### Abstract

A method for determining monitor units (MU) for electron beams using a 3D pencil beam dose algorithm is described. A set of correction factors (OF_{c}) to the pencil beam dose is generated that transforms the output into dose per MU. The OF_{c}s depend on cone selection, field size, and source-to-surface distance (SSD) for a given electron energy. The dose per MU is determined by scaling the dose by the OF_{c}. The OF_{c} value is determined using a measured set of relative output factors (OF_{rel}) for various field sizes and SSD. The pencil-beam algorithm is used to compute the "raw" value (OF_{p}) to each of the OF_{rel} measurement points. The OF_{c} is the ratio of the measured OF_{rel} to the calculated OF_{p}. The OF_{c} value for irregularly-shaped electron fields or for electron fields at extended SSD may be interpolated from the OF_{c} table. The interpolation over SSD is performed linearly using the central axis SSD. The interpolation over field size is more complicated and uses the minimum area-circumscribing rectangle around the field shape. Due to the relative flatness of the OF_{c}, the interpolation is less error prone than the more common direct interpolation of the output factors. Computations were performed in the ADAC Pinnacle^{3} Planning System. Measurements were obtained using a Varian 2300c for 6 and 15 MeV. The results show that this method can predict the dose per MU within 1% to 2% for clinical fields and within 3% to 4% for extreme field shapes.

Original language | English (US) |
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Pages (from-to) | 209-213 |

Number of pages | 5 |

Journal | Medical Dosimetry |

Volume | 27 |

Issue number | 3 |

DOIs | |

Publication status | Published - Nov 26 2002 |

Externally published | Yes |

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### Keywords

- Correction factor
- Electron dose calculation
- Pencil beam

### ASJC Scopus subject areas

- Radiological and Ultrasound Technology
- Oncology
- Radiology Nuclear Medicine and imaging