[en] A series of symposia on dosimetry in medicine and biology have been held by the IAEA in co-operation with WHO. The present symposium was the first one focusing on ''Dosimetry in Radiotherapy''. The papers presented reflected the different steps in the calibration chain such as the calibration standards established by the National Standards Laboratories and the conversion of the reading of calibrated instruments to the desired quantity, i.e. absorbed dose to water at a reference point in the user's beam at the radiotherapy clinic. The programme further examined the procedures necessary for optimization of the treatment of the patient, such as treatment planning methods, dose distribution studies, new techniques of dose measurement, improvements in the physical dose distributions/conformation therapy and special problems involved in total body treatments. Results of quality assurance in radiotherapy were presented from local hospitals as well as from national and international studies. Refs, figs and tabs

[en] The absorbed dose to a medium irradiated by orthovoltage X-ray beams (100-300 kV) is usually determined according to the formalism recommended by the ICRU. The absorbed dose to water can be calculated from the reading of an ionization chamber, the exposure calibration factor and the factor F, which converts exposure to absorbed dose to water for the specified radiation quality. In a review of published data of dose measurements in orthovoltage X-ray beams and using this formalism, large differences can be observed. In a proposed new formalism, the calibration factor for ⁶⁰Co gamma rays is also applied for measurements in orthovoltage beams. Experiments using different types of ionization chambers and analysed according to both the new and the ICRU formalisms are described. The results have been compared with those recently obtained by other groups using different calibration or measurement procedures. (author). 12 refs, 2 figs, 2 tabs

[en] Poster presentation. 3 refs, 1 tab

[en] The EGS Monte Carlo radiation transport system has been used to model the therapy beam from an AECL ⁶⁰Co unit. Photon contamination is shown to be mostly due to the source capsule and to have little effect on the depth-dose curve. The calculations for broad beams show that there are many sources of electron contamination (collimators, air, source capsule) which vary in importance as a function of SSD. The effects of filters and magnets on electron contamination have been studied. The calculations are shown to be in good agreement with experimental results except very close to the surface (<0.5 mm) where the calculations underestimate the measurements. (author). 13 refs, 8 figs, 1 tab

[en] A method of calculating dose distribution over the beam axis in a heterogeneous medium (polystyrene, bone-equivalent medium) irradiated with high-energy electrons has been established from experimental data using the 'small beam' model. The method, which uses a decomposition of the dose breakdown due to radiation scattering, has made it possible to account for the overdose in the exposed bone-equivalent medium and the effect of the bone-equivalent medium on the percentage depth dose in the polystyrene downbeam of the heterogeneity. The scattering functions in media made of homogeneous polystyrene, in bone-equivalent materials and in heterogeneous media were established for 9 MeV, 15 MeV and 25 MeV electrons and various circular fields. The scattering functions for heterogeneous media were recalculated by combining the scattering functions for homogeneous polystyrene and bone-equivalent media, and the percentage depth doses over the beam axis derived from them. The uncertainties in the percentage depth doses for the polystyrene downbeam of the heterogeneity were less than 3%, 2% and 0.5% for 9 MeV, 15 MeV and 25 MeV electrons, respectively. Uncertainty in the region upbeam of the heterogeneity is cut to under 1% by taking into account the effect of electrons, backscattered upbeam by the bone-equivalent medium. (author). 7 refs, 11 figs, 1 tab

[en] The Spencer-Attix cavity theory is extended to the case of a non-medium equivalent dosimeter wall for electron irradiation. An expression is derived for the case of a thick wall in which all the δ-rays depositing energy in the cavity are generated. An approximate expression is given for a new quantity S_med,wall,c^SA3 = D_med/D_c for an intermediate wall thickness, consisting of the weighted sum of the s-ratios for the 'thick' and 'thin' wall cases. This is cast in the form of a correction factor to the conventional S-A s-ratio and contains the quantities S_med,c^SA/S_med,c^BG, S_wall,c^SA/S_wall,c^BG and ε, the proportion of the δ-ray cavity dose from wall-generated δ-rays, for which an expression containing a cutoff energy related to the wall thickness, Δ_wall, is given. Values of these quantities are given for wall materials ranging from A-150 plastic (I = 65.1 eV) through water (I = 75.0 eV) to aluminium (I = 166 eV) for 5, 10 and 20 MeV electrons as a function of depth, Δ_wall and S-A cavity cutoff, Δ_c, derived from electron fluence spectra generated using the EGS4 user code FLURZ. The predictions of the theory are compared with an experiment involving graphite and aluminium walled chambers in a 20 MeV electron beam. The correct trend is predicted, but the closeness of the agreement depends critically on the values chosen for Δ_c and Δ_wall. Correction factors to the S_water,air^SA-values given in dosimetry protocols to be applied to two commercially available thimble chambers, with A-150 and C-552 walls, respectively, are given; these do not deviate from unity by more than 0.5% for 10 MeV electrons at the reference depth. The limitations inherent in this simple theory, such as its restriction to wall thicknesses that do not significantly disturb the primary electron fluence, and the neglect of δ-ray scattering differences between wall and cavity material, are discussed. The new theory should be viewed as a predictor of the order of magnitude of 'wall' effects rather than yielding exact numbers. (author). 34 refs, 9 figs, 7 tabs

[en] Poster presentation. 1 fig

[en] Radioactive implants in interstitial radiotherapy using the Paris system were initially to be for lines which were radioactive along the whole of their lengths. The use of 'source trains' made up of a set of individual sources a given interval apart led to study being made of the conditions for which discrete sources could replace continuous sources in the Paris system. A systematic study of the dose distributions around both types of source was carried out. The distributions were calculated, on a VAX 8600 computer using a program devloped by the Gustave-Roussy Institute, taking into account oblique filtration and attenuation by the tissues. The program makes it possible to calculate volumes within an isodose of a chosen value on the basis of a dose matrix with a basic grid size of 2 mm. The geometrical conditions for which the dose distributions round continuous sources and discrete series of individual sources are not significantly different, were calculated beforehand; this gave a simple equivalence rule between a continuous source of length L and total activity dK/dt_N and n individual sources of activity dK/dt_Ns with a spacing of s between them: L=n·s and dK/dt_N=n·dK/dt_Ns. On this basis, a more thorough study was made of the dose distributions using the volume-dose distributions for the main volumes used in the Paris system to describe radioactive implantation. The study made it possible to compare continuous sources with series of individual sources of equivalent length for some source devices of similar configurations to those used in clinical practice according to the rules of the Paris system. (author). 6 refs, 6 figs, 3 tabs

[en] Poster presentation. 2 refs

[en] To achieve brachytherapy efficiency in the clinical routine, some quality assurance procedures have been developed to control the dose distribution algorithm and the nominal parameters of ¹³⁷Cs low dose rate sources. Experimental methods, based on the dose measurements in air, in water and in perspex phantom by means of TLDs 100 LiF and an ionization chamber, are described. The source characterization is obtained by means of an optimization process to evaluate a physical parameter involved in the theoretical algorithm, and the dose measurement in a well defined point. (author). 7 refs, 4 figs, 1 tab