Mechanical and radiation isocenter coincidence: an experience in linear accelerator alignment

Area of Science:

• Medical Physics
• Radiation Oncology

Background:

• Accurate isocenter alignment is critical for effective radiation therapy.
• Linear accelerator (linac) commissioning involves precise calibration of multiple isocenters.

Purpose of the Study:

• To investigate a 4mm discrepancy observed between the mechanical and radiation isocenters of a newly commissioned linac.
• To determine the cause and clinical significance of isocenter noncoincidence.

Main Methods:

• Alignment of linac laser lines with optical and radiation isocenters.
• Use of a mechanical checkout jig to assess isocenter coincidence.
• Extensive measurements using standard and custom-designed devices.

Main Results:

• A 4mm discrepancy was detected when the gantry rotated from 0 to 180 degrees.
• The mechanical isocenter was within the 1mm tolerance, but noncoincidence with the radiation isocenter caused the clinical error.
• Measurements confirmed the observed discrepancy and its origin.

Conclusions:

• The noncoincidence of mechanical and radiation isocenters, despite the mechanical isocenter being within tolerance, leads to a clinically significant 4mm discrepancy.
• Current commissioning procedures may not adequately detect such subtle isocenter errors.
• Recommendations are provided for future linac commissioning to ensure accurate radiation delivery.