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Comparative Study
. 2009 Nov;36(11):4991-9.
doi: 10.1118/1.3238465.

Kilovoltage beam Monte Carlo dose calculations in submillimeter voxels for small animal radiotherapy

Magdalena Bazalova  1 , Hu Zhou, Paul J Keall, Edward E Graves
Affiliations
Comparative Study

Kilovoltage beam Monte Carlo dose calculations in submillimeter voxels for small animal radiotherapy

Magdalena Bazalova et al. Med Phys. 2009 Nov.

Abstract

Purpose: Small animal conformal radiotherapy (RT) is essential for preclinical cancer research studies and therefore various microRT systems have been recently designed. The aim of this paper is to efficiently calculate the dose delivered using our microRT system based on a microCT scanner with the Monte Carlo (MC) method and to compare the MC calculations to film measurements.

Methods: Doses from 2-30 mm diameter 120 kVp photon beams deposited in a solid water phantom with 0.2 x 0.2 x 0.2 mm3 voxels are calculated using the latest versions of the EGSnrc codes BEAMNRC and DOSXYZNRC. Two dose calculation approaches are studied: a two-step approach using phase-space files and direct dose calculation with BEAMNRC simulation sources. Due to the small beam size and submillimeter voxel size resulting in long calculation times, variance reduction techniques are studied. The optimum bremsstrahlung splitting number (NBRSPL in BEAMNRC) and the optimum DOSXYZNRC photon splitting (Nsplit) number are examined for both calculation approaches and various beam sizes. The dose calculation efficiencies and the required number of histories to achieve 1% statistical uncertainty--with no particle recycling--are evaluated for 2-30 mm beams. As a final step, film dose measurements are compared to MC calculated dose distributions.

Results: The optimum NBRSPL is approximately 1 x 10(6) for both dose calculation approaches. For the dose calculations with phase-space files, Nsplit varies only slightly for 2-30 mm beams and is established to be 300. Nsplit for the DOSXYZNRC calculation with the BEAMNRC source ranges from 300 for the 30 mm beam to 4000 for the 2 mm beam. The calculation time significantly increases for small beam sizes when the BEAMNRC simulation source is used compared to the simulations with phase-space files. For the 2 and 30 mm beams, the dose calculations with phase-space files are more efficient than the dose calculations with BEAMNRC sources by factors of 54 and 1.6, respectively. The dose calculation efficiencies converge for beams with diameters larger than 30 mm.

Conclusions: A very good agreement of MC calculated dose distributions to film measurements is found. The mean difference of percentage depth dose curves between calculated and measured data for 2, 5, 10, and 20 mm beams is 1.8%.

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Figures

Figure 1
Figure 1
Two approaches for DOSXYZNRC dose calculation for small animal radiotherapy based on a microCT scanner: using two phase-space files (a) and using the BEAMNRC simulation source (b).
Figure 2
Figure 2
The efficiency of dose calculations as a function of bremsstrahlung splitting number NBRSPL using DBS for different beam sizes in theBEAMNRC simulation source dose calculation. The symbols represent values calculated using Eq. 1.
Figure 3
Figure 3
Dose calculation efficiency as a function of photon splitting number Nsplit for dose calculation with phase-space files (a) and using the BEAMNRC simulation source (b). Calculated efficiencies in (b) using the data from (a) are represented by the symbols for the 2 (◆), 5 (◼), 10 (▲), and 20 (●) mm beams.
Figure 4
Figure 4
Efficiency of MC dose calculation with BEAMNRC source and with phase-space files: the CPU time for a 3 GHz machine (a) and the number of histories (b) to achieve 1% statistical uncertainty. The ordinates are logarithmic.
Figure 5
Figure 5
Depth dose curves for a 1 min treatment simulated with phase-space files (lines) and measured with EBT Gafchromic films (markers). The scanner isocenter is at 0 mm depth.
Figure 6
Figure 6
MC simulated profiles (lines) and measured profiles with films (symbols) for the 20 mm beam at 0.1 (◆), 1.0 (▲), and 2.0 (◼) cm depths parallel (a) and perpendicular to the heel effect (b), and for 10 (▲), 5 (◼), and 2 (◆) mm beams at 1 mm depth parallel (c) and perpendicular to the heel effect (d) in solid water phantom positioned 2.5 cm above the isocenter. Solid lines show the percentage difference between MC and film data. The treatment time is 1 min.
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