SU-E-T-300: Monte Carlo Simulation of Single-Plane Magnetically Focused Narrow Proton Beams.

Purpose To investigate narrow, elongated magnetically focused proton beams and compare their properties with passively collimated beams using Monte Carlo simulation. METHODS We performed Geant4 Monte Carlo simulations involving a single quadrapole focusing magnet, thereby creating a flattened beam with an elongated elliptical cross section. The parameters of the magnet were chosen to mimic k=3 (quadrapole) Halbach cylinders that are available commercially as assemblies of rare earth permanent magnetic materials. For comparison, simulations were also performed with the same beam line components and passively collimated beams (using an elliptically shaped collimator). To facilitate fair comparison, efforts were made to closely match the planned treatment volumes (PTV) for each simulation case in dose, volume, and major and minor diameters of the elliptically shaped PTV at Bragg depth. RESULTS Magnetic focusing delivered significantly better dose localization to the target over collimated beams which are the current beam delivery modality. Compared to collimated beams, the magnetically focused beams showed a 31% smaller therapeutic ratio, a 31% smaller integrated dose, a 34% smaller entrance dose, a 30% larger peak-to-entrance central depth dose ratio, a 37% smaller penumbra volume, and were 35% more efficient in dose delivery (based on proton number). CONCLUSIONS The clinically relevant advantages of the magnetically focused beams compared to the collimated beams (the current standard of care) can be attributed to the preferential directional acceleration of protons due to the magnetic field. Our simulations suggest such magnets can be used to deliver tissue sparing doses to normal and at-risk tissue, and enhanced dose to elongated, narrow targets. Future work to characterize and test prototype magnets is in progress. Such beams my find application in novel proton treatments including application to the spinal cord.