SU‐E‐T‐394: Effects of Various Encapsulation Materials to the Dose Fall‐Off for Beta‐Emitting Brachytherapy Seed

Purpose: A beta‐emitting glass seed without encapsulation has been proposed for the brachytherapy treatment of prostate cancer. Beta sources have several desirable properties. Beta sources have short range and are easily shielded, lowering extraneous dose to the medical staff as well as the patient. Even though there are several advantages of using beta‐emitting glass seeds, the dose fall‐off is considerable. This study aims at evaluating the effects of dose fall‐off and bremsstrahlung using encapsulation. Methods: MCNPX2.6 Monte Carlo transport code was used to calculate the quantitative dosimetric parameters of the seeds. The radioisotope used for seed was 142Pr. The 142Pr decays by beta emission with a 2.162 MeV endpoint energy is followed by emission of a 1.575 MeV gamma‐ray with 3.7% abundance. The gamma dose was not included in this research except the contributions of bremsstrahlung because the encapsulation does not attenuate the 1.5 MeV gamma much. Various encapsulation materials were investigated including silver, titanium, and tungsten. A simulation model of glass seed with encapsulation was developed within MCNPX2.6. Dosimetry data in water were calculated in radial distances from the glass seed from 0.1 cm to 1.0 cm by using the mesh tally feature in MCNPX2.6. Results: The relative dose of titanium and tungsten to water at 0.2 cm were 84.5 % and 21.9 % while the relative dose at 0.6 cm was 60.4% and 12.4% respectively. Considering the reduction of dose at the target point (0.6 cm), it was found that titanium is the most appropriate material. The contribution of bremsstrahlung was insignificant. Conclusions: The effects of source encapsulation on dose fall‐off were studied. The encapsulation has effectively made the dose profiles uniform. The bremsstrahlung effect was insignificant. From this study, the dosimetric parameters would be derived for clinical use.