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An Effective Atomic Number of the Compounds in Proton Beam Therapy

V Moskvin

V Moskvin1,3*, M Suga2, C Cheng1,3, I Das1,3, (1) Indiana University- School of Medicine, Indianapolis, IN, (2) Osaka University, Osaka, (3) Indiana University Health Proton Therapy Center, Bloomington, IN

SU-E-T-723 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Purpose: The effective atomic number (Zeff) of a material is defined, conventionally, from the X-Ray interactions with the material. The interaction of protons in a medium is fundamentally different from photons. Hence applicability of the photon-based definition may not be justified in proton therapy. The purpose of this study is to define the Zeff of a medium for proton beam therapy.

Methods: A robust definition of Zeff is proposed through the proton range shift ΔR(t) in water due to the presence of a layer of a material of thickness t. This definition assures experimental verification of Zeff. In this study, the proton range shift was calculated with the TRIM (the Transport of Ions in Matter) code for a set of compounds. The ICRU 46 elemental composition for human tissue, dental implant materials and some plastics commonly used in radiotherapy applications were used for Zeff calculation for proton energies from 5 MeV to 250 MeV. The Zeff for protons is compared to those from conventional definitions.

Results: The weighted atomic densities Zeff for known material composition assures invariance of the Zeff with initial proton energy in the model. The Zeff for protons differs from those defined for photons substantially. The semi-empirical model proposed for the calculations of Zeff, is based on the measurable values of the range shift in water in the presence of a material of unknown elemental composition. Zeff may be calculated from the equation Zeff(x) = a exp(-b x), where x = ΔR(t)/t.

Conclusion: A robust definition of effective atomic number of a given medium for proton interaction is proposed based on the measurable value of range shift in water in the presence of the material layer.

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