Background
Methods
Dose planning method
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It uses simplified cross section models that are accurate for the energy range typically employed in conventional radiotherapy and for low atomic numbers, such as those encountered inside the patient body. For example, the Klein-Nishina differential cross section [7] is used to describe photon incoherent (Compton) scattering, thus neglecting Doppler broadening and binding effects, which are non-negligible for high Z elements or low energies. Similarly, the Møller differential cross section [8] is used to describe electron inelastic collisions with atomic electrons, thus assuming that the target particle is free and at rest. This, again, is valid for low atomic numbers and high energies.
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Photon transport is simulated detailedly using the delta scattering, or Woodcock tracking technique [9], which completely avoids the need to consider intersections with voxel walls.
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For electrons, DPM employs the standard condensed history model, falling into what has been called a mixed scheme for the treatment of energy losses by Berger [10]. It treats large energy transfer collisions detailedly and uses the continuous slowing down approximation to describe the effect of small energy loss interactions. For condensing angular deflections, the code is based on a refinement of the Kawrakow and Bielajew [11] formulation of the Lewis multiple-scattering theory [12], which allows fast random sampling of the scattering angle. The algorithm further relies on the small angle approximation, under which all materials can be characterized by means of a single scattering angle distribution.
DPM improvements
Parallelization of DPM
Mixed geometry model
Dynamic geometry
pDPM benchmarks
Photon transport in a MLC
Photon transport in a multi-layer phantom
Simulation of photon beams in clinical cases
Simulation times
/O2 /Qipo /QxP
for PENELOPE and /Qopenmp
for pDPM. PENELOPE is a serial code, hence, simulations were carried out by simultaneously running 32 instances of the code (each one with different initial random number seeds) and letting the operating system (Windows Server 2016) deal with the task assignment to the CPU cores. In order to provide a source of particles for each PENELOPE instance, the source phase-space file must be partitioned prior to starting the simulation. For the phase space used in this work this partitioning process took approximately 15 min. This time was not taken into account in the benchmark. Conversely, pDPM genuinely runs in parallel, hence, partitioning of the phase-space file is not necessary. The simulations with pDPM used 32 threads. In all cases the simulation time reported corresponds to that required to reach an average standard statistical uncertainty of 1%. The reported dose statistical uncertainties are computed using voxels that score more than 50% of the maximum dose.Results
Photon transport in a MLC
Test case | α [%] | Δ[%] | α[%] | Δ[%] |
---|---|---|---|---|
Described in section Photon transport in a multi-layer phantom | ||||
(all voxels) | 14.2 | -5.0 | 17.0 | 0.2 |
(in the air layer) | 97.0 | -5.3 | 0 | 0 |
(excluding the air layer) | 32.1 | -0.2 | 20.4 | 0.2 |
Described in “Photon transport in a MLC” section | 26.0 | -0.4 | 13.3 | 0.3 |
Head&Neck | 32.4 | -0.8 | 17.8 | 0.7 |
Lung | 36.6 | -0.8 | 11.7 | 0.5 |
Brain | 30.5 | -0.6 | 7.0 | 0.7 |
Prostate | 28.1 | -0.4 | 18.2 | 0.4 |
Photon transport in a multi-layer phantom
Simulation of photon beams in clinical cases
Region | Γ1,1 [%] | Γ2,1 [%] |
---|---|---|
Prostate | ||
Body | 99.8 | 100 |
PTV | 99.6 | 100 |
Rectum | 99.7 | 100 |
Bladder | 100 | 100 |
H&N | ||
Body | 99.6 | 100 |
PTV1 | 98.0 | 100 |
PTV2 | 96.2 | 100 |
Spine | 100 | 100 |
Left Parotid | 99.2 | 99.9 |
Brain | ||
Body | 99.7 | 100 |
PTV1 | 99.4 | 100 |
PTV2 | 99.1 | 100 |
Brain stem | 99.6 | 100 |
Lung | ||
Body | 99.6 | 100 |
PTV | 99.2 | 100 |
Simulation times
Simulation time [min] | Speedup | |||||
---|---|---|---|---|---|---|
pDPM | ||||||
Test case | Voxel size [cm3] |
penelope
| Original voxel | Coarse voxel | Original voxel | Coarse voxel |
Described in “Photon transport in a multi-layer phantom” section | 0.5×0.5×0.25 | 37 | 9.5 | - | 3.9× | - |
Described in “Photon transport in a MLC” section | 0.2×0.2×0.5 | 324 | 129 | - | 2.5× | - |
Head&Neck VMAT, 194 CP | 0.19×0.15×0.19 | 1061 | 140 | 42 | 7.6× | 25.3× |
Lung VMAT, 194 CP | 0.19×0.14×0.19 | 331 | 28 | 14 | 11.8× | 23.6× |
Brain VMAT, 354 CP | 0.11×0.2×0.11 | 687 | 117 | 34 | 5.8× | 20.2× |
Prostate IMRT, 621 CP | 0.18×0.25×0.18 | 472 | 64 | 45 | 7.3× | 10.5× |