Serial MR Imaging Post Trigeminal Radiosurgery - Identifying Radiosurgical Shot Accuracy in Euclidean Space

Abstract

and intrafraction imaging. Maximum tolerated misalignment prior to treatment initiation was sub-mm & sub-degree. Following initial setup , 6 intrafraction CBCT's were acquired (1) Pretreatment localization scan (2) post-residual errors correction pretreatment check scan (verification) (3) four Intrafraction scans, each scan after delivery of every 4000 mu. For intrafraction motion analysis, CBCT data scans of same patients with more than 0.5 0 & 0.5mm mm vector shifts was considered. Free form Intensity based deformation algorithm was used to deform structures, dose & plan with intrafraction CBCT scan which revealed > 0.5 0 , o.5mm vector shifts. Intermodality and intramodality deformation errors were quality assured using registration refine tools prior to data collection. Clinical relevance was measured as the Target percent underdosaged & max dose increase to OAR. In brainstem the volume doses D 1% , D 2% were taken for analyzing the higher doses in the brain stem. In addition, max temporal doses and mean cochlear doses analyzed. The p-values were calculated using two tailed Student's Ttest and tabulated in appropriate tables. Statistical comparison was done using SPSS software using paired t test. Results: Mean rotational errors observed between planning CT and CBCT were in Pitch (meane0.4 , SD AE 0.372) , Yaw (meane0.370, SD AE 0.22) and roll (meane0.380, SD AE 0.383) respectively. The mean 3D intrafraction shift was 0.26 mm (SD,0.26 mm) and The max brainstem voxel dose D 1% variation was 3% over planned (range:2-5%).The temporal lobe and cochlear doses were unaffected. A loss of prescribed isodose coverage of 3% (SD, 0.08) was found without 6DOF re-positioning. Conclusion: Using CBCT DIR for intrafraction motion analysis, 0.5 mm vector translation error and 0.5 degree rotation error about each axis should be used as the maximum tolerated misalignment before initiation of frameless functional SRS treatment. We observed that patient compliance is important as few patients fall asleep and move involuntarily during the procedure.