SU-E-J-56: Localization Error in Gamma Radiosurgery with 3T MR Due to Fiducial Box Chemical Shift from Incorrect...

Area of Science:

• Medical Physics
• Radiotherapy Physics
• Medical Imaging

Background:

• Gamma radiosurgery (GSR) relies on accurate target localization using MRI fiducial boxes.
• Incorrect filling materials in fiducial boxes can lead to chemical shift artifacts in MRI.
• These artifacts may cause significant geometric errors in treatment planning.

Purpose of the Study:

• To evaluate localization and positioning errors in GSR due to incorrect liquid filling material (baby oil) in MRI fiducial boxes.
• To assess the utility of a 3D MAGIC polymer gel dosimeter for quality assurance in GSR.

Main Methods:

• Two phantoms (grid and skull) were used with 'baby oil' fiducial markers and a MAGIC gel capsule simulating an intracranial target.
• Phantoms underwent stereotactic MR and CT scanning for GSR treatment planning.
• 3D dose distribution was read out using the MAGIC gel in a 3T MRI scanner; geometric errors were compared to direct measurements.

Main Results:

• A 3.8mm shift in the phase encoding direction was observed for the 'baby oil' fiducial marker in 3T MRI-based GSR planning.
• A 3.7mm shift in Teflon rod position was confirmed by comparing CT and MR images.
• These findings indicate significant geometric inaccuracies caused by the incorrect filling material.

Conclusions:

• Inappropriate filling materials in GSR MRI fiducial boxes can cause substantial target localization and positioning errors.
• The study demonstrates that MAGIC polymer gel is a valuable tool for 3D quality assurance in GSR.
• Accurate material selection for fiducial boxes is critical for safe and effective radiosurgery.