Build-up modification of commercial diodes for entrance dose measurements in 'higher energy' photon beams

Background and Purpose: Several commercially available p-type diodes do not provide sufficient build-up for in-vivo dosimetry in 'higher' energy photo n beams, and only limited information could be found in the literature desc ribing the correction factor variation and/or the achievable accuracy for i n-vivo dosimetry methods in this energy range. The first aim of this study is to assess and analyze the variation of diode correction factors for entr ance dose measurements at higher photon energies. In a second step the tota l build up thickness of the diode has been modified in order to minimize th e correction factor variation. Materials and Methods: Diode correction factors accounting for non-referenc e conditions (field size, source surface distance, tray, wedge, and block) are determined in 18-25 MV photon beams provided by different treatment uni ts for Scanditronix p-type diodes recommended for higher energy photon beam s: old type and new type EDP-20, and EDP-30 diodes. Hemispherical build-up caps of different materials (copper, iron, lead) are used to increase the t otal build-up thickness. Perturbation effects with and without additional b uild-up caps are assessed for the three diode types, Results: For unmodified diodes field size correction factors (CFS) vary bet ween 1.7% and 6%, dependent on diode type and treatment unit. For example. for an old type EDP-20 the C-FS variation at 18 MV is much higher on a GE l inac (5%) as compared to the Philips machine (1.7%). Depending on diode typ e, this variation can be reduced to 1-2% when adding additional build-up. T he variation of source to surface distance correction factors is almost ind ependent of build-up thickness. By adding additional build-up the influence of trays and blocks can be almost eliminated. Conclusions: The correction factor variation of unmodified diodes reflects the variation of the electron contamination with treatment geometry. A tota l build-up thickness of 30 mm is found to be the 'best compromise' for the three types of diodes investigated when measuring entrance doses in the ene rgy range between 18 and 25 MV. (C) 1999 Elsevier Science Ireland Ltd. All rights reserved.