Monte Carlo dosimetry of a new 192Ir pulsed dose rate brachytherapy source.

A new microSelectron pulsed dose rate source has been designed, containing two active pellets instead of one inactive and one active pellet contained in the old design, to facilitate the incorporation of higher activity up to 74 GBq (2 Ci). In this work, Monte Carlo simulation is used to derive full dosimetric data following the AAPM TG-43 formalism, as well as the dose rate per unit air kerma strength data in Cartesian, "away and along" coordinates for both source designs. The calculated dose rate constant of the new PDR source design was found equal to lambda=(1.121 +/- 0.006) cGy h(-1) U(-1) compared to lambda = (1.124 +/- 0.006) cGy h(-1) U(-1) for the old design. Radial dose functions of the two sources calculated using the point source approximated geometry factors were found in close agreement (within 1%) except for radial distances under 2 mm. At polar angles close to the longitudinal source axis at the sources' distal end, the new design presents increased anisotropy (up to 10%) compared to the old one due to its longer active core. At polar angles close to the longitudinal source axis at the sources' drive wire end however, the old design presents increased anisotropy (up to 18%) due to attenuation of emitted photons through the inactive Ir pellet. These differences, also present in "away and along" dose rate results, necessitate the replacement of treatment planning input data for the new microSelectron pulsed dose rate source.