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Geometric Optimizations of a Directional High Dose Rate Brachytherapy Source Using Monte Carlo


A Heredia

A Heredia1*, D Henderson1,2, B Thomadsen1,2, (1) Department of Medical Physics, University of Wisconsin-Madison, Madison, WI 53705, (2) Department of Engineering Physics, University of Wisconsin-Madison, Madison, WI 53706

SU-E-T-336 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Purpose:
The purpose of this work is to present results from geometric optimizations of a directional high dose rate brachytherapy (d-HDR) source using MCNP5. Three brachytherapy source designs are presented here; 1)cylindrical d-HDR (Cyl.1) , 2) D-shaped d-HDR version 1 (D.1), and 3) D-shaped d-HDR version 2 (D.2).

Methods:
The sources were modeled using MCNP5 version 1.51. Tally volumes (1mm³) were placed around the seed at radii ranging from 0.25-100cm and fifty polar and azimuthal angles, in 7.2° increments ranging from 0°-352.8°. A cylindrical mesh superimposed over the simulated geometry was also employed.

The d-HDR active volumes consisted of a palladium-103 (Pd-103) pellet ranging from 0.105-0.150 cm in diameter, with a 0.003 cm thick osmium foil for shielding, within a .001 cm titanium encapsulated volume.

Results:
The dose rate constant, Λ, for the Cyl.1, D.1, and D.2 was 0.686 cGy h-1 U-1, 0.671 cGy h-1 U-1, and 0.657 cGy h-1 U-1, respectively. The optimized D.2 design showed an improvement over the Cyl.1 and D.1, having a flatter dose profile for a 165° span, where the relative dose at the radial distance was max. The Cyl.1 design had a rounder dose profile showing only a 60° plateau section, resulting in a lower dose conformality. The D.1 design showed the presence of two significant horns at 120° and 225°. This was attributed to the self attenuation within the active volume, resulting in a lower relative dose along the widest dimension.

Conclusion:
Geometric optimizations of a new directional high dose rate source have been presented. The results show an improved geometry in the D-shaped d-HDR version 2 (D.2) compared to both Cyl.1 and D.1 seed designs. The D.2 seed has the potential to improve dose conformality and current patient outcomes, having flexibility for use in both intracavitary and interstitial brachytherapy applications.

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