ADVANTAGES OF MCNPX-BASED LATTICE TALLY OVER MESH TALLY IN HIGH-SPEED MONTE CARLO DOSE RECONSTRUCTION FOR PROTON RADIOTHERAPY

Abstract

Monte Carlo simulations are increasingly used to reconstruct dose distributions in radiotherapy research studies. Many studies have used the MCNPX Monte Carlo code with a mesh tally for dose reconstructions. However, when the number of voxels in the simulated patient anatomy is large, the computation time for a mesh tally can become prohibitively long. The purpose of this work was to test the feasibility of using lattice tally instead of mesh tally for whole-body dose reconstructions. We did this by comparing the dosimetric accuracy and computation time of lattice tallies with those of mesh tallies for craniospinal proton irradiation. The two tally methods generated nearly identical dosimetric results, within 1% in dose and within 1 mm distance-to-agreement for 99% of the voxels. For a typical craniospinal proton treatment field, simulation speed was 4 to 17 times faster using the lattice tally than using the mesh tally, depending on the numbers of proton histories and voxels. We conclude that the lattice tally is an acceptable substitute for the mesh tally in dose reconstruction, making it a suitable potential candidate for clinical treatment planning.

Keywords: dose reconstruction; lattice tally; mesh tally.

Fig. 1

Geometric model of proton therapy unit and the voxelized phantom oriented for the superior spinal proton field. The beam delivery system includes a vacuum window (A), a beam profile monitor (B), a range modulator wheel (C), a second scatterer (D), a range shifter assembly (E), backup and primary monitors (F), the snout (G), the range compensator (H), the treatment couch (I), and the patient (J).

Fig. 2

Schematic illustration of setup of the voxelized patient phantom.

Fig. 3

Probability P(γ > x), where x is a certain calculated γ value, that a voxel’s γ value will exceed the γ criteria (1% of dose and 1-mm distance-to-agreement) as a function of the x value, using the dose distribution from the mesh tally dosimetric results as the reference. Probability curves are plotted separately for two cranial fields (beams 1 and 2) and three spinal fields (beams 3, 4, and 5).

Fig. 4

Computation time (t) of different tallies for a spinal field in CSI proton therapy as a function of (a) the number of particles simulated and (b) the number of voxels in the lattice unit cell. The ratio of computation time is shown as dashed lines.