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Importance of Secondary Particles Produced in a Tungsten Multislit Collimator for Proton Minibeam Therapy

S Mossahebi

S Mossahebi1*, J Snider1 , F Dilmanian2 , S Krishnan3 , J Eley1 , (1) University of Maryland School of Medicine, Baltimore, MD, (2) Brookhaven National Laboratory, Upton, NY, (3) UT MD Anderson Cancer Center, Houston, TX


SU-I-GPD-T-199 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: The purpose of this study was to investigate secondary particle contamination for proton minibeam arrays produced using single proton pencil beams and a heavy-metal multislit collimator.

Methods: A 5-cm-thick tungsten multislit collimator was implemented at the Maryland Proton Treatment Center (Baltimore, MD) to produce planar proton minibeams of 300-micron beams with 1-mm center-to-center spacing. Secondary contamination from charged particles was measured with radiochromic films (Gafchromic EBT3, Ashland Inc., Covington, KY) and quantified as the peak-to-valley dose ratio of the proton minibeam array at 1 cm downstream of the multislit collimator in air. Neutron exposures were also measured, this time at 60 cm downstream of the multislit collimator. The measurement used the Wide Energy Neutron Detection Instrument (FHT 762 WENDI-2, Thermo Fisher Scientific, Halethorpe, MD), positioned beyond a phantom of 30-cm-thick solid water, which stopped the primary protons. We then compared the resulting neutron equivalent doses for collimated minibeams against those for uncollimated proton pencil beams, which was normalized to the number of protons reaching the phantom.

Results: Radiochromic films revealed peak-to-valley dose ratios of 25.2 ± 4.6, 30.6 ± 3.1, 19.8 ± 2.0, and 15.0 ± 2.0 for incident proton energies of 80, 100, 120, and 140 MeV, respectively. Per proton reaching the phantom, the ratios of neutron dose equivalent for collimated minibeams compared with uncollimated proton pencil beams were 1.69 ± 0.08, 4.50 ± 0.09, and 4.09 ± 0.14 for proton energies of 100, 120, and 140, respectively.

Conclusion: Our findings suggest that levels of secondary particle contamination from a multislit collimator are generally small (3%-7% of the primary peak dose) for proton minibeam therapy at energies relevant for brain tumor treatments but will worsen at higher energies.

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