A Monte Carlo Study of the Relationship between the Time Structures of Prompt Gammas and the In-vivo Radiation Dose in Proton Therapy

For in-vivo range verification in proton therapy, attempts have been made to measure the spatial distribution of the prompt gammas generated by the proton-induced interactions and to determine the proton dose distribution. However, the high energies of prompt gammas and background gammas are still problematic in measuring the distribution. In this study, we suggested a new method for determining the in-vivo range by utilizing the time structure of the prompt gammas formed during the rotation of a range modulation wheel (RMW) in passive scattering proton therapy. To validate the Monte Carlo code simulating the proton beam nozzle, we compared the axial percent depth doses (PDDs) with the measured PDDs for varying beam range from 4.73 to 24.01 cm. Also, we assessed the relationship between the proton dose rate and the time structure of the prompt gammas in a water phantom. The results of the PDD showed agreement within relative errors of 1.1% in the distal range and 2.9% in the modulation width. The average dose difference in the modulation was assessed as less than 1.3% by comparison with the measurements. The time structure of prompt gammas was well-matched, within 0.39 ms, with the proton dose rate, and this enabled an accurate prediction of the in-vivo range.