3D isocentricity analysis for clinical linear accelerators

Area of Science:

• Medical Physics
• Radiation Oncology
• Radiotherapy Physics

Background:

• Accurate isocentricity is crucial for precise radiation delivery in cancer treatment.
• Conventional methods for measuring linear accelerator (linac) isocentricity often rely on 2D projections, limiting comprehensive 3D analysis.
• The PRESAGE dosimeter offers a novel 3D volumetric measurement capability.

Purpose of the Study:

• To develop and validate a three-dimensional (3D) concurrent isocentricity measurement technique for clinical linear accelerators (linacs).
• To utilize the PRESAGE 3D dosimeter for a comprehensive assessment of rotational centers in a Varian TrueBeam linac.
• To establish a new definition and measurement for linac isocenter location and size in 3D space.

Main Methods:

• Irradiation of the PRESAGE 3D dosimeter at various gantry and couch angles on a Varian TrueBeam linac.
• 3D image reconstruction with submillimeter resolution using filtered back-projection after irradiation.
• Analysis of beam trajectories and rotational centers using star-shot planes and orthogonal views to define isocenter location and size.

Main Results:

• Table and gantry rotational centers were measured relative to setup lasers with displacements of 0.60 mm and 0.75 mm, respectively.
• The distance between table and gantry rotational centers was found to be 0.49 mm.
• The linac's radiation isocenter was determined to be at (-0.07, -0.17, 0.51) mm relative to setup lasers, with a size described by semi-axes of 0.13 mm and 0.23 mm.

Conclusions:

• The study successfully demonstrates a 3D concurrent measurement of rotational centers using a single PRESAGE dosimeter setup.
• The proposed method provides a more comprehensive evaluation of linac isocentricity compared to traditional 2D film measurements.
• A novel definition for the linac isocenter and its 3D size was proposed and validated.