The measurement of linear accelerator isocenter motion using a three-micrometer device and an adjustable pointer

Area of Science:

• Medical Physics
• Radiotherapy Technology
• Quality Assurance in Radiation Oncology

Background:

• Linear accelerators (LINACs) are crucial for external beam radiotherapy.
• Precise beam-target alignment is essential for effective and safe radiation delivery, especially in stereotactic radiosurgery (SRS).
• Gantry and treatment table motions can introduce positional uncertainties affecting treatment accuracy.

Purpose of the Study:

• To measure the small motions of major axes of a linear accelerator during gantry and treatment table rotation.
• To improve beam-target alignment during stereotactic radiosurgery (SRS).

Main Methods:

• Utilized an adjustable front pointer and a three-micrometer device to measure gantry isocenter motion and table rotational axis wobble.
• Defined nominal gantry and table isocenters.
• Applied gantry motion path and table isocenter coordinates to offset simulated target coordinates, compensating for gantry sag.
• Examined target simulation films to document alignment improvements.

Main Results:

• Achieved an overall measurement precision of 0.2 mm for gantry and table isocenter coordinates.
• Observed gantry isocenter motion forming a pinched loop (max 1 mm point-to-point distance) over 360-degree rotation.
• Found table axis motion to be negligible compared to gantry isocenter motion reproducibility.
• Established a table isocenter invariant to table rotation.

Conclusions:

• The three-micrometer device and adjustable front pointer are effective tools for 3D mapping of isocenters and their motions.
• These measurement techniques are valuable for linear accelerator quality assurance.
• Improved beam-target alignment during SRS and other high-dose external beam therapies can be achieved.