SU-E-T-319: Monte Carlo Characterization of a New Directional Pd-103 High Dose Rate Source for Brachytherapy Application.

PURPOSE Standard brachytherapy sources emit radiation in a non-preferential direction away from the source. Though treatment outcomes are good, this can lead to late skin and subcutaneous toxicities in sensitive structures. Proposed low dose rate directional sources for interstitial brachytherapy, showed to have an improvement in the dose uniformity within the subcutaneous volume, reduction in skin dose, and reduction in volume receiving dose outside the target. The objective of this work is to demonstrate the potential use of a new Pd-103 directional seed for application in high dose rate brachytherapy treatments. METHODS Monte Carlo simulations were performed using the MCNP5 F6 energy deposition tallies as well as the MCNP5 F4 flux density tallies placed around a partially shielded Pd-103 source at angles(deg): 0, 45, 90, 135, 180, 225, 270, 315 as well as radial distances (cm): 0.5, 1, 1.5, 2, 3, 4, 5, 6, 7, 8, 9, 10. The source that modeled was a Pd-103 core using TG-43U recommended nuclear data, with a diameter = 1.4 mm, active length = 1.0 cm. In addition osmium metal with .03 mm thickness was used as shielding material to cover half of the cylindrical surface of the Pd-103 volume. The seed was encapsulated using .05 mm thick titanium. RESULTS MCNP models show that the dose to the radial distances, corresponding to the osmium shielded side, are dramatically reduced to less than 4% of the total dose. CONCLUSIONS The potential of a Pd-103 directional brachytherapy source has been studied. The results show that a seed with a partially shielded volume can be exploited to reduce side effects associated with radiation therapy to sensitive structures surrounding target volumes.