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Toward Magnetically Focused Proton Radiosurgery

G McAuley

GA McAuley1*, PQ McGee1 , JM Slater1 , JD Slater1,2 , AJ Wroe1,2 , (1) Loma Linda University, Loma Linda, CA, (2) Loma Linda University Medical Center, Loma Linda, CA


TH-AB-605-11 (Thursday, August 3, 2017) 7:30 AM - 9:30 AM Room: 605

Purpose: To investigate and demonstrate the advantages of magnetic focusing for small volume proton radiosurgery

Methods: Triplet combinations of quadrupole k=3 Halbach cylinders with various field gradients (100 to 250 T/m) were evaluated for the irradiation of small (≤ 10 mm diameter) targets. The magnets were placed on a track aligned with the proton beam and unmodulated 127 MeV and 157 MeV protons with initial beam diameters of 3 to 20 mm were delivered to a water tank. Depth and transverse dose distributions were measured using a PTW PR60020 diode detector and EBT3 film, respectively. Magnetic system parameters were adjusted to produce beam spots at the Bragg depth of low eccentricity, and each focused beam was paired for comparison to an unfocussed collimated beam that best matched the full width of the transverse dose profile at 90% of maximum dose. Monte Carlo simulations were also performed to both inform optimal experimental configurations and for comparison with experimental data.

Results: Results thus far using 150 T/m and 250 T/m gradient magnets and 8 to 20 mm initial diameter 127 MeV beams produced target beam spots with ~ 2.5 to 4 mm diameters. Importantly, peak to entrance dose ratios were up to 68% larger compared to spot size matched collimated beams. In addition, the focused beams were up to 3.4 times more efficient per MU in dose to target delivery. Additional results using different magnet combinations will also be presented.

Conclusion: Our results suggest that triplet magnetic focusing reduces entrance dose and increases beam delivery efficiency for small (≤ 10 mm diameter) targets compared to unfocused collimated beams. Potential patient benefits include reduced dose to normal tissue and decreased treatment times leading to an improved patient experience and reduced target motion. Such improvements would be immediately applicable to clinical proton radiosurgery.

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