Sperduto_Khan's Treatment Planning in Radiation Oncology, 5e

CHAPTER 20 Treatment Planning Algorithms: Photon Dose Calculations 447

1.20E-11

1.00E-11

A Dose/incident fluence Gy-cm 2 FIGURE 20.7. Comparison of EGS4/PRESTA with Attila for a percent depth–dose calculation in a heterogeneous phantom. (Re printed from Gifford KA, Horton JL, Wareing TA, et al. Comparison of a finite-element multigroup discrete-ordinates code with Monte Carlo for radiotherapy calculations. Phys Med Biol . 2006;51:2253–2265, with permission of IOP Publishing. All rights reserved.) B 0 3 5 7 1012 151719 2224 2729 8.00E-12 6.00E-12 4.99E-12 2.00E-12 0.00E+00 H 2 O AI Lung H 2 O EGS4/PRESTA Atilla Depth (cm)

geometries. In their work, Attila was compared with EGSnrc MC simulations for a 6-MV photon beam from a Varian 2100 for a prostate and a head and neck case previ ously treated within their department. For both the BTE and MC calculations, CT datasets were converted into a material map of four materials with fixed densities: air, adipose tissue, soft tissue, and bone. In their comparison, calculations were performed with the same beam geom etries as those used clinically, with the exception of beam modulation which was removed for this comparison. Dose calculation differences were investigated along with the resolution of various discretization variables required for accurate Attila calculations. Figure 20.8 displays the material map for an axial slice through the center of the PTV for the head and neck case.

external photon beam calculation. Differences in doses were compared, along with relative calculation speeds. The photon dose calculation comparison was made comparing Attila versus EGS4 for an 18-MV photon beam from a Varian 2100 accelerator. A narrow beam geometry was used to highlight any differences in regions of elec tron disequilibrium. In addition, a heterogeneous multislab phantom was used, which consisted of water, lung, and alu minum. The Attila calculation was divided into 24 and 36 photon and electron energy groups, respectively. A compari son of depth–doses between these two calculations is shown in Figure 20.7. The agreements here were also good, with a root mean square difference of 0.7% of the maximum dose. Vassiliev et al. 45 extended these comparisons to include external beam calculations of heterogeneous patient

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A B FIGURE 20.8. A: Dose field calculated by Attila for a head-and-neck case on the axial plane through isocenter. Pixels, where the dose difference between Attila and Monte Carlo (EGS) exceeds 3%/3 mm, are shown in black on A and B . B: Material map through the axial plane containing the isocenter for the dose distribution calculated in A . (Adapted from Fig. 5 in [25] Safai S, Bortfeld T, Engelsman M. Comparison between the lateral penumbra of a collimated double-scattered beam and uncollimated scanning beam in proton radiotherapy. Phys Med Biol . 2008;53:1729–1750.)