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Dosimetric Uncertainties Associated with High Z-Materials in Dose Calculation Algorithms for Therapeutic High Energy Proton Beams

I Ali1*, N Alsbou2, S Ahmad1, (1) University of Oklahoma Health Sciences, Oklahoma City, OK, (2) Department of Engineering and Physics, University of Central Oklahoma, Edmond, OK

Presentations

(Sunday, 7/12/2020)   [Eastern Time (GMT-4)]

Room: AAPM ePoster Library

Purpose: To investigate quantitatively the scattering of high energy 250MeV proton beam on high-z materials and determine associated dosimetric uncertainties in dose deposition calculated by the dose calculation algorithms used in proton therapy.
Methods: The dose distributions from the scattering of energetic proton beams obtained from the MEVION-S250 proton therapy system were measured. The MEVION system produces proton beams with large and small field sizes (25 and 14cm diameters), ranges from 5-32cm and modulation 2-20cm. Phantoms with a variety of high-z material which was embedded in solid water phantom (30x30x30cm3) at depth ranging from 1-19 cm and irradiated with nearly 250 MeV proton beams (15x15cm2). The dose distributions deposited by the proton beams were detected downstream with high sensitivity computed radiography detectors.
Results: Energetic protons scatter strongly on high-z materials that were implanted in solid-water phantoms. The high-z materials resulted interference-patterns in the dose deposition in the medium which was not uniform with large variation of nearly 10% in local regions between the peaks and valleys downstream the heterogeneity. These dose variations resulted from strong constructive and destructive interference-patterns from the interference of the primary and scattered protons. The interference-patterns in dose deposition patterns resulted from the wave nature of the proton interactions with medium that is not considered by conventional dose calculation algorithms such as pencil beam convolution or Monte-Carlo. The scatter kernels used by these algorithms should consider the quantum nature of proton interactions particularly with high z-materials to calculate dose accurately.
Conclusion: Large deviation in the dose deposition by protons interacting with high-z heterogeneities which were represented by interference-patterns due to overlap of primary and scatter protons. The conventional dose calculation algorithms used in proton therapy did not account for these interference-patterns and new algorithms that consider the wave nature of protons has to be developed.

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