Room: Exhibit Hall | Forum 4
Purpose: Spot scanning proton beam therapy requires several time consuming measurements to verify accuracy and quality on a regular basis. We propose to combine a series of tests into a single set of measurements performed using a commercial, conical scintillator phantom.
Methods: The Logos XRV-100 Beam Metrology system, a conical scintillator phantom, was used to measure spot size, radiation/imaging isocenter coincidence, and gantry rotational isocenter accuracy (gantry starshot) using a single setup and data acquisition session on a spot scanning proton therapy system. Single spots were delivered to the phantom varying the energy and gantry angle per spot and analyzed together. Baseline data was acquired for seven energy/gantry angle combinations to be varied monthly covering energies 69.4, 84.5, 100, 139.1, 180.4, 200.4, 221.3 MeV and gantry angles 0, 30, 60, 90, 120, 150, 180 degrees. The proposed method has been successfully implemented in our practice.
Results: The combined measurement required approximately 30 minutes to complete. Spot size, isocenter coincidence, and starshot analysis were performed with submillimeter precision. Spot sizes agreed with Lynx data (IBA dosimetry) within <1 mm and demonstrated reproducibility of 0.3 mm. Gantry/imaging isocenter coincidence reproducibility was <0.35 mm, which may be partially attributable to spot position variability. In addition to performing prior QA tests, this detector was able to measure changes in spot size with gantry angle, a known effect that was not measurable with the prior equipment.
Conclusion: The measurement precision and reproducibility achieved points to the suitability of the XRV-100 system for monthly spot scanning proton beam therapy QA. Efficient collection of increased data for the required monthly tests in one simple setup demonstrates the utility of switching to a conical scintillator phantom. Alternating energy and gantry angle in the same setup allows for more detailed monitoring of beam characteristics such as spot size.