Table of contents

By measuring the transient changes in each of the polarised components of fluorescence during the application of a pulsed electric field to a solution of macromolecules, a method has been developed for evaluating the binding characteristics of fluorescent chemotherapeutic agents to DNA. It is shown that whereas quinacrine and berberine intercalate the DNA helix, hydroxystilbamidine does not. Furthermore, measurements on both the native and the diol-epoxide forms of benzo(a)pyrene show that the former is consistent with an intercalation type of binding, whilst the latter appears to be more inclined to the major DNA axis and may possibly be associated with the external helical grooves. The significance of these findings is discussed.

Experiments are presented on the fluorescent properties of haematoporphyrin-derivative in vitro and in an animal tumour. The experiments show that, firstly, an argon laser can be used as an effective excitation source and, secondly, that the in vivo fluorescence spectrum is different from that in vitro.

The possibility of subcutaneous tumour detection of haematoporphyrin-derivative (HPD) and light has been studied by calculations on a model. The tumour model consists of a skin layer and a (subcutaneous) tumour layer. The Kubelka-Munk theory (1948, 1954) has been applied to estimate the HPD fluorescence intensities from the two layers. Literature values have been used for the optical parameters of the tissues. The results are that the depth at which a subcutaneous tumour can be detected is almost independent of the excitation wavelength and the tumour thickness. Only superficial tumours (less than 2.3 mm subcutaneously) can be detected as HPD fluorescence from the overlying skin tends to become more intense than from the tumour.

A new approach to calculating the transmission and reflection of ultraviolet radiation from skin is described. The method is based upon a numerical solution of the equation governing radiation transport in matter. By comparing the calculations with experimental measurements of the optical properties of human skin described elsewhere, it has been possible to derive absorption and scattering coefficients for stratum corneum. The model has been used to investigate the photoprotective function of the thickness and melanin content of stratum corneum, and yields results which concur with experimental studies on the ultraviolet optics of stratum corneum carried out by other workers.

The per cent ionisation curves for the 4 MV photon beam from a Clinac 4 exhibit pronounced enhancement with field size in the build-up region. The source and nature of this increase is investigated by analysing the radiation components of the 4 MV photon beam with a 0.23 T magnetic field. The build-up ionisation curves were measured at 86 cm SSD with a parallel-plate ionisation chamber embedded in a polystyrene phantom. The surface ionisation increases from 18% to 32% as the field size is increased from 4.3 cm*4.3 cm to 9.7 cm*32.3 cm. With the electromagnetic placed beside the treatment head, the field size dependence reduces from 16% to 24%. This and similar results at 113.2 cm SSD clearly demonstrate that electrons from the Clinac 4 treatment head are responsible for approximately half the observed increase in the surface ionisation with field size.

The method described enables isodose distributions and output factors of treatment fields can be predicted with good accuracy, without the need for any dose measurement in the actual field. A Gaussian pencil beam model is employed with two different pencil beams for each electron beam energy. The values of the parameters of the pencil beam dose distributions are determined from a set of measurements of broad beam distributions; in this way the influence of electrons scattering by the applicator walls is taken into account. The dose distribution of electrons scattered from high atomic number metal frames, which define the treatment field contour at the skin, is calculated separately and added. This calculation is based on experimentally derived data. The method has been tested for beams with 6, 10, 14 and 20 MeV electron energy. The distance between calculated and measured isodