# Program Information

## Towards a Spectroscopic Determination of Dose to Water

### M Malin*, L DeWerd, University of WI-Madison, Madison, WI

SU-E-T-537 Sunday 3:00PM - 6:00PM Room: Exhibit HallPurpose: To determine the spatial resolution required in the measurement of the energy spectrum emitted from the encapsulation of low-energy, photon-emitting brachytherapy sources to enable accurate dose-to-water calculations by propagating the measured spectrum through a water medium using Monte Carlo techniques.

Methods: A computational model of an Oncura 6711 I-125 brachytherapy source was created in MCNP5. The intensity of the fluence leaving the encapsulation of the source was quantified as a function of position along the long axis of the source (z) and the angle of emission at a given z with respect to the surface of the source (designated in spherical coordinates by polar and azimuthal angle). Phase space files recorded as photons left the encapsulation were altered to generate a uniform distribution of the fluence emitted along either z, the polar angle, or the azimuthal angle. The other variables in the phase space file were left unaltered. The dose distribution in water was calculated for both the model and the phase space files and compared to determine if future measurements would require capturing the spatial variation in all three dimensions.

Results: The fluence leaving the surface of the source is highly anisotropic in all three dimensions (z, polar angle, azimuthal angle). Phase space files that assume a uniform distribution of fluence emitted from any of the three dimensions do not generate accurate dose distributions in water at distances less than 8 cm from the source.

Conclusion: Accurate dose calculations require modeling the highly anisotropic fluence distribution emitted from the encapsulation of the source as a function of z and polar and azimuthal angles. Functional forms of these distributions will be determined and fit to spectroscopic measurements. This hybrid Monte Carlo-experimental fluence distribution will then be used to directly validate source models used in model-based dose calculation algorithms.

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