Assessment of Brain Tumor Displacements after Skullbased Registration: A CT/MRI Fusion Study

Purpose: To assess brain tumor displacements between skull based and soft-tissue based matching during CT-MRI fusion for a total of 35 brain lesions. Methods: Twenty-five patients who underwent CT and MRI scans in the same day were retrospectively recruited into the study. Semi-automatic skull based fusion was first performed and reviewed. A secondary fine-tuning of the fusion was performed, if mismatch was observed in the tumor or neighboring soft-tissues. Two physicists fine-tuned the secondary fusion until the best match could be agreed upon. The resulting rotations and translations after fine-tuning indicated local displacements between the two fusions. We further created a PTV to evaluate the coverage of the GTV after soft-tissue based fusion. Results: In 29 of the 35 lesions, minor to no mismatch was found between the soft-tissue and skull based fusions. The translational and rotational shifts were 0.05±0.63 mm (LR), 0.01±0.79 mm (AP), 0.37±1.01 mm (SI); -0.15±0.67° (pitch), -0.19±0.34° (yaw), and -0.12±0.49° (roll). In the remaining 6 lesions, noticeable displacements were observed between the two fusions. For the outlier lesion, the GTV was nearly missed by the PTV, and for the rest of the 5 lesions, the mean coverage of the GTV was 98.9%. Conclusion: In a small portion of lesions, our study showed noticeable brain tumor displacement with typical patient setup in CT and MRI scans between skull based and soft-tissue based fusion. Careful review of skull based fusion is suggested and adding a margin to the GTV is recommended, if fusion deviations are found.