David Greene

AN EXPERIMENTAL UNIT FOR FAST NEUTRON RADIOTHERAPY.

Abstract The possible application of 14 MeV neutrons produced by the deuterium-tritium reaction to radiotherapy is discussed. The basic neutron generator is a sealed tube device developed by the Services Electronic Research Laboratory and this is described, as is also an associated shielding and collimating system. It is shown that the neutron beams produced by this device are adequately defined for radiotherapy. The dose-rate at the end of the collimator is equivalent to about 5 rads/minute of X rays. Percentage depth doses for a limited range of field sizes have been measured at 50, 75 and 100 cm SSD, and the values are intermediate between these for 250 kVp X rays and 60Co γ rays. It is shown that there is a slight “build-up” effect at the surface, extending to a depth of 2 mm.

Isodose Curves in Non-uniform Phantoms

A method of using an isodose plotter to produce isodose charts in a water phantom containing bone, lung-equivalent material, or air cavities is described. It is shown that for 60Co γ rays and 4 MV X radiation standard isodose charts can be modified to fit the experimental results by the use of a simple set of rules to an absolute accuracy of about 2 per cent. Some isodose charts for conventional quality radiation in non-uniform phantoms are also discussed, but here the effects produced by the non-uniformity are more complex than with megavoltage radiation and corrections derived from simple rules are of limited value. In the second part of the paper, the rules for modifying megavoltage isodose charts are used to prepare some treatment plans for real body sections, and these are compared with conventional treatment plans where it is assumed that the body section consists entirely of soft tissue. It is shown that there is probably no clinically significant difference between the conventional and corrected d...

The Use of a Linear Accelerator “Simulator” in Radiotherapy

In this hospital it is the usual practice, when tumour localisation procedures have been completed, for the initial “set up” of a radiotherapy treatment to be done by the radiotherapist. This may involve either the final inspection and fitting of the patients mould, or the application of skin marks to specify the fields to be applied. Such marks may include the placing of the skin point of reference for a pin-and-arc treatment, or the field outline (including the outline of any organ to be shielded) in a regional treatment. In a large field treatment it may also be necessary to make a decision about the SSD to be used, while for pin-and-arc treatment on an isocentrically mounted machine it will be necessary to measure the SSD and the mean angle of incidence for each field. When each field of a treatment has been set up it is useful to be able to take a radiograph with the therapy beam as a final check on its accuracy. In so far as the ultimate success of the treatment depends on these processes, it is im...

Some measurements on the absorption of 4 MV x rays in concrete.

Measurements have been made on the absorption, in concrete, of the primary and secondary radiation from a 4 MV linear accelerator. The results give tenth-value layers for primary and secondary radiation of 12 in. and 9 in. respectively, in concrete of density 2.2 g/c.c. The distribution of tube leakage radiation, and of secondary radiation scattered from a phantom in the beam have also been examined, and it is shown that scattered radiation is only significant in the forward direction.

A Study of the Potential Value of High Energy Electron Therapy in Comparison with Megavoltage X-ray Therapy

The procedure followed in this paper has been to take a series of typical lesions and to consider how they might be treated with 4 MV X rays, or with electrons whose energy could be varied up to a maximum of 35 MeV. The resultant treatment plans are discussed and compared. It is concluded that, for lesions of the head and neck, similar field arrangements are required for electron and X-ray treatments, and that the resultant combined isodoses are very similar, while for deep lesions of the trunk, the X-ray treatment plans are much better than those for electron treatments. These conclusions are slightly weighted in favour of the electron treatment, because here the combined isodoses are based on idealised isodose charts which ignore beam edge effects. On the assumption that high energy electrons have no specific biological advantage over X rays, it is concluded that electron beam therapy has no advantages over megavoltage X-ray therapy.

Performance of a linear accelerator in clinical service.

The 4 MV linear accelerator (Miller, 1953, 1954) at this Centre has now been in routine clinical service for five years. In view of the increasing use of this type of equipment, it may be of interest to give an account of its technical performance, and of the arrangements that have been adopted for routine maintenance. It should be stated that this was a prototype machine and some of the faults to be mentioned arose from this fact. The main facts about the performance of the machine are summarised in Table I. This gives, for each year, the number of hours for which the machine produced X rays (“output hours”), the number of exposures made, the number of hours for which the valve filaments were switched on (“filament hours”), the number of days on which it was not possible to produce X rays (“days lost”), and the number of valves replaced. These facts may be placed in context by quoting the numbers of patients treated and the total numbers of exposures given. In the five years there were only eight days wh...

Observations on the Effect of Chamber Size on Measurements at the Edge of an X-ray Beam

Measurements have been made with a 3 mm diameter and a 6 mm diameter chamber to determine the dose distributions at the beam edge for (1) 300 kVp radiation in air, under conditions of known geometry; (2) for 300 kVp radiation in a water phantom where the beam was defined by a standard applicator; (3) for 4 MV radiation in a water phantom. For the “in air” experiments it was possible to compare the geometrical penumbra, the calculated response of the chambers to this dose distribution, and the measured dose distribution. For a 20 MV beam the dose distribution at the beam edge, as measured by the 6 mm diameter chamber, has been compared with that determined by measuring induced activity in a copper strip irradiated in the same position. For the “in air” observations reasonable agreement was found between the calculated and experimental results. For the measurements made in water it was found that the two measuring systems used always gave the same information and, therefore, that the resolving power of the ...

Dosimetry intercomparison between UK and Japanese institutes

In April 1983, an exposure meter from the Christie Hospital was taken to Japan to make an intercomparison of exposure measurements for 60Co gamma rays with the Japanese centres listed in Table I. The exposure meter was a field instrument, a Farmer ionisation chamber with a home-made electrometer which had been intercompared with the local secondary standard (Nuclear Enterprises Type 2561). The secondary standard had been calibrated at the National Physical Laboratory (NPL) in January, 1983. After the intercomparison in that same month, the constancy of the field instrument was checked weekly, using a 90Sr source, and this was continued after the instrument had been brought from Japan.

The Design of a Treatment Room to House a Gantry-mounted 4 MeV Linear Accelerator

The logical development of an optimum design for a treatment room to contain a 4 MeV linear accelerator is presented. Although this design was developed around a particular machine (the A.E.I. “Orthotron”), it is basically applicable to other comparable megavoltage X-ray units intended for treating large numbers of patients. Topics considered include access to the treatment room, patient viewing system, and detailed specification of protective barriers.

II—Physical Aspects

In the previous symposium on the relative biological efficiency of 4 MV X rays, this term was defined as the ratio: The purpose of the present series of experiments is to compare the efficiencies of 4 MV and 20 MV radiation. The relative biological efficiency in this case is defined by the ratio: In accordance with this definition, it was necessary to deliver a series of doses, measured in rads, to the various biological specimens, using 4 MV and 20 MV radiations. As an interim measure, a series of equal ionization doses was delivered at the two qualities, and these doses were subsequently converted to rads. This was possible because the R.B.E. as here defined does not require that equal doses be delivered at the two qualities. As is required in experiments of this type, all variables, oter than that being investigated, the radiation quality, were made the same for the to machines. There is one exeption to this statement, namely, that the 4 MV linear accelerator operates at 350 pulses second, while the 20...