An algorithm to include the bremsstrahlung contamination in the determination of the absorbed dose in electron beams.

None of the existing protocols or codes of practice for high-energy electron dosimetry take any account of the accelerator-generated bremsstrahlung always present in electron beams. This results in a systematic error in the derivation of the absorbed dose. The purpose of this study is to draw attention to this omission which affects the absorbed dose calibration. A method based on available experimental data is presented for dealing with this deficiency in electron dosimetry. A re-defined algorithm for absorbed dose derivation accounting for this bremsstrahlung component is proposed. The question of omission of the bremsstrahlung contamination is important in comparing ionization methods with other dosimetric methods such as calorimetry or the use of ferrous sulphate.

Polarity effect of plane-parallel ionization chambers in electron radiation.

Although plane-parallel ionization chambers have been in use for some time, there is still much to be learned about their performance characteristics. This work is concerned with the polarity effect in electron beams about which there is little published data. The investigations involved several popular ionization chambers; the PTW Markus chamber, the NACP chamber and its Calcam version, the Vinten-631 chamber, and the NE 2571 (Farmer-type) thimble chamber. The chambers were irradiated in electron beams of nominal energies between 4 MeV and 18 MeV. It was found in this study that the NACP, Markus and Vinten chambers require a correction of the order of 0.2% in the energy range between 4.5 MeV and 18 MeV. The overall behaviour of the Calcam chamber was similar with the exception of energies below 4 MeV. The depth dependence of the polarity effect seemed closely related to the mean beam energy at the depth of measurement. There is some evidence that the effect is also dependent on the angular spread of the electron beam and its spectrum. The authors considered how best to quantify the polarity effect practically, and propose that it should be expressed as a correction factor to be applied to readings with one particular chamber bias.

Ion recombination corrections for plane-parallel and thimble chambers in electron and photon radiation.

The aim of this work was to investigate the collection efficiency of several popular ionization chambers used in high-energy electron and photon dosimetry. The recombination effect was evaluated in plane-parallel type ion chambers (Markus, NACP, Calcam and Vinten-631) and in a thimble-type chamber (NE2571). The chambers' response was studied in a pulsed accelerator beam, for both photons and electrons, and in continuous radiation from a 60Co machine. The study involved both theoretical and experimental evaluation of the recombination effect utilizing the methods described in the literature. The main conclusions to be drawn from this work are as follows: (i) For the Farmer NE2571 0.6 cm3 chamber used here, the correction factors required are of the order of 0.1% for 60Co radiation, 0.5% for 5 and 8 MV x-rays and 10 and 12 MeV electrons and 0.9% for 15 MV x-rays and 15 and 18 MeV electrons, all at typical clinical dose rates of 200-400 cGy min-1 (0.017 cGy pulse-1). (ii) The recombination effect for the plane-parallel chambers was found to be smaller than for the thimble chamber, with values of < 0.1% in continuous radiation and 0.2-0.4% in pulsed radiation.

A dosimetric intercomparison of megavoltage photon beams in UK radiotherapy centres.

A dosimetry intercomparison has been carried out for all 64 radiotherapy centres in the UK. Doses were measured with an ionization chamber in an epoxy resin water-substitute phantom of relatively simple geometry. Reference-point measurements were made for all MV photon beams. For 61 Co-60 beams, a mean ratio of measured-to-stated dose of 1.002 was observed with a standard deviation of 0.014, whilst for 100 MV x-ray beams, the corresponding figures were 1.003 and 0.015. 97% of beams lay within a +/- 3% deviation. One measurement was instrumental in discovering a large discrepancy. Doses were also investigated in two planned three-field distributions at one beam quality in each centre. One of these was in a homogeneous phantom, whilst the second included a lung-equivalent insert. Doses were measured at the central point and at four other points in the high dose volume. In both situations, the mean ratio of measured-to-calculated doses for all points was 1.008, with standard deviations of 0.027 and 0.035 for the uniform and non-uniform phantoms, respectively. Discrepancies over 5% were followed up. The work must be viewed in the context of other international intercomparisons and is an essential part of wider radiotherapy audit processes.

Determination of absorbed dose in high-energy electron and photon radiation by means of an uncalibrated ionization chamber.

