[en] In the calculation of monitor units and treatment times of MV photon beams the head scatter factor S_c accounts for the change in photon fluence when the collimator setting and thus the field size is changed. S_c can be measured with a narrow beam-coaxial phantom [1] as a function of the settings of the X and Y collimator blocks. Although S_c is expected to have a symmetrical nature S_c turns out to be an asymmetrical function of these settings. Elongated fields in X and Y directions can have S_c values which differ several per cent. In order to obtain sufficient accuracy usually S_c is measured for a matrix of field sizes of for instance X=2 up to 40 cm and Y=2 up to 40 cm. The reason of this asymmetry originates from the fact that the X and Y blocks are located in different planes. So the aperture - looking upstream from the measurement point to the blocks - is an asymmetrical function of the collimator setting. Our approach is to correct the asymmetry in S_c by considering S_c as a function of the corrected field dimensions either C_fX and Y or X and C_fY. The factor C_f has a value between 0.0 and 1.0 depending on the beam energy and the accelerator design. An iteration algorithm based on the Clarkson approach is used to calculate a best fit C_f value from S_c values of square and some elongated fields. Effectively the S_c matrix is reduced to a one dimensional function S_c(r), with r the average radius of fields with dimensions X=C_fa and Y=a or X=a and Y=C_fa. The algorithm is applied to calculate S_c of rectangular shaped fields. The accuracy of the method, based on one single - a priori - unknown, parameter C_f is well within measurement accuracy (0.5 %). Besides, a substantial reduction of the number of S_c measurements, necessary to cover the clinically used fields, is achieved this way. The method is validated for symmetrical beams with energies ranging from 6 to 25 MV of accelerators of several manufacturers and also for asymmetrical wedged fields of the 6 MV beams of a Philips SL15 and a Saturne-41 machine

[en] In a number of cases it is necessary to estimate the radiation dose to organs far away from the target volume of a patient receiving radiotherapy, e.g. the dose to the gonads or to the fetus of a pregnant patient, often called the peripheral dose. For an accurate calculation of the peripheral dose due to brachytherapy, the tissue scatter and absorption as a function of distance r, T(r) needs to be taken into account. Historically, T(r) data from measurements and Monte Carlo calculations have been fitted to various algorithms (Meisberger, van Kleffens, and others). These calculational methods are, however, inappropriate in the range of distances above 10 cm. In the present work, results of measurements and EGS calculations are reported to determine the dose at distances up to 60 cm from Cs-137, Ir-192 and Co-60 sources. The influence of the size of the phantom was investigated. Several mathematical models to fit the data were investigated. The model of Kornelsen and Young, T(r) = (1 + k_a.(μ.r)^kb).exp(-μ.r), proved to give the best fit to the results. Parameters for this model were derived for the three isotopes. The total reference air kerma (TRAK), which is the sum of the products of the reference air kerma rate of the sources of an implant and the duration of the application, can then be used: with a simple application of the inverse square law, neglecting effects from patient size and tissue inhomogeneities, a good approximation of the dose-in-patient in the range 10 cm < r < 60 cm is obtained from: D(r) = TRAK ^* (100/r)^2* T(r). A few experiments in a Rando Alderson phantom are presented to support the measurements and the calculations

No abstract available

[en] Eight cardiac pacemakers were irradiated in a cobalt-60 beam. Two out of six demand-type pacemakers showed an alarming decrease in pulse repetition frequency when irradiated to dose levels that are used in radiotherapy. Two modern programmable pacemakers showed a failure at a dose of 97 and 147 Gy, respectively. The dose levels at which these failures occurred were low enough to recommend that cardiac pacemakers should always be kept outside the radiation beam. The signals induced by electromagnetic interference (EMI) from two linear accelerators were measured using a simulation model of a pacemaker. In the laboratory, 22 modern-type pacemakers were tested with these signals to determine the sensitivity for the electromagnetic fields in the treatment rooms. It was observed that an inhibition of one pacemaker pulse was to be expected on one of the two linear accelerators when switching the machine on and off. No permanent effects were found. These findings resulted in the recommendation in our department not to use this treatment machine for radiation therapy of pacemaker-bearing patients. (orig.)

[en] Using a minimal set of measured data, the collimator scatter correction factor of an asymmetrical collimated rectangular field (X₁,X₂;Y₁,Y₂) can be calculated from the product of one-dimensional factors, in combination with a correction term: S_c(X₁,X₂;Y₁,Y₂)=S_cx1(X₁)S_cx2(X₂)S_cy1(Y₁)S_cy2(Y₂)+cδ(X;Y). Two forms of the function δ(X;Y) were investigated. (Copyright (c) 1999 Elsevier Science B.V., Amsterdam. All rights reserved.)

