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Localization Error in Gamma Radiosurgery with 3T MR Due to Fiducial Box Chemical Shift From Incorrect Liquid Filling Materials

X Ding*, J Bourland, W Dolesh, Wake Forest Univ, WINSTON-SALEM, NC

SU-E-J-56 Sunday 3:00:00 PM - 6:00:00 PM Room: Exhibit Hall

Purpose: Use of a 3D MAGIC (Methacrylic and Ascorbic acid in Gelatin Initiated by Copper) polymer gel dosimeter to evaluate potential localization and positioning errors due to the use of incorrect liquid filling material (baby oil instead of copper sulfate) for a gamma radiosurgery (GSR) MRI fiducial box.

Method and Materials: Two phantoms were used to evaluate geometric errors due to chemical shift for 'baby oil' fiducial markers: 1) a cylindrical grid phantom with Teflon rod insert; 2) a human skull phantom with a MAGIC (Gelatin 9%; Methacrylic acid 4%; CuSO4 0.1mM; Ascorbic acid 2mM; Glucose 10%) gel capsule to simulate an intracranial target. Both phantoms were stereotactically MR and CT scanned for GSR treatment planning using a 'baby oil' GSR MRI fiducial box and a standard CT fiducial box. The gel capsule was given 15Gy @ 50% isodose line using the 4mm collimator based on 'baby oil' fiducial box MR images. Then, the skull phantom with capsule was scanned in 3T GE Signa® EXCITETM clinical scanner (Spin Echo pulse sequence) for 3D dose distribution read-out. Dose distribution errors were analyzed based on gel results for the combined MR and CT plans. For comparison, deviation of grid phantom teflon rods were directly measured from MR and CT stereotactic images.

Results: Gel dose profile measurements indicate a 3.8mm shift in the phase encoding direction for the 'baby oil' fiducial marker for 3T MRI based GSR treatment planning. A 3.7mm shift in Teflon rod position was also confirmed through direct comparison of CT and MR images.

Conclusion: Both results show significant target localization and positioning errors can be caused if inappropriate filling materials are used for GSR MRI fiducial boxes. This study also indicates that MAGIC polymer gel can be used as a versatile tool for GSR 3D quality assurance.

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