Investigation of in vivo beam range verification in carbon ion therapy using the Doppler Shift Effect of prompt gamma: A Monte Carlo simulation study

Abstract

Carbon ion therapy has received increasing attention because of its excellent physical dose distribution and high biological effect comparing against the conventional radiotherapy. However, the range uncertainty is still a major issue that limits further improvement of the treatment efficiency. Hence, we propose a novel method for reducing the range uncertainty in carbon ion therapy based on the Doppler Shift Effect of prompt gamma (PG), which can quantify the average carbon energy by the significant energy shift of the 4.44 MeV PG in the spectrum and then calibrate the range. Specifically, the influences of heterogeneity, initial carbon beam configuration, and detected direction are thoroughly discussed using the Monte Carlo simulation in this paper. Results show a moderate difference (<3.5%) between the Monte Carlo simulation and the analytical calculation, which verify the theoretical feasibility and veracity of the proposed method. The different initial energies (200, 225, 250, 300, and 400 MeV/u) and energy spreads (0%, 0.5%, 1%, 2%) of carbon beam have a not obvious influence on the accuracy of this method. When the detected direction changes from 90° to 50°, the PG energy shift overturns from “red shift” to “blue shift” and the difference value increases significantly. Moreover, for inhomogeneous phantoms, this method can distinguish the different ranges with blocks filled by different materials (bone, adipose, and air) in the beam path. Finally, through the analysis of range verification error with cavities of different thicknesses (0–30 mm) in the water phantom, the precision of the proposed method in theory is determined to be 0.008%–6.253%, and this value can be less than 3% with an appropriate detection site. These results indicate that the measurement of the PG energy shift could be applied to reduce the range uncertainty on the basis of the Doppler Shift Effect in carbon ion therapy.