Build-up modi®cation of commercial diodes for entrance dose measurements in `higher energy' photon beams Dietmar Georg a, * , Bie De Ost b , Marie-The Âre Áse Hoornaert b , Pierre Pilette b , Jan Van Dam b , Michel Van Dycke b,1 , Dominique Huyskens a a Department of Oncology, Section Radiation Physics, U.Z. Gasthuisberg Leuven, Herestraat 49, 3000 Leuven, Belgium b Belgian Hospital Physicist Association, P.O. Box 23, 1950 Kraainem, Belgium Received 11 September 1998; received in revised form 5 March 1999; accepted 13 April 1999 Abstract Background and Purpose: Several commercially available p-type diodes do not provide suf®cient build-up for in-vivo dosimetry in `higher' energy photon beams, and only limited information could be found in the literature describing the correction factor variation and/or the achievable accuracy for in-vivo dosimetry methods in this energy range. The ®rst aim of this study is to assess and analyze the variation of diode correction factors for entrance dose measurements at higher photon energies. In a second step the total build up thickness of the diode has been modi®ed in order to minimize the correction factor variation. Materials and Methods: Diode correction factors accounting for non-reference conditions (®eld size, source surface distance, tray, wedge, and block) are determined in 18±25 MV photon beams provided by different treatment units for Scanditronix p-type diodes recommended for higher energy photon beams: 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 total build-up thickness. Perturbation effects with and without additional build-up caps are assessed for the three diode types. Results: For unmodi®ed diodes ®eld size correction factors (C FS ) vary between 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 linac (5%) as compared to the Philips machine (1.7%). Depending on diode type, this variation can be reduced to 1±2% when adding additional build-up. The variation of source to surface distance correction factors is almost independent of build-up thickness. By adding additional build-up the in¯uence of trays and blocks can be almost eliminated. Conclusions: The correction factor variation of unmodi®ed diodes re¯ects the variation of the electron contamination with treatment geometry. A total build-up thickness of 3O mm is found to be the `best compromise' for the three types of diodes investigated when measuring entrance doses in the energy range between 18 and 25 MV. q 1999 Elsevier Science Ireland Ltd. All rights reserved. Keywords: In-vivo dosimetry; Entrance dose measurements; Electron contamination 1. Introduction The usefulness of in-vivo dosimetry as a means for qual- ity assurance and quality improvement in radiotherapy has been demonstrated in numerous publications [5,10,11,15± 17,23±25,27,30]. Moreover, in-vivo dosimetry is recom- mended for advanced radiotherapy techniques, such as conformal radiotherapy or beam intensity modulation, espe- cially when performing dose escalation studies [7,8,12,18]. Within recent years p-type diodes have become the most popular and widely applied tool for in-vivo dosimetry. Besides advantages like linear dose rate response in clini- cally applied radiation qualities, high radiation sensitivity, operating simplicity, small size, mechanical stability, the main advantage of using diodes is the on line information provided [9,11,14,26,31±33]. Diodes have been used in a variety of applications, e.g. entrance dose measurements, target dose and midplane dose determination, total body irradiation and monitor unit veri®cation [6±9,15± 19,21,23,24,26,36]. Most of these methods have been described for photon beams up to 10 MV, including exam- ples of numerical values for the respective correction factors to be applied in this energy range. Concerning in-vivo dosimetry in `higher energy photon beams' (18±25 MV) only limited information could be found in the literature, describing either the correction factor variation and/or the achievable accuracy for in-vivo dosi- Radiotherapy and Oncology 51 (1999) 249±256 0167-8140/99/$ - see front matter q 1999 Elsevier Science Ireland Ltd. All rights reserved. PII: S0167-8140(99)00058-4 * Corresponding author. 1 B.de O., M.-T.H., P.P., J.v.D. and M.v.D. are members of the In-Vivo Working Group of the BHPA.