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Pre-Treatment Positional Shift Differences Between CBCT and Leksell Frame G MRI Fiducial Coordinates

J Stanford , J Treas*, A Youssef , J Sheehan , Holy Spirit Cancer Center - A Geisinger Affiliate, Mechanicsburg, PA


(Sunday, 7/29/2018) 3:00 PM - 6:00 PM

Room: Exhibit Hall

Purpose: Evaluate position differences for patients undergoing Gamma Knife (GK) treatment by comparing the proposed translational shifts between Leksell Frame G and GK Icon CBCT coordinates.

Methods: A total of 27 patients underwent GK treatments with G-Frame using the Gamma Knife Icon. Each patient received MR and was placed into Leksell coordinates with MR fiducial indicator box. A CBCT was performed on 22 patients prior to treatment. Using the GammaPlan (GP) TPS, the MR with frame G localization was set as the reference set and CBCT data set were registered thereto. Following registration, the GP suggested translational couch shifts in the X, Y and Z directions to reposition the couch/patient. All shifts were recorded and analyzed.

Results: Ignoring directional trends, 22 patients were demonstrated absolute mean (SD) shift displacements (mm) in the X, Y, Z planes of 0.21(0.19), 0.31(0.20) and 0.47 (0.30) respectively. Min and max shift ranges of 0.01/0.68, 0.02/0.78 and 0.08/1.12 for each plane were noted. All plane shifts were combined as an overall displacement with mean, minimum and maximum values of 0.68, 0.15 and 1.15 mm. Direction shifting trend was only noted in Y plane with modal values in X, Y and Z planes of 0.09, -0.33 and .12 respectively. One patient unknowingly bumbed their frame causing a small movement, the pre-treatment CBCT was performed with shift of 7.9mm. Frame was reset, patient reimaged, re-planned and another CBCT taken with an overall shift of 0.54mm. Original large shift was not included in the above analysis since the patient was not treated.

Conclusion: Pre-treatment CBCT shifts are reasonable, but may have greater significance for smaller targets. The potential safety benefit using CBCT with G-frame prior to treatment as a secondary check, may reduce medical event arising from geometrical miss; however further investigation in this area is warranted.


Gamma Knife, Immobilization, Patient Positioning


TH- External beam- photons: gammaknife

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