A method of scaling the 3D electron pencil-beam dose calculation to obtain accurate monitor units for irregularly-shaped electron beams

Todd R. McNutt, Wolfgang A. Tomé

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A method for determining monitor units (MU) for electron beams using a 3D pencil beam dose algorithm is described. A set of correction factors (OFc) to the pencil beam dose is generated that transforms the output into dose per MU. The OFcs 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 OFc. The OFc value is determined using a measured set of relative output factors (OFrel) for various field sizes and SSD. The pencil-beam algorithm is used to compute the "raw" value (OFp) to each of the OFrel measurement points. The OFc is the ratio of the measured OFrel to the calculated OFp. The OFc value for irregularly-shaped electron fields or for electron fields at extended SSD may be interpolated from the OFc 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 OFc, the interpolation is less error prone than the more common direct interpolation of the output factors. Computations were performed in the ADAC Pinnacle3 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 languageEnglish (US)
Pages (from-to)209-213
Number of pages5
JournalMedical Dosimetry
Issue number3
Publication statusPublished - Nov 26 2002
Externally publishedYes



  • Correction factor
  • Electron dose calculation
  • Pencil beam

ASJC Scopus subject areas

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

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