The aim of this study was to develop a dosimetric method based on an ionization chamber which has an uncalibrated sensitive volume but which behaves as a Bragg-Gray cavity in high-energy radiation. The new type of chamber developed in the course of this study has a variable volume and is constructed from water-similar materials. It can be used in a water phantom directly in a beam of a therapy megavoltage machine under clinical conditions. The chamber allows absorbed dose to be determined from first principles, overcoming many of the problems encountered with conventional dosimetry based on calibrated chambers. The study involved an intercomparison of the performance of the new chamber in high-energy electron and photon radiation with the conventional calibrated chambers employed according to the established dosimetry protocols. Good agreement was found between these dosimetric methods and it may therefore be concluded that the method developed in this work can be successfully employed for absolute dosimetry. The new chamber is a promising device for research in various aspects of dosimetry.

Electron backscattering. Implication to electron dosimetry.

Plane-parallel electron chambers have structures made of plastic materials of low atomic number which do not provide the same scattering into the sensitive volume as water and suitable correction factors are therefore required to allow for this effect. Hunt et al. (1988) measured electron backscatter in low atomic number materials and this data was, in this work, analyzed together with the experimental data obtained for higher atomic number materials published earlier (Klevenhagen et al., 1982). It was found that both sets of data correlate well and fit to an empirical exponential expression for which the necessary calculation parameters were derived. This expression enables suitable backscatter correction factors to be calculated for materials of atomic numbers between 4 and 82, including plastics, and electron energies between 3 and 35 MeV. The deficiency of backscatter was evaluated for the most popular materials used in the construction of electron chambers and this data may be used for corrections.

An afterloaded applicator for the interstitial irradiation of brain tumours.

A stereotactically implanted afterloaded system for the interstitial radiotherapy of brain tumours is described. The technique has the advantages of accuracy of dosimetry, no radiation hazard in the operating theatre and ease of afterloading with a source which may be inserted and removed at will. Caesium-137 is used, but the system could equally well be used with other isotopes.

Backscattering in electron beam therapy for energies between 3 and 35 MeV.

Whenever a heterogeneity is present in an electron beam treatment field during radiotherapy, there is the possibility of tissue overdosage at the tissue-heterogeneity interface due to electrons backscattered from the heterogeneity. Measurements of this effect were made in a polystyrene phantom using a purpose-built thin-window parallel-plane ionisation chamber. Materials of various atomic numbers were used as scatterers and the investigations were made over a wide range of electron energies. Electron backscatter factor (EBF), defined as the ratio fo dose at the interface surface with and without the scatterer present, was found to increase with increasing atomic number and decrease with increasing beam energy. Both of these relationships were found to be non-linear. The EBF dependence on the scatterer thickness was also investigated. All data in this work were expressed in relation to the beam energy incident on the scatterer in preference to the nominal beam energy set on the accelerator. This approach enables the dose enhancement at an interface to be predicted from a knowledge of the heterogeneity (atomic number and thickness,), its depth in tissue and the beam energy being used for treatment. The results of this work were compared with the published data and an explanation is offered to account for the difference.

Use of the Curietron at The London Hospital.

The Curietron is a remotely controlled afterloading machine for gynaecological low dose rate intracavitary radiotherapy. The Curietron and its installation are described together with specially designed applicators intended to produce a source geometry similar to the Manchester configuration. Some of the practical problems met are discussed. The principal value of the Curietron lies in the marked reduction of radiation exposure to all staff concerned.

Behavior of p-n junction silicon radiation detectors in a temperature-compensated direct-current circuit.

The behavior of silicon p-n junction radiation detectors used in the direct-current short-circuit mode without bias was examined under such load resistance which ensures operation in a temperature-compensated state. The objective was twofold; to check whether the compensation is achieved and to investigate the extent of compensation shown at temperatures other than that initially selected as the operation point. It was found that the detector performance in various thermal conditions can be predicted from a knowledge of the behavior of the individual detector and circuit parameters and that it is possible to stabilize the detector response within +/- 2% over a relatively wide temperature range: 18 degrees-40 degrees C. However, in the case of devices which show thermal currents of large temperature sensitivity, compensation at small and at large dose rates needs to be considered separately.

An experimental and calculated dose distribution in water around CDC K-type caesium-137 sources.

The radiation distribution in water around caesium-137 K-type sources has been measured and the experimental results used to provide data for an expression for dose calculations which may be conveniently applied in computer programs. The calculated absorbed dose rate obtained in this manner is estimated to be within 3% of the actual dose rate for any point in water up to 8 cm from the source. It is also suggested that the strength of a brachytherapy source be expressed in terms of an experimental exposure rate at some well-defined distance since this quantity may be determined more precisely and with less ambiguity than source activity.