Dosimteric Impact of Distortion Correction of MR Image Set for Cranial Stereotactic Radiation Treatments
Purpose: For cranial stereotactic radiation therapy, magnetic resonance imaging (MR) is used to contour targets. Any geometric distortion in MR image sets can creates uncertainty in stereotactic delivery of the treatment. Brainlab has released a novel MR distortion correction module based on CT image set information. In this study we evaluated the consequent impact of Brainlab MR distortion correction module on target dose.
Methods: A retrospective analysis of MR distortion correction on previously treated twenty cranial targets with seven single and thirteen multi met targets was done. All stereotactic cases were done with high resolution MRI and CT protocols. Fusion and contouring was done by the same physician. In Brainlab elements, cranial distortion correction was applied to the MR image set based on original planning MR-CT fusion. To reduce user variability, the same physician recontoured the targets on corrected MR image set. The original treatment plans were evaluated for coverage of recontoured gross tumor volumes (GTV), planning target volumes (PTV) and inverse Paddick Conformity indices (iCI).
Results: The median distortion displacement of targets center of mass was 1.12 mm (max 2.57 mm). Eighteen out of twenty GTVs achieved 99.5% (p<0.05) or more coverage by prescription dose because of planning margins (1 mm to single targets and 0.5 mm – 2 mm margins to multi-met targets). Minimum dose to GTVs relative to prescription dose reduced to 94% or less for seven targets. Sixteen planning target volumes fail to meet the criterion of 95% of volume covered by prescription dose. The average iCI of corrected PTVs after the distortion corrected increased by 0.24 (p<0.01), indicating reduced conformity.
Conclusion: This study indicates that MR distortion in certain cases can cause geometric miss of SRS targets. Brainlab’s MR distortion correction module can improve MR-CT fusion and therefore treatment accuracy.
Sandhu R, Knill C, Seymour Z, Grills IS, Chinnaiyan P. Dosimetric impact of distortion correction of MR image set for cranial stereotactic radiation treatments. Poster and Abstract. American Association of Physicists in Medicine Virtual Meeting, July 2020