The volume effect of detectors in the dosimetry of small fields used in IMRT

Wolfram U. Lauba, Tony Wong

Research output: Contribution to journalArticlepeer-review

221 Scopus citations


In this study we investigate the effect of detector size in the dosimetry of small fields and steep dose gradients with a particular emphasis on IMRT measurements. Comparisons of calculated and measured cross-profiles and absolute dose values of IMRT treatment plans are presented. As a consequence of the finite size of the detector that was used for the commissioning of the IMRT tool, local discrepancies of more than 10% are found between calculated cross-profiles of intensity modulated beams and intensity modulated profiles measured with film. Absolute dose measurements of intensity modulated fields with a 0.6 cm3 Farmer chamber show significant differences of more than 6% between calculated and measured dose values at the isocenter of an IMRT treatment plan, Differences of not more than 2% are found in the same experiment for dose values measured with a 0.015 cm3 pinpoint ion chamber. A method to correct for the spatial response of finite-sized detectors and to obtain the "real" penumbra width of cross-profiles from measurements is introduced. Output factor measurements are performed with different detectors and are presented as a function of detector size for a 1 × 1 cm2 field. Because of its high spatial resolution and water equivalence, a diamond detector is found to be suitable as an alternative to other detectors used for small field dosimetry as there are photographic and photochromic film, TLDs, or water-equivalent scintillation detectors.

Original languageEnglish (US)
Pages (from-to)341-347
Number of pages7
JournalMedical physics
Issue number3
StatePublished - Mar 1 2003
Externally publishedYes


  • IMRT
  • Quality assurance
  • Small field dosimetry
  • Volume effect

ASJC Scopus subject areas

  • Biophysics
  • Radiology Nuclear Medicine and imaging


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