[en] In radiotherapy simple procedures for the calculation of dose or monitor units (MU) are required, for example, for direct (fixed SSD) beam irradiation techniques. It can also be used as independent checks of the calculations performed by the more sophisticated treatment planning (TP) systems. The algorithms used in MU calculations must comply with accuracy requirements comparable with those for TP systems. This thesis deals with the accuracy achievable in general in dose calculations for megavoltage photon beams. In the first section, the dose calculation procedure is discussed as part of the full dosimetry chain, starting with beam calibration up to the clinical dose delivery. The different aspects of a MU calculation program are discussed from the viewpoint of a separation of the dose variation with field size into a head scatter and a phantom scatter component. For this purpose, the parameters are defined at the reference depth in the phantom of 10 cm, while for the measurements a mini-phantom is used. A consistent formalism for MU calculation is described. In the second section several aspects of a calculation procedure, such as the concept of equivalent squares, the dose variation due to the presence of blocking trays, and the influence of asymmetric field setting on the head scatter component are dealt with. The third section of the thesis deals with the quality assurance of treatment planning systems. An extension of the AAPM TG 53 test set is proposed, while the results of its application to seven commercially available TP systems are presented. Finally, tolerances to be used with a comparison of dose calculations with measurements in different clinical situations are discussed

[en] Collimator scatter correction factors S_c have been measured with a cylindrical mini-phantom for five types of dual photon energy accelerators with energies between 6 and 25 MV. Using these S_c-data three methods to parametrize S_c of square fields have been compared including a third-order polynomial of the natural logarithm of the fieldsize normalised by the fieldsize of 10 cm². Also six methods to calculate S_c of rectangular fields have been compared including a new one which determines the equivalent fieldsize by extending Sterling's method. The deviation between measured and calculated S_c for every accelerator, energy and all methods are determined resulting in the maximum and average deviation per method. Applied to square fields the maximum and average deviation were for the method of Chen 0.64% and 0.15%, of Szymzcyk 0.98% and 0.21%, and of this work 0.41% and 0.10%. For the rectangular fields the deviations were for the method of Sterling 1.89% and 0.50%, of Vadash 1.60% and 0.28%, of Szymczyk et al. 1.21% and 0.25%, of Chen 1.84% and 0.31% and of this work 0.79% and 0.20%. Finally, a recommendation is given how to limit the number of fields at which S_c should be measured

[en] Mailed dosimetry, using thermoluminescent dosimeters, can play an important role in quality assurance procedures in radiotherapy. In 1989 a pilot study was started with the main aim to show feasibility of this method for the multicentre EORTC trial 22881 on the conservative management of breast carcinoma. 2 anthropomorphic breast phantoms and 6 breast carcinoma-patients were irradiated according to prescription of the protocol. TLD measurements of the entrance and exit dose were performed in 6MV tangential X-ray beams. It proved to be possible to correlate the dose measured in the entrance and exit points of the beams to the calculated dose closely under the surface. A thickness of at least 5mm bolus material must be applied over the dosimeters and a distance of at least 3cm from the lateral and medial field borders must be maintained in order to reach a clinically acceptable accuracy in the measurements. (author). 5 refs.; 1 fig.; 2 tabs

No abstract available

[en] Thirteen radiotherapy departments in the Netherlands and Belgium participated in an intercomparison of the calibration procedures in use for the quality control of iridium-192 high dose rate sources. A measuring device from 1 of the departments was taken to the 12 other participants, where 1-3 sources were measured by both the local and the visiting physicist's team. Also, 1 ¹⁹²Ir HDR reference source was taken to 7 other - Dutch - departments to serve as a stability check of the measuring device. The results of the local calibration were compared with the values obtained by the visiting team. A mean deviation of +1.3% was found in 19 observations, with values ranging from -0.4% to +3.0%. In total, 16 sources were checked. The measured reference air kerma rate of the available sources was compared with the value stated on the source calibration certificate: the values on the certificate were on the average +1.8% higher than the measured values, ranging from -4.2% to +6.8%. In 1 instance the reference source was also measured by The Netherlands Measurements Institute (NMI), the national standards laboratory in the Netherlands. A good agreement was found between the NMI calibration and the reference air kerma rate determined with the measuring device used in this project