[en] A new IAEA Code of Practice for parallel-plate ion chambers complementing and updating TRS-277 is scheduled for β-testing within 1995. The working group is formed by P. Andreo (chairman), P. Almond, O. Mattsson, A. Nahum and M. Roos. Its initial contents was presented during ESTRO-94 (Granada), emphasizing the method of choice for calibration using high-energy electron beams. Alternative methods in ⁶⁰Co beams are also included in the protocol together with an extensive discussion on their limitations and increased uncertainty. These are mainly caused by the large variation of the factors k_m and p_wall for plane-parallel chambers of the same design and discrepancies between Monte Carlo calculated and experimental values. Perturbation effects in electron beams have received considerable attention, yielding changes in the previous outline. The present knowledge of correction factors for backscatter effects due to the different materials in commercial chambers is inconclusive and a recommendation on numerical values cannot be given. In consistency with the actual trends for calibrating ion chambers in terms of absorbed dose to water, both N_D,air and N_D,w formalisms are included. New symbols are adopted where the first subscript denotes the calibration quantity and the second is the medium where the quantity is measured; if needed a third index indicates the quality of the beam used for calibration. Adopting N_D,w it is no longer possible to sustain the concept of a global central electrode correction factor for cylindrical chambers as in TRS-277, as only the contribution arising during in-phantom measurements is relevant. The major consequence is that the N_D,air factor includes k_cel and therefore it is truly a factor characteristic of the ion chamber volume. The practical (numerical) consequences of these changes are almost negligible in most cases but it is a conceptual modification. Updated values for p_cel are included. Note that although the Code of Practice is addressed to plane-parallel ionization chambers, thimble chambers are used as reference for certain types of calibration

[en] In order to compute stopping-power ratios water/air for use in clinical proton dosimetry a Monte Carlo code has been developed. The main difference between the present code and other codes for proton transport is the inclusion of the detailed production of secondary electrons along the proton track. For this purpose the code is a Class-II type, where single proton-electron collisions yielding energy losses larger than a specific cut-off are considered individually. Proton multiple scattering is sampled from the complete Moliere distribution. To take into account in an approximate way the effect of inelastic nuclear collisions the fraction of the incident energy that is converted to kinetic energy of charged particles in the interaction is deposited on the spot. The energy that goes to neutral particles is assumed to leave the scoring geometry without any energy deposition. Stopping-power ratios are calculated in-line, i.e. during the transport, thereby reducing the uncertainty of the calculated value. The production and transport of the secondary electrons is used to determine an additional contribution to the stopping-power ratios obtained using the proton spectra alone

[en] Water is in all dosimetry protocols suggested as the standard phantom material in calibration of electron and photon beams. In some situations, especially for calibration in low-energy electron beams, solid plastic phantoms may be used. Corrections then have to be applied to account for differences in the electron fluence in water and the solid material. This correction is applied somewhat differently in the protocols. The IAEA protocol defines a correction factor h_m as the ratio of the signal measured with a plane parallel chamber placed at dose maximum in water to that measured at dose maximum in another phantom material, while the AAPM protocol defines Oe^water_med as the ratio of the electron fluence in water to that in a solid phantom. These differences have been often neglected in comparisons between experimentally and analytically determined values. The two definitions imply that different measuring geometries must be used for experimental determination of the ratios. Following the IAEA protocol, chamber dependent h_m-factors have to be used. The reason for this is that the ionization due to backscattered electrons will depend on the chamber construction. Many plane parallel chambers have thick back bodies and the backscatter will be typical for the chamber material instead of the phantom material. Also the front wall may have an effect on the ionization in the chamber cavity that may vary with phantom material. The fluence factor Oe^water_med, as defined by the AAPM-protocol, depends on the phantom material and can either be calculated or measured with a perturbation free chamber. This work presents experimental fluence correction factors according to the two definitions. Measurements were made in common solid plastic phantoms (polystyrene, PMMA, Solid water^TM and Plastic water^TM) with a large plane parallel ionization chamber. The chamber was used either in a nearly perturbation free geometry (AAPM) or in geometries simulating commercial chambers (IAEA)

[en] There are at least three reasons for having a good method to determine the beam quality in a photon beam. Firstly, the beam quality must be specified for each treatment field when used in the clinic. Secondly, for good characterisation of the beam in the dose planning system the beam quality must in some way be known. Thirdly, the beam quality is of importance when determining correction factors for different kind of dosimetry purposes. There are a number of methods available to achieve a measure of the beam quality. These can be divided into two groups, namely broad beam and narrow beam methods. In the first category TPR_(20(10)) is the most widespread one although other methods such as the depth of 80 % dose level, relative depth dose at 10 cm depth, D_10cm, and spectral reconstruction have been used. In the narrow beam category, at least two techniques have been proposed. One is based on a collimated narrow beam and the other is based on a narrow phantom system in a broad beam. A potential advantage with the narrow beam technique is that the beam quality variation across the beam can be determined, whereas the broad beam technique provides some sort of average beam quality for a 10 cm x 10 cm beam. The collimated narrow beam technique has been validated by both experimental and Monte Carlo based methods and used to determine the radial energy variation in a number of clinical beams. The size of this energy variation and potential errors introduced in different situations by disregarding this effect will be discussed. The TPR_(20(10))-method, the D_10cm-method, the spectral reconstruction method and the collimated narrow beam method can all be used to determine the water to air stopping power ratio. A comparison between these methods shows discrepancies in the order of up to 2 % in the high energy region as compared to experimentally determined s_w,air data

