WE‐G‐500‐04: A Novel Technique for In‐Vivo and Real‐Time Range Verification Based On the Characteristic Prompt Gamma Time‐Structure of Passively Modulated Proton Beams

Purpose: We propose a novel technique for in‐vivo and real‐time range verification based on the unique temporal dependence of prompt‐gamma produced by passively modulated proton beams Methods: For passively modulated proton beams, the dose delivered to a medium is periodic with the period of the Range Modulator Wheel (RMW) rotation. It has been confirmed experimentally that such periodic dose‐rate functions, measured inside a medium, have a unique correlation with their point of measurement along a Spread‐Out Bragg Peak (SOBP). Therefore, also the time‐structure of prompt‐gammas, generated by proton induced nuclear interactions in the target and detected outside the medium, is expected to be periodic and correlated with their source‐location along the SOBP. To verify such correlation, we employed the Monte Carlo method using TOol for PArticle Simulation (TOPAS). The simulated detection system is constituted by a single‐slit collimated detector placed at 90° from the beam direction. Discrimination of prompt‐γ from background radiation is done trough energy and time‐of‐flight (TOF) selectionResults: We verified that the time‐pattern of prompt‐gamma rate functions (i.e. gamma‐counts vs. RMW rotation‐time) is periodic and correlated with the detection depth along the SOBP. Utilizing this feature it is feasible to predict the range of a SOBP within few millimeters with a single point of prompt‐gamma measurement. The influence of the prescribed dose and collimation setup over the detection statistics is discussed in order to assess the potential clinical application of the technique. TOF selection and background subtraction can significantly increase the signal to noise ratio Conclusion: Our results show that the range of a SOBP can be determined, with a single point of measurement, by analyzing the time‐pattern of prompt‐gamma detected with a single‐slit detection system. This novel technique potentially represents a powerful tool for real‐time range verification for passively modulated proton therapy systems.