EXACTRAC x-ray and beam isocenters-what's the difference?

Area of Science:

• Medical Physics
• Radiation Oncology
• Image-Guided Therapy

Background:

• Accurate patient positioning is critical in radiation therapy.
• Image-guidance systems (IGS) are essential for precise tumor targeting.
• The Brainlab Exactrac system is widely used for image guidance.

Purpose of the Study:

• To assess the geometric accuracy of the Exactrac system's isocenter relative to the linear accelerator's radiation isocenter.
• To develop and apply a method for correcting discrepancies between these isocenters.

Main Methods:

• Five Varian linear accelerators with Exactrac robotic systems were evaluated.
• A Winston-Lutz phantom and in-house software were used for deviation analysis.
• MV-images and stereoscopic X-ray images were acquired to measure isocenter deviations.

Main Results:

• 3D deviations between the Exactrac and linac isocenters ranged from 0.31 to 1.07 mm.
• A novel calibration method reduced a 0.90 mm deviation to 0.23 mm on one linac.
• Routine linac maintenance complicated initial measurements.

Conclusions:

• Isocenter deviations between the linac and Exactrac systems can be clinically significant.
• The proposed calibration method effectively reduces these geometric discrepancies.