SU-E-J-49: Reliability Study of ExacTrac- System Image Isocenter Using an On-Demand QA Procedure

Area of Science:

• Medical Physics
• Radiation Oncology
• Quality Assurance

Background:

• Accurate patient positioning is critical for effective radiation therapy.
• Image-guided radiation therapy (IGRT) relies on precise alignment between imaging systems and treatment beams.
• ExacTrac® is a widely used system for image-guided patient positioning.

Purpose of the Study:

• To develop a rapid quality assurance (QA) procedure to verify ExacTrac® image isocenter congruence with linear accelerator (linac) radiation isocenter.
• To assess the accuracy and reliability of the ExacTrac® image isocenter using the developed QA method.

Main Methods:

• A pointer-based system using a tungsten ball was employed for alignment.
• ExacTrac® software calculated the image isocenter relative to the ball using kV images.
• Linac radiation isocenter was determined via portal images at four gantry angles.
• In-house software analyzed ball center and radiation center for deviation calculation.

Main Results:

• A simple QA procedure was established, executable in under 20 minutes.
• The procedure confirmed ExacTrac® image isocenter accuracy against the linac radiation isocenter.
• Mean position errors were -0.27±0.30 mm (lateral), 0.52±0.24 mm (vertical), and 0.18±0.22 mm (longitudinal).
• Maximum deviations observed were 0.86 mm (lateral), 0.89 mm (vertical), and 0.54 mm (longitudinal).

Conclusions:

• Weekly QA data demonstrated high reliability of the ExacTrac® image isocenter with sub-millimeter accuracy.
• This imaging alignment precision is essential for image-guided stereotactic body radiation therapy (SBRT).
• The developed QA procedure is suitable for on-demand use to maintain ExacTrac® isocenter accuracy.