Poster - Thurs Eve-31: Clinical implementation and experience with EPID-based precision isocentre localization

Area of Science:

• Medical Physics
• Radiation Oncology
• Radiotherapy Technology

Background:

• Modern linear accelerators utilize complex mechanical movements (gantry, collimator, table) defining multiple isocenters.
• Traditional film-based and manual isocenter localization methods are time-consuming and struggle to correlate individual motion impacts.

Purpose of the Study:

• To develop and implement an Electronic Portal Imaging Device (EPID)-based technique for precise isocenter localization.
• To automatically assess isocenter position and motion relative to the mean for each treatment unit axis.

Main Methods:

• Utilized a radio-opaque marker placed at the isocenter.
• Acquired multiple MV images to track the marker's projected position.
• Developed a MatLab code for automated image analysis and isocenter determination.

Main Results:

• Measurements from 18 machines across two vendors and clinics showed mean isocenters within a 2mm sphere.
• Identified instances where combined gantry, table, and collimator rotations resulted in treatment isocenters exceeding 2mm displacement.
• Observed minor shifts in gantry-isocenter location post-equipment upgrade.

Conclusions:

• The EPID-based technique provides efficient (<1 hour) and clinically relevant information on isocenter accuracy.
• This method is vital for monitoring treatment unit performance and ensuring radiotherapy precision.
• Automated isocenter analysis aids in quality assurance and identifying potential treatment deviations.