Program Information
Monte Carlo Analysis of Magnetically Focused Beams for Proton Radiosurgery
GA McAuley1*, JM Slater1 , S Heczko1 , T Nguyen1 , JD Slater1,2 , A Wroe1,2 , (1) Loma Linda University, Loma Linda, CA, (2) Loma Linda University Medical Center, Loma Linda, CA
Presentations
SU-K-108-12 (Sunday, July 30, 2017) 4:00 PM - 6:00 PM Room: 108
Purpose: A preliminary Monte Carlo investigation of the advantages in dose distribution and delivery efficiency of proton beams focused by a triplet of quadrupole magnets
Methods: Monte Carlo simulations were performed using various configurations of a triplet of quadrupole magnets. Magnet parameters were selected to match what can be commercially manufactured as assemblies of rare-earth permanent magnetic materials adhered into Halbach cylinders (eg, field gradients of 100 to 250 T/m, bore diameters of 10 to 20 mm). Focused unmodulated 127 MeV proton beams (90% dose range of 9.88 cm in water) were target matched with passive collimated beams (the current beam delivery method for proton radiosurgery) and properties of transverse dose, depth dose and volumetric dose distributions were compared.
Results: Magnetically focused beams delivered beam spots of low eccentricity to Bragg depth with full widths at the 90% reference dose contour from ~ 2 to 5mm. Focused beams showed 27 to 102% larger peak-to-entrance dose ratios compared to unfocused beams. When focused initial beam diameters were larger than matching unfocused beams (11 of 12 cases), the result was an increase in dose delivery efficiency of 1.5 to 4.2x. Peak-to-entrance and efficiency benefits tended to increase with larger magnet gradients and larger initial diameter focused beams. Focusing tended to shift dose from penumbra (80%-20%) to below the 20% isodose contour compared to unfocused beams.
Conclusion: This preliminary Monte Carlo study suggests focusing protons immediately upstream of tissue entry using permanent magnet assemblies can produce beams with larger peak-to-entrance dose ratios and dose delivery efficiencies compared to collimated beams. These improvements have clinical implications but still require experimental verification before application to the practice of proton radiosurgery.
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