# Patient-dependent beam-modifier physics in Monte Carlo photon dose calculations

## Abstract

Model pencil-beam on slab calculations are used as well as a series of detailed calculations of photon and electron output from commercial accelerators to quantify level(s) of physics required for the Monte Carlo transport of photons and electrons in treatment-dependent beam modifiers, such as jaws, wedges, blocks, and multileaf collimators, in photon teletherapy dose calculations. The physics approximations investigated comprise (1) not tracking particles below a given kinetic energy, (2) continuing to track particles, but performing simplified collision physics, particularly in handling secondary particle production, and (3) not tracking particles in specific spatial regions. Figures-of-merit needed to estimate the effects of these approximations are developed, and these estimates are compared with full-physics Monte Carlo calculations of the contribution of the collimating jaws to the on-axis depth-dose curve in a water phantom. These figures of merit are next used to evaluate various approximations used in coupled photon/electron physics in beam modifiers. Approximations for tracking electrons in air are then evaluated. It is found that knowledge of the materials used for beam modifiers, of the energies of the photon beams used, as well as of the length scales typically found in photon teletherapy plans, allows a number of simplifying approximations to bemore »

- Authors:

- Lawrence Livermore National Laboratory, Livermore, California 94550 (United States)

- Publication Date:

- OSTI Identifier:
- 20216380

- Resource Type:
- Journal Article

- Journal Name:
- Medical Physics

- Additional Journal Information:
- Journal Volume: 27; Journal Issue: 5; Other Information: PBD: May 2000; Journal ID: ISSN 0094-2405

- Country of Publication:
- United States

- Language:
- English

- Subject:
- 62 RADIOLOGY AND NUCLEAR MEDICINE; RADIOTHERAPY; MONTE CARLO METHOD; SIMULATION; PHOTON BEAMS; COLLIMATORS; ACCELERATORS; ELECTRON BEAMS; SECONDARY EMISSION; RADIATION DOSES; THEORETICAL DATA

### Citation Formats

```
Schach von Wittenau, A. E., Bergstrom, P. M. Jr., and Cox, L. J.
```*Patient-dependent beam-modifier physics in Monte Carlo photon dose calculations*. United States: N. p., 2000.
Web. doi:10.1118/1.598959.

```
Schach von Wittenau, A. E., Bergstrom, P. M. Jr., & Cox, L. J.
```*Patient-dependent beam-modifier physics in Monte Carlo photon dose calculations*. United States. doi:10.1118/1.598959.

```
Schach von Wittenau, A. E., Bergstrom, P. M. Jr., and Cox, L. J. Mon .
"Patient-dependent beam-modifier physics in Monte Carlo photon dose calculations". United States. doi:10.1118/1.598959.
```

```
@article{osti_20216380,
```

title = {Patient-dependent beam-modifier physics in Monte Carlo photon dose calculations},

author = {Schach von Wittenau, A. E. and Bergstrom, P. M. Jr. and Cox, L. J.},

abstractNote = {Model pencil-beam on slab calculations are used as well as a series of detailed calculations of photon and electron output from commercial accelerators to quantify level(s) of physics required for the Monte Carlo transport of photons and electrons in treatment-dependent beam modifiers, such as jaws, wedges, blocks, and multileaf collimators, in photon teletherapy dose calculations. The physics approximations investigated comprise (1) not tracking particles below a given kinetic energy, (2) continuing to track particles, but performing simplified collision physics, particularly in handling secondary particle production, and (3) not tracking particles in specific spatial regions. Figures-of-merit needed to estimate the effects of these approximations are developed, and these estimates are compared with full-physics Monte Carlo calculations of the contribution of the collimating jaws to the on-axis depth-dose curve in a water phantom. These figures of merit are next used to evaluate various approximations used in coupled photon/electron physics in beam modifiers. Approximations for tracking electrons in air are then evaluated. It is found that knowledge of the materials used for beam modifiers, of the energies of the photon beams used, as well as of the length scales typically found in photon teletherapy plans, allows a number of simplifying approximations to be made in the Monte Carlo transport of secondary particles from the accelerator head and beam modifiers to the isocenter plane. (c)},

doi = {10.1118/1.598959},

journal = {Medical Physics},

issn = {0094-2405},

number = 5,

volume = 27,

place = {United States},

year = {2000},

month = {5}

}