An Integrated Approach to Verify Spot Position and Isocenter in Image-Guided Proton Therapy

Area of Science:

• Medical Physics
• Radiotherapy
• Quality Assurance

Background:

• Proton therapy offers precise dose delivery but requires rigorous quality assurance.
• Conventional 2D QA methods may not fully capture 3D dose distributions and positional accuracy.
• Accurate verification of spot positions and isocenter is critical for effective proton therapy.

Purpose of the Study:

• To develop and validate a novel 3D quality assurance (QA) method for proton therapy.
• To utilize an X-ray computed tomography-based polymer gel dosimeter (XCT-PGD) integrated with a cone-beam computed tomography (CBCT) system.
• To enable comprehensive verification of spot positions and isocenter accuracy in proton therapy.

Main Methods:

• Irradiation of XCT-PGD using a PROBEAT-M1 proton therapy system with spot scanning beams.
• Acquisition of pre- and postirradiation CBCT scans to determine spot positions.
• Validation against Winston-Lutz tests and log file analysis, with 3D visualization of beam trajectories.

Main Results:

• The novel method showed excellent agreement with conventional techniques.
• Isocenter position accuracy within 0.39 mm compared to Winston-Lutz tests.
• Spot position accuracy within 0.37 mm compared to log file analysis.
• Simultaneous verification of multiple spot positions and beam trajectories in 3D.

Conclusions:

• Combining XCT-PGD with CBCT offers a robust 3D verification method for proton therapy delivery accuracy.
• This approach provides comprehensive spatial information and minimizes setup uncertainties.
• The method is suitable for annual QA and commissioning in proton therapy facilities.