MRI is widely used in radiotherapy planning due to its high soft-tissue contrast, but geometric distortions can compromise target localization accuracy.
Methods
Two phantoms – a commercial anthropomorphic STEEV phantom and a custom-made phantom fabricated from ABS plastic – were scanned on MRI and CT. Distortion was measured via linear distances between periodic structures in a DICOM viewer, with CT as the reference. Dose modelling was performed to assess the effect of target displacement on D99% coverage.
Results
STEEV phantom showed ≤0.95 mm maximum distortion (≤0.2 mm within 30 mm from isocenter). The custom phantom showed 0.30 ± 0.05 mm in the central zone (within 50 mm) and up to 3.1 mm at the periphery (102 mm from center). For a 0.1 cm 3 target, a 1 mm shift reduced D99% from 95% to 82%; for a 5.2 cm 3 target the reduction was negligible.
Conclusions
Distortion increases with distance from isocenter and is clinically significant for small targets (<1.6 cm 3 ). The custom ABS phantom offers a cost-effective tool for routine quality assurance. The proposed method is accessible for resource-limited settings.
