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

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

A PROBEAT-M1 proton therapy system was used to irradiate an XCT-PGD with spot scanning beams from 4 gantry angles (45°, 180°, 270°, and 315°). Three spot positions, including the isocenter position, were evaluated. Pre- and postirradiation CBCT scans were acquired to determine the spot positions. The accuracy of the proposed method was validated against conventional Winston-Lutz tests and beam delivery log file analysis. Spot positions were analyzed using 3-dimensional visualization of the beam trajectories within the gel dosimeter.

The proposed method demonstrated excellent agreement with conventional techniques. The isocenter position determined using our method was within 0.39 mm that from the Winston-Lutz test. Spot position from our method was within 0.37 mm that from log file analysis. Three-dimensional detection using the gel dosimeter enabled simultaneous verification of multiple spot positions and beam trajectories, providing comprehensive spatial information not available through conventional 2-dimensional methods.

Combining an XCT-PGD with CBCT provides a robust and comprehensive method for 3-dimensional verification of the accuracy of proton therapy delivery. This novel approach offers advantages over conventional QA techniques by enabling simultaneous evaluation of spot positions and isocenter accuracy while minimizing setup uncertainties. The method is particularly suitable for annual QA and commissioning procedures in proton therapy facilities.