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The Effects of a Dynamic Collimation System On Proton Pencil Beams to Improve Lateral Tissue Sparing in Spot Scanned Proton Therapy

P Hill

P Hill*, D Wang, R Flynn, D Hyer, University Of Iowa, Iowa City, IA


SU-E-T-321 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Purpose: To evaluate the lateral beam penumbra in pencil beam scanning proton therapy
delivered using a dynamic collimator device capable of trimming a portion of the
primary beam in close proximity to the patient.

Methods: Monte Carlo simulations of pencil beams were performed using MCNPX. Each
simulation transported a 125 MeV proton pencil beam through a range shifter, past a
collimator, and into a water phantom. Two parameters were varied among the
simulations, the source beam size (sigma in air from 3 to 9 mm), and the position of
the edge of the collimator (placed from 0 to 30 mm from the central axis of the beam).
Proton flux was tallied at the phantom surface to determine the effective beam size
for all combinations of source beam size and collimator edge position.

Results: Quantifying beam size at the phantom surface provides a useful measure to
compare performance among varying source beam sizes and collimation conditions. For a
relatively large source beam size (9 mm) entering the range shifter, sigma at the
surface was found to be 10 mm without collimation versus 4 mm with collimation.
Additionally, sigma at the surface achievable with collimation was found to be smaller
than for any uncollimated beam, even for very small source beam sizes. Finally, the
lateral penumbra achievable with collimation was determined to be largely independent
of the source beam size.

Conclusion: Collimation can significantly reduce proton pencil beam lateral penumbra.
Given the known dosimetric disadvantages resulting from large beam spot sizes,
employing a dynamic collimation system can significantly improve lateral tissue
sparing in spot-scanned dose distributions.

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