Quantifying isocenter measurements to establish clinically meaningful thresholds.

Area of Science:

• Medical Physics
• Radiation Oncology
• Radiosurgery Technology

Background:

• High-precision stereotactic radiosurgery (SRS) demands stringent isocenter accuracy in linear accelerators.
• Understanding and quantifying sources of uncertainty in isocenter definition is critical for patient safety and treatment efficacy.

Purpose of the Study:

• To statistically analyze isocenter congruency verification tests to establish meaningful tolerance levels for SRS.
• To quantify individual contributors to isocenter uncertainty and determine clinically relevant action levels for linear accelerators.

Main Methods:

• Examined 149 isocenter congruency tests using a laser-defined isocenter and electronic portal imaging device (EPID).
• Quantified spatial corrections from radiation exposures under various gantry, collimator, and couch positions.
• Isolated uncertainties from daily setup errors, gantry sag, multileaf collimator (MLC) offset, and couch walkout.

Main Results:

• Variations in test tool positioning averaged 0.38 mm.
• Gantry sag contributed 0.4 mm, MLC offset 0.16 mm, and couch walkout 0.72 mm to isocenter uncertainty.
• Recommended a schedule action level of 1.25 mm and an immediate action level of 1.50 mm for daily isocenter congruency tests.

Conclusions:

• Established statistically derived, clinically meaningful action levels for isocenter congruency in SRS programs.
• These tolerance levels provide a numerical threshold specific to the linear accelerator, guiding prompt corrective actions by medical physicists.
• Ensures the integrity of isocenter alignment for safe and effective SRS and stereotactic body radiosurgery (SBRT) delivery.