Europium-155 as a source for dual energy cone beam computed tomography in adaptive proton therapy: A simulation study

To investigate the feasibility of a 3D imaging system utilizing a ¹⁵⁵ Eu source and pixelated cadmium-zinc-telluride (CZT) detector for applications in adaptive radiotherapy. Specifically, to compare the reconstructed stopping power ratio (SPR) values of a head phantom obtained with the proposed imaging technique with theoretical SPR values.

A Geant4 Monte Carlo simulation was performed with the novel imaging system. The simulation was repeated with a typical 120 kV X-ray tube spectrum while maintaining all other parameters. Dual energy ¹⁵⁵ Eu source cone beam computed tomography (CBCT) images were reconstructed with an iterative projection algorithm known as total variation superiorization with diagonally relaxed orthogonal projections (TVS-DROP). Single energy 120 kV source CBCT images were also reconstructed with TVS-DROP. Reconstructed images were converted to SPR with stoichiometric calibration techniques based on ICRU 44 tissues. Quantitative accuracy of reconstructed attenuation coefficient images as well as SPR images were compared.

Images generated by gamma emissions of ¹⁵⁵ Eu showed superior contrast resolution to those generated by the 120 kV spectrum. Quantitatively, all reconstructed images correlated with reference attenuation coefficients of the head phantom within 1 standard deviation. Images generated with the ¹⁵⁵ Eu source showed a smaller standard deviation of pixel values. Use of a dual energy conversion into SPR resulted in superior SPR accuracy with the ¹⁵⁵ Eu source.

¹⁵⁵ Eu was found to display desirable qualities when used as a source for dual energy CBCT. Further work is required to demonstrate whether the simulation results presented here can be translated into an experimental prototype.