[en] Background and purpose: The IAEA/WHO TLD postal programme for external audits of the calibration of high-energy photon beams used in radiotherapy has been in operation since 1969. This work presents a survey of the 1317 TLD audits carried out during 1998-2001. The TLD results are discussed from the perspective of the dosimetry practices in hospitals in developing countries, based on the information provided by the participants in their TLD data sheets. Materials and methods: A detailed analysis of the TLD data sheets is systematically performed at the IAEA. It helps to trace the source of any discrepancy between the TLD measured dose and the user stated dose, and also provides information on equipment, dosimetry procedures and the use of codes of practice in the countries participating in the IAEA/WHO TLD audits. Result: The TLD results are within the 5% acceptance limit for 84% of the participants. The results for accelerator beams are typically better than for Co-60 units. Approximately 75% of participants reported dosimetry data, including details on their procedure for dose determination from ionisation chamber measurements. For the remaining 25% of hospitals, who did not submit these data, the results are poorer than the global TLD results. Most hospitals have Farmer type ionisation chambers calibrated in terms of air kerma by a standards laboratory. Less than 10% of the hospitals use new codes of practice based on standards of absorbed dose to water. Conclusion: Despite the differences in dosimetry equipment, traceability to different standards laboratories and uncertainties arising from the use of various dosimetry codes of practice, the determination of absorbed dose to water for photon beams typically agrees within 2% among hospitals. Correct implementation of any of the dosimetry protocols should ensure that significant errors in dosimetry are avoided

[en] In a recent study the absorbed dose was determined in a 170 MeV proton beam using seven different ionization chambers. The absorbed dose obtained with two IC-18 chambers, a chamber type commonly used as reference in proton beams, was found to be up to 1.5 % lower than that obtained with a Farmer NE-2571, when calibration factors in terms of N_K were used. When the IC-18 chambers instead were calibrated in a high-energy electron beam, using the Farmer NE-2571 as a reference, consistent results were obtained in the proton beam within the estimated experimental uncertainties. This finding questions the value of the product k_attk_m for the IC-18 chamber given by the IAEA Code of Practice which was used in this comparison, and points at possible chamber-to-chamber variations that theoretical k_attk_m factors cannot predict. This second conclusion is confirmed by the results obtained at a recent ionization chamber intercomparison between different proton therapy centers and was presented at the XXII PTCOG meeting

[en] Within the frame of our programme on realistic Monte Carlo calculations for dosimetry and treatment planning, the detailed simulation of the treatment head of radiotherapy accelerators has been focused on a MM50 Racetrack microtron. The project includes the improvement of bremsstrahlung target design and the effect of neutron production on the irradiation of patients with high-energy scanned beams. Most applications by other authors have so far used the EGS4 Monte Carlo system.; their implementation has been usually based on the coding of the specific geometry of each machine. Our project concentrates in using 'public' and standardized software tools for both geometry description and simulation to minimize manpower and economical requirements. Accelerator head structures are described using the Combinatorial Geometry (CG) package where 3D 'bodies' are combined using Boolean algebra; this enables an almost universal characterization that can be easily adapted to any accelerator treatment head. Graphical visualization is accomplished with the code SABRINA, which processes both CG (and specific inputs for the MCNP Monte Carlo system); SABRINA runs in almost every UNIX workstation. Simulations are performed with the ACCEPT code of the ITS 3.0 MC system, fully based on CG inputs and containing 'state-of-the-art' bremsstrahlung cross-sections; it also allows a direct implementation of the purging magnet in the MM50 head. Our interest in neutron production will consider the utilization of MCNP when a planned version containing the electron transport of ITS 3.0 becomes a reality. All the software is available in Europe through the NEA data bank. This presentation will describe the selected tools and actual status of the project. Support from K Van Riper (LANL) and R Kensek and J Halbleib (Sandia Labs) is greatly acknowledged

[en] The parallelism between existing air kerma formalisms based on the 'absorbed dose to air' chamber factor, N_D (or N_gas), and the more simple approach based on calibrations in terms of absorbed dose to water, N_w, is described. The importance of avoiding steps performed by the user that introduce avoidable uncertainties in the dosimetric procedure, is emphasized. Radiation beam quality factors normalized to ⁶⁰Co γ-rays have been calculated from a comparison between the two formalisms, and sets of tables produced for the different ionization chambers included in the 1987 IAEA Code of Practice. The calculated set of data is compared with existing experimental determinations at Primary Standard Dosimetry Laboratories, showing agreement within estimated uncertainties. Problems associated with the calibration of ionization chambers in terms of absorbed dose to water in high-energy photon beams are discussed. (author)

[en] Extrapolated ranges of electrons R_ex,t and R_ex,p have been determined from transmission and projected-range straggling curves, respectively. Data on the two kinds of curve have been obtained by Monte Carlo calculations for 0.1-50 MeV electrons incident on elemental absorbers of atomic numbers between 4 and 92. The two sets of extrapolated ranges, as well as another set, R_ex,q, determined from charge-deposition curves (Nucl. Instrum. Methods B 119 (4) (1996) 463), have been found practically the same. Appreciable differences of R_ex,t from R_ex,p and R_ex,q have been observed only for electrons of energies below 2 MeV incident on absorbers of the highest atomic numbers. The cause of these differences has been traced to the backscattering of electrons from the incident surface, phenomenon affecting only R_ex,t among the extrapolated ranges determined by the three methods. The fact that the extrapolated range is in most cases independent of the method of determination establishes the usefulness of this quantity. An analytic expression fitted to the Monte Carlo results of R_ex,t is given

[en] Within the IAEA structure, the Division of Human Health contributes to the enhancement of the capabilities in Member States to address needs related to prevention, diagnosis and treatment of health problems through the development and application of nuclear and radiation techniques within a framework of quality assurance. In view of the increasing cancer incidence rates in developing countries the activities in improving management of cancer have become increasingly important. This review will outline the IAEA's role in cancer management focusing on activities related to improving radiotherapy worldwide