Impact of iterative Metal Artifact Reduction (iMAR) on External Beam Radiotherapy: A Phantom Sttudy
Keywords:
Iterative Metallic artifact reduction (iMAR), Hounsfield unit correction, Dose calculation accuracyAbstract
Background: Metallic implants introduce significant artifacts in computed tomography (CT) images, leading to Hounsfield Unit (HU) inaccuracies and subsequent errors in external beam radiotherapy dose calculations.
Objective: This study evaluated the impact of iterative metal artifact reduction (iMAR) on CT image quality, electron density representation, and dosimetric accuracy using a phantom-based model.
Materials & Methods: Initial CT simulator performance was validated against AAPM Task Group 66 standards using a Catphan® 600 phantom. To assess HU recovery, carbon steel rods were inserted into a CIRS Gammex phantom in various configurations to simulate clinical scenarios (e.g., spinal lesions, prosthetic hips). Images were reconstructed with and without the iMAR algorithm across five smoothness levels. Dosimetric impact was evaluated using a CIRS pelvic phantom and the Eclipse treatment planning system. Dose distributions for 6 MV and 10 MV photon beams were calculated using the Grid-Based Boltzmann Solver (GBBS) algorithm. Accuracy was quantified using point dose comparisons and gamma index analysis.
Results: CT quality assurance confirmed all parameters were within clinical tolerances. In the presence of metal, uncorrected images exhibited severe streaking and photon starvation. However, iMAR significantly restored HU fidelity and anatomical visualization, particularly in complex multi-metal geometries. Dosimetrically, iMAR-corrected datasets yielded point doses closer to the reference data, reducing the percentage dose difference (from 0.66% to 0.44% at 6 MV and 1.82% to 0.64% at 10 MV). While both corrected and uncorrected plans passed the 3%/3 mm gamma criteria (>99%), the stricter 2%/2 mm criteria revealed a substantial decline in uncorrected passing rates (~87–89%). The application of iMAR restored these passing rates to >97% and significantly reduced standard deviation, indicating improved procedural consistency.
Conclusion: The iMAR algorithm effectively mitigates metal-induced artifacts and restores CT number accuracy. Its implementation is critical for high-precision EBRT planning, as it ensures dose calculation stability and superior gamma passing rates under stringent clinical criteria.
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