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Factors Contributing to the Increased Proton RBE at the Distal Edge of Bragg Peak

C Cheng

C-W Cheng1,2*, M Takashina3, M Suga3, IJ Das1, 2, V Moskvin1, (1) Indiana University- School of Medicine, Indianapolis, IN, (2) Indiana University Health Proton Therapy Center, Bloomington, IN,(3) Osaka University, Osaka, Japan

SU-E-T-483 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Purpose: RBE in proton beam is variable and it is expected that the values are different at Bragg peak. This is investigated using Monte Carlo simulation. The objective is to determine the physical quantities contributing to the spatial variation of RBE in proton beam.

Methods: The Monte Carlo codes PHITS and FLUKA are used in this study. A mono-energetic un-modulated proton beam is incident on a water phantom. The average proton energy along the proton beam path, the proton flux, Φ(#protons/cm2/incident proton) and the linear energy transfer, LET (keV/μm) integrated over the energy range 0-Ep,max are computed at selected depths from d=0 to 1cm beyond the Bragg peak for incident proton energies 50-250 MeV. The quantity LET*Φ /ρ which is a direct measure of energy absorbed/unit mass at each point of the proton beam path is also determined.

Results: The proton energies at the BP are 4, 4, 12, 19 and 25 MeV for incident proton energies of 50, 100, 150, 200 and 250 MeV respectively. As the proton energy decreases with depth, the LET increases, while the proton flux decreases due to the broadening of the proton spectrum with depth. Thus the product, LET*Φ attains a maximum somewhere on the distal edge of the BP (or the SOBP). Lower energy protons have a higher LET*F at the distal edge than that higher energy beams.

Conclusion: RBE is related to the relative dose (energy absorbed/unit mass) to a reference radiation for the same biological endpoint. The variation of LET*Φ indirectly shows that the increase in RBE at the distal edge is due to the two competing factors in play, the LET and the proton flux as a function of depth. Strictly speaking, secondary electrons contribute to the₂ LET and flux, and will affect the RBE, to a small extent.

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