Assessment of the use of prompt gamma emission for proton therapy range verification

[摘要] PURPOSE: Prompt gamma rays emitted from proton-nucleus interactions in tissue present a promising non-invasive, in situ means of monitoring proton beam based radiotherapy. This study investigates the fluence and energy distribution of prompt gamma rays emitted during proton irradiation of phantoms. This information was used to develop a correlation between the measured and calculated gamma emission and the proton beam range, which would allow treatments to more effectively exploit the sharp distal falloff in the dose distributions of protons. METHOD & MATERIALS: A model of a cylindrical Lucite phantom with a monoenergetic proton beam and an annular array of ideal photon tallies arranged orthogonal to the beam was developed using the Monte Carlo code MCNPX 2.6.0. Heterogeneous geometries were studied by inserting metal implants into the Lucite phantom, and simulating a phantom composed of bone and lung equivalent materials and polymethyl methacrylate. RESULTS: Experimental and computational results indicated a correlation between gamma emission and the proton depth-dose profile. Several peaks were evident in the calculated energy spectrum and the 4.44 MeV emission from 12C was the most intense line having any apparent correlation with the depth dose profile. Arbitrary energy binning of 4-5 MeV and 4-8 MeV was performed on the Monte Carlo data; this binned data yielded a distinct emission peak 1cm proximal to the Bragg peak. In all cases in the Lucite phantom the position of the Bragg peak;;s 80% distal falloff corresponded with the position of the 4-8MeV binned 50% distal falloff. The 4-5MeV binning strategy was successful with the heterogeneous phantom in which the proton beam entered lung and stopped in bone. However, the density disparity between the bone and lung equivalent materials rendered this technique unsuccessful for the heterogeneous phantom in which the beam entered bone and stopped in lung. For this 1.4MeV binning was conducted, assessing the 1.37 MeV characteristic gamma peak of 24Mg, which was only present in the lung slab. CONCLUSIONS: The results are promising and indicate the feasibility of prompt gamma emission detection as a means of characterizing the proton beam range in situ. This study has established the measurement and omputational tools necessary to pursue the development of this technique.

[发布机构] Massachusetts Institute of Technology