Parameterization of brachytherapy source phase space file for Monte Carlo-based clinical brachytherapy dose calculation.

A common approach to implementing the Monte Carlo method for the calculation of brachytherapy radiation dose deposition is to use a phase space file containing information on particles emitted from a brachytherapy source. However, the loading of the phase space file during the dose calculation consumes a large amount of computer random access memory, imposing a higher requirement for computer hardware. In this study, we propose a method to parameterize the information (e.g., particle location, direction and energy) stored in the phase space file by using several probability distributions. This method was implemented for dose calculations of a commercial Ir-192 high dose rate source. Dose calculation accuracy of the parameterized source was compared to the results observed using the full phase space file in a simple water phantom and in a clinical breast cancer case. The results showed the parameterized source at a size of 200 kB was as accurate as the phase space file represented source of 1.1 GB. By using the parameterized source representation, a compact Monte Carlo job can be designed, which allows an easy setup for parallel computing in brachytherapy planning.

Image registration and calculation of a biologically effective dose for multisession radiosurgical treatments. Technical note.

The purpose of this study was to develop techniques for registering image sets associated with staged or multifraction radiosurgical treatments of large targets with the Leksell gamma knife to transform shot coordinates between treatment sessions and produce cumulative dose distributions and to investigate the theoretical biological effects of such protracted treatments by means of such concepts as the linear-quadratic model and biologically effective dose. An image registration technique based on normalized mutual information was adapted to produce one fused-image study from an imaging series acquired during distinct treatment sessions. A spreadsheet computer program was developed to determine coordinate transformations between the associated stereotactic coordinate systems based on digitized coordinates of fiducial markers appearing on the fused images. Coordinates of shots used during one treatment session could then be transformed to the stereotactic space of another session, and cumulative dose distributions could be computed. The procedure was applied to the two-stage treatment of a giant arteriovenous malformation (AVM). Overall uncertainty in each transformed shot position is approximately 0.7 mm. An effective single-fraction dose (D(eff)) was defined and computed for the two-stage AVM treatment. The simple summed dose distribution was compared with the D(eff) distribution. Because dose values differ significantly in overlap regions between the individual distributions, the clinical usefulness of the simple cumulative distribution is dubious. It may be useful for a future update of the GammaPlan treatment planning software to generate effective single-session dose distributions for such cases.