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

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.