CT-based dosimetry calculations for 125I prostate implants

To evaluate the Monte Carlo code MCNP4B for low-energy brachytherapy calculations, including the effects of interseed attenuation and patient specific heterogeneities, on the calculated dose distribution from transperineal implantation of 125I.

The Monte Carlo code MCNP4B was used to model and benchmark the absolute dose distribution from two 125I brachytherapy seeds (model 6711 and 6702). Based upon the physical source model, the total photon intensity and differential energy spectrum were evaluated as a function of angle from the transverse bisector of the source. These spectral and intensity data were reformatted to produce probability distributions for sampling from a virtual point source. The virtual source model and a modified version of MCNP4B is then used for simulating arbitrary brachytherapy source configurations within a homogeneous or heterogeneous patient specific computed tomography (CT)-based lattice geometry.

Comparison with TG-43 data and the Monte Carlo calculations is excellent with MCNP4B predicting the radial dose function for the 125I 6711 and 6702 sources within 6% for all data points tested. Attenuation effects from neighboring seeds were investigated for pre- and postimplant seed distributions and found to be negligible. Preliminary dosimetry analysis of postimplant seed distributions comparing homogeneous water versus heterogeneous CT simulation geometries indicates an average decrease of approximately 5.6% for the volume of tissue irradiated to a prescription isodose line of 144 Gy.