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Evaluation of Heterogeneities in Proton Therapy

W Harris1*, W Nie2, A Kassaee1, A Kassaee1, (1) University of Pennsylvania, Philadelphia, PA, (2) University of Nebraska, Omaha, NE

Presentations

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

Room: AAPM ePoster Library

Purpose: To study the effect of heterogeneous materials in proton beams by evaluating dose distributions from measured Gafchromic film, calculations using Treatment Planning System (TPS) and calculations using Monte Carlo simulation.

Methods: A rectangular rod phantom generated with 3D printer with dimensions of 14cm x 10cm and thickness of 2cm, with 6 rods of various diameters, was scanned and imported into a TPS using Proton Convolution Superposition dose calculation and TOPAZ Monte Carlo software for dose calculations. The phantom is robust since any configuration of different materials can be implemented in various positions and diameters. Ten cm of solid water was placed upstream and downstream the phantom, and films were placed downstream the phantom every 0.5cm depth to evaluate the dose distribution. Calculations of the dose distribution at the film locations based on the TPS and Monte Carlo were compared with measurements of the film based on two delivered treatment plans: 1) a single AP field resulting in uniform SOBP along the entire phantom and 2.5cm downstream the phantom and 2) a single AP field resulting in a single energy Bragg Peak dose distribution with maximum dose 2.5 cm downstream the phantom. Ten variations of rod materials including solid water, air, brass, aluminum, copper and titanium were evaluated. The peaks and valleys of the dose distribution in planes through profiles were compared.

Results: Preliminary results show inadequate estimation of the dose profiles based on TPS when heterogeneities of high Z material are introduced in the phantom. The maximum dose difference between TPS and measured data was 60% in scenarios with complex heterogeneities including titanium rods and air. Monte Carlo simulations resulted in good agreement with measurement data.

Conclusion: The robust phantom has a variety of interchangeable rods and can be used to validate treatment planning algorithms for proton dose calculations.

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