Distortion of magnetic resonance images used in gamma knife radiosurgery treatment planning: Implications for acoustic neuroma outcomes

Objective: To quantify the image distortion of our series of acoustic neuromas treated with gamma knife radiosurgery. Study Design: Retrospective chart and digital radiographic file review with quantitative assessment of gamma knife treatment plans. Setting: Tertiary referral center. Patients: Patients undergoing gamma knife radiosurgery for the treatment of acoustic neuromas. Intervention: Gamma knife radiosurgery. Main Outcome Measures: Gamma knife treatment plans containing magnetic resonance images were reviewed at each axial, sagittal, and coronal slice. The length of the greatest displacement of the treatment plan was measured and the volume of the treatment plan that fell outside of the internal auditory canal calculated. Known clinical measurements of audiometric, vestibular, facial, and trigeminal nerve functions were then compared with current measurements of tumor size. Results: Twenty-two of the 23 patients had measurable image shifts on the axial images. The range of the image shift was 0 to 5.8 mm, with a mean shift of 1.92 +/- 1.29 mm (+/- standard deviation). Tumor volumes of the treatment plan that fell outside of the internal auditory canal ranged from 0 to 414 min 3, with a mean of 90.5 mm(3). The mean percentage that fell outside of the internal auditory canal was 16.7% of total tumor volume (range, 2.4-77.6%). We could not draw any consistent correlations between degree of image shift and continued tumor growth or objective examination values. Conclusion: We have demonstrated a small but potentially significant shift in the treatment plan of gamma knife radiosurgery when based on magnetic resonance images. Although the image shift does not seem to affect the growth of the acoustic neuromas or auditory or facial nerve function, longer term follow-up is required to fully appreciate the true impact of this image shift.