Monte Carlo dosimetry of the microselectron HDR 192Ir brachytherapy source using MCNP4A

This study examines the transverse radial dose distribution around the Nucletron MicroSelectron high dose rate (HDR) 192Ir brachytherapy source using the Monte Carlo radiation transport code MCNP (Monte Carlo N Particle) version 4A. The geometry modeled consisted of an identical simulation of the geometry of the MicroSelectron HDR source within the centre of a cylindrical water phantom of 80 cm diameter and height. Doses were calculated at 0.1 cm intervals in the 1 cm closest to the source, whilst extending to 1 cm intervals 15 cm from the source. Conversion to the clinically relevant unit of air-kerma strength to describe source activity enables absolute comparison of Monte Carlo dose calculations to current treatment planning computer results. Most HDR brachytherapy planning computers utilise algorithms based on a point source with attenuation and scatter corrections based on the Van Kleffen and Starr or Meisberger equations. Significant differences between the planning computer algorithms and the Monte Carlo dose calculations occur in the near field (radius less than 1 cm) and the far field (radius greater than 8 cm) which in some cases may have important clinical consequences. These results are consistent with Monte Carlo calculations of previous authors examining the MicroSelectron HDR 192Ir source and also show that far field uncertainties are increased as the size of the phantom is reduced to a more realistic patient size.