title: The volume effect of detectors in the dosimetry of small fields used in IMRT.
abstract: 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 x 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.
authors.affiliation: Department of Medical Physics, University of Tübingen, Germany. wlaub@lhs.org
authors.forename: Wolfram U
authors.initials: WU
authors.lastname: Laub
authors.forename: Tony
authors.initials: T
authors.lastname: Wong
mesh_terms: D003198:Computer Simulation; D019544:Equipment Failure Analysis / Q000379:methods; D005373:Film Dosimetry; D008954:Models, Biological; D011786:Quality Control; D011874:Radiometry / Q000295:instrumentation* / Q000379:methods / Q000592:standards; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted / Q000295:instrumentation / Q000379:methods*; D020266:Radiotherapy, Conformal / Q000295:instrumentation* / Q000379:methods / Q000592:standards; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity; D014159:Transducers