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Intercomparison of normalized head-scatter factor measurement techniques

D M Frye1, B R Paliwal, B R Thomadsen

  • 1University of Wisconsin Hospital, Madison, USA.

Medical Physics
|February 1, 1995
PubMed
Summary
This summary is machine-generated.

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Comparing measurement techniques for photon beams, the miniphantom method showed discrepancies, especially at higher energies (10- and 24-MV). This technique may exclude contamination electrons, impacting accuracy in radiotherapy calculations.

Area of Science:

  • Medical Physics
  • Radiation Oncology

Background:

  • Accurate measurement of head-scatter factors is crucial for radiotherapy dose calculations.
  • Traditional methods using buildup caps may include contributions from contamination electrons, particularly at higher photon beam energies.

Purpose of the Study:

  • To compare normalized head-scatter factors measured using cylindrical beam coaxial miniphantoms and graphite buildup caps.
  • To investigate the influence of contamination electrons on these measurements at various photon beam energies and field sizes.

Main Methods:

  • Measurements of normalized head-scatter factors were performed using miniphantoms and buildup caps for 4-, 6-, 10-, and 24-MV photon beams.
  • Electromagnet measurements were utilized to identify and quantify contamination electron contributions.

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Main Results:

  • Good agreement between miniphantoms and buildup caps was observed for 4- and 6-MV photon beams.
  • Miniphantoms yielded lower normalized head-scatter factors than buildup caps for 10- and 24-MV beams at large field sizes.
  • Miniphantoms showed higher factors than buildup caps at small field sizes.
  • Contamination electrons were identified in buildup cap measurements at 24 MV, but not in miniphantom measurements.

Conclusions:

  • The miniphantom technique may underestimate head-scatter factors at higher energies due to the exclusion of contamination electrons.
  • This exclusion can lead to inaccuracies in derived parameters like tissue-maximum ratios and phantom scatter factors.
  • Potential inaccuracies in monitor unit calculations for radiotherapy treatments are highlighted.