The Status of High-Energy Photon and Electron Beam Dosimetry Five Years After the Implementation of the Iaea Code of Practice in the Nordic Countries

Abstract

The status of the dosimetry of high-energy photon and electron beams is analysed, taking into account the main developments in the field since the implementation of the IAEA Code of Practice in the Nordic countries. In electron beam dosimetry, energy-range relationships are discussed; Monte-Carlo results with different codes are compared with the experimentally derived empirical expression used in most protocols. Updated calculations of water-to-air stopping-power ratios following the changes in the Monte-Carlo code used to compute actual sw,air values are compared with the data included in most dosimetry protocols. The validity of the commonly used procedure to select stopping-power ratios for a clinical beam from the mean energy at the phantom surface and the depth of measurement, is analysed for ‘realistic’ electron beams. In photon beam dosimetry, calculated correction factors including the effect of the wall plus waterproofing sleeve and existing data on the shift of the effective point of measurement of an ionization chamber, are discussed. New calculations of medium-to-air stopping-power ratios and their correlation with the quality of the beam obtained from the convolution of Monte-Carlo kernels are presented together with their possible practical implications in dosimetry. Trends in Primary Standard Dosimetry Laboratories towards implementing calibrations in terms of absorbed dose to water are presented, emphasising controversial proposals for the specification of photon beam qualities. Plane-parallel ionization chambers are discussed regarding aspects that affect determinations of absorbed dose, either through the different methods used for the calibration of these chambers or by means of correction factors. Recent studies on the effect of the central electrode in Farmer-type cylindrical chambers are described.