This addendum to the code of practice for the determination of absorbed dose for x-rays below 300 kV has recently been approved by the IPEM and introduces three main changes: (i) Due to a lack of available data the original code recommended a value of unity for k(ch) in the very-low-energy range (0.035-1.0 mm Al HVL). A single table of k(ch) values, ranging from 1.01 to 1.07, applicable to both designated chamber types is now presented. (ii) For medium-energy x-rays (0.5-4 mm Cu HVL) methods are given to determine the absorbed dose to water either at 2 cm depth or at the surface of a phantom depending on clinical needs. Determination of the dose at the phantom surface is derived from an in-air measurement and by extending the low-energy range up to 4 mm Cu HVL. Relevant backscatter factors and ratios of mass energy absorption coefficients are given in the addendum. (iii) Relative dosimetry: although not normally forming part of a dosimetry code of practice a brief review of the current literature on this topic has been added as an appendix. This encompasses advice on techniques for measuring depth doses, applicator factors for small field sizes, dose fall off with increasing SSD and choice of appropriate phantom materials and ionization chambers.

An investigation has been carried out into the properties of the BANG polymer gel and its use in the dosimetry of low dose rate brachytherapy. It was discovered that the response of the gel was reproducible and linear to 10 Gy. The gel was found to be tissue equivalent with a response independent of energy to within experimental accuracy (standard error of measurement +/- 5%). The slope of the calibration curve was found to increase from 0.28 +/- 0.01 s-1 Gy-1 to 0.50 +/- 0.02 s-1 Gy-1 for an increase in monomer concentration from 6 to 9%. Absorbed dose distributions for a straight applicator containing 36 137Cs sources were measured using the gel and the results compared with measurements made with thermoluminescent dosemeters (TLDs) and calculated values. Good agreement was found for the relative measurements. The root mean square residual percentage errors were 3%, 1% and 4% for the gel and the two groups of TLDs, respectively. There were some significant differences in absolute values of absorbed dose in the gel, possibly owing to the effects of oxygen. Measurements of a complex gynaecological insert were also made and compared with isodose curves from a planning system (Helax TMS), and in areas unaffected by oxygen diffusion the isodose levels from 100 to 50% agreed to within less than 0.5 mm.

United Kingdom dosimetry codes of practice have traditionally specified one electrometer for use as a secondary standard, namely the Nuclear Enterprises (NE) 2560 NPL secondary standard therapy level exposure meter. The NE2560 will become obsolete in the foreseeable future. This report provides guidelines to assist physicists following the United Kingdom dosimetry codes of practice in the selection of an electrometer to replace the NE2560 when necessary. Using an internationally accepted standard (BS EN 60731:1997) as a basis, estimated error analyses demonstrate that the uncertainty (one standard deviation) in a charge measurement associated with the NE2560 alone is approximately 0.3% under specified conditions. Following a review of manufacturers' literature, it is considered that modern electrometers should be capable of equalling this performance. Additional constructural and operational requirements not specified in the international standard but considered essential in a modern electrometer to be used as a secondary standard are presented.

The beam and performance characteristics of a new orthovoltage X-ray unit, the Pantak DXT-300 have been evaluated. Data were collated for four qualities: 3.27 mmAl, 7.15 mmAl, 1.65 mmCu and 3.51 mmCu half value layer (HVL) (SE = 0.04 mm). Parameters which were investigated included beam quality, central axis depth dose, relative output, backscatter factors, field uniformity, peripheral dose and head leakage. The calibration procedure and the performance of the dosimetry system have also been described.

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When making in vivo measurements, particularly of doses to patients undergoing total body irradiation, considerable differences are often found between the results obtained using semiconductors and thermoluminescent dosemeters (TLD) (Naudy, 1981; Aukett, 1985; Welsh & Stedeford, 1986, personal communication). The variation in sensitivity of lithium fluoride with dose and with photon energy was established when the use of the material was first suggested (Cameron et al, 1961) and is now well known (Fowler & Attix, 1966; McKinlay, 1981). Variations with dose-rate have been observed for semiconductor detectors (Lavin, 1968; Grüssel & Rikner, 1984). The response of semiconductor detectors also varies with photon energy (Wright & Gager, 1977; Dixon & Ekstrand, 1982; Johansson, 1982; Rikner, 1985). Corresponding variations in sensitivity with depth might be observed for both thermoluminescent and semiconductor dosemeters. Such effects have already been observed for different types of semiconductor detectors (Gager et al, 1977; Rikner & Grüssel, 1983). In order to investigate this possibility the two types of detector were compared with an ionization chamber at a series of depths including both build-up and build-down regions. Measurements of the variation in sensitivity with dose and dose-rate were also made so that these could be eliminated from the results. The 6 MV linear accelerator used at this centre for total body irradiation was used throughout. The semiconductors used were Therados type EDP 10 used in conjunction with a DPD 5 dose monitor.

This new code of practice for the determination of absorbed dose for x-rays below 300 kV has recently been approved by the IPEMB and introduces the following changes to the previous codes: (i) The determination of absorbed dose is based on the air kerma determination (exposure measurement) method. (ii) An air kerma calibration factor for the ionization chamber is used. (iii) The use of the F (rad/röentgen) conversion factor is abandoned and replaced by the ratio of the mass-energy absorption coefficients of water and air for converting absorbed dose to air to absorbed dose to water. New values for ratios of these coefficients are recommended. Perturbation and other correction factors are incorporated in the equations. (iv) New backscatter factors are recommended. (v) Three separate energy ranges are defined, with specific procedures for each range. These ranges are: (a) 0.5 to 4 mm Cu HVL; for this range calibration at 2 cm depth in water with a thimble ion chamber is recommended. (b) 1.0 to 8.0 mm Al HVL; for this range calibration in air with a cylindrical ion chamber and the use of tabulated values of the backscatter factor are recommended. (c) 0.035 to 1.0 mm Al HVL; for this range calibration on the surface of a phantom with a parallel-plate ionization chamber is recommended. Contents 1 Data 3.1 Mass-energy absorption coefficient ratios, water to air, ( μen /ρ) w/air 3.1.1 Medium energies (0.5-4 mm Cu HVL) 3.1.2 Low energies (1.0-8 mm Al HVL) 3.1.3 Very low energies (0.035-1.0 mm Al HVL) 3.2 Back-scatter factor, B w 3.3 The overall chamber correction factor, k ch 3.3.1 Medium energies (0.5-4 mm Cu HVL) 3.3.2 Very low energies (0.035-1.0 mm Al HVL) Appendix A. Theoretical basis for the new air kerma based code A.1 Background A.2 Medium-energy x-rays (0.5-4 mm Cu HVL) A.2.1 General details A.2.2 Air kerma at the centre of a hole in the phantom (K air,hole ) A.2.3Air kerma at the chamber centre in the undisturbed medium (K air,z ) A.2.4Correction for the effect of the chamber stem A.2.5Air kerma to water kerma conversion A.

Significant discrepancies of up to 10% exist between backscatter factors (BSF) recommended in a recent IAEA dosimetry Code of Practice (1987) compared with those published in Br. J. Radiol. Supplement 17 (1983), for the x-ray quality range 0.1-2.0 mm Al HVL. In an attempt to resolve this discrepancy, BSFs have been measured using thermoluminescence dosimetry (TLD) with small lithium borate chips, in order to minimise displacement effects associated with the use of larger volume ionisation chambers. Although subject to uncertainties inherent in the TLD calibration and readout process, the results indicate that the BJR (1983) data overestimate BSFs in this quality range. Broad agreement with the IAEA data is indicated, for the limited number of x-ray qualities and field sizes used.