Two-dimensional measurement of the prompt-gamma distribution for proton dose distribution monitoring

In proton therapy, accurate monitoring of the in-vivo proton dose distribution is essential in order to deliver the planned dose to the tumor volume within a minimal safety margin. Recently, a strong correlation between the distributions of the proton dose and the prompt gammas was found, and various prompt-gamma distribution-measurement systems, including collimation-based systems, Compton cameras, knife-edge imaging systems, and ion vertex imaging systems, have been proposed. In the present study, the feasibility of proton dose distribution monitoring was tested using a two-dimensional measurement system for prompt gammas. The measurement system, developed in the present study, incorporates a vertically-aligned one-dimensional array of gamma sensors, a parallel multi-hole collimator, a precision movement system, and a digitizer- and LabVIEW-based automatic data acquisition system. A 45-MeV proton beam of 0.5 nA was delivered to a polymethyl methacrylate (PMMA) phantom, and the two-dimensional prompt-gamma distribution was measured using the developed system. The proton beam range could be quantitatively determined to within a 1.6-mm error by sigmoidal curve-fitting with the Boltzmann equation. A comparison of the prompt-gamma distribution as measured by our detection system with the proton dose distribution as measured independently by using Gafchromic EBT films positioned inside the PMMA phantom showed good agreement. Both results imply that it is, indeed, possible to confirm the patient’s proton dose distribution by using two-dimensional prompt-gamma measurements.