Room: Stars at Night Ballroom 2-3
Purpose: We will present our clinical prototype system for in vivo range verification of proton pencil-beams that integrates prompt gamma-ray spectroscopy detectors with a precise positioning robot.
Methods: The clinical prototype system to measure the range of proton pencil-beams during treatment delivery has been developed and constructed in-house during the past years. Spectroscopy of proton-induced prompt gamma-rays is performed for every delivered pencil-beam. By analyzing the spectra using a nuclear reaction model, the absolute range of the proton beams is determined. The detector consists of an array of large LaBrâ‚ƒ scintillators, mounted behind a tungsten collimator in a rotating frame. It has now been joined with a mobile 6-axis positioning robot for alignment with the patient, which was calibrated using a coordinate measuring machine. The system will be positioned superior of the patient in the proton therapy gantry for in vivo measurements during brain and head & neck treatments.
Results: Phantom experiments showed a range verification precision of 1.1 mm at a 95% confidence level for a dose of 0.9 Gy delivered to a cylindrical volume with a radius and depth of 10 mm. The mean systematic uncertainty was 0.5 mm and the mechanical positioning error was measured as < 0.3 mm.
Conclusion: With millimeter accuracy on both the range verification and the detector positioning, the clinical prototype prompt gamma-ray spectroscopy system is expected to provide the most accurate in vivo measurements of proton beams to date. The clinical data will be used to better understand the magnitude of the proton range uncertainty and to investigate methods to increase the treatment precision.
Funding Support, Disclosures, and Conflict of Interest: This work was supported in part by the Federal Share of program income earned by Massachusetts General Hospital on C06-CA059267, Proton Therapy Research and Treatment Center, and by the National Cancer Institute grant U19 CA021239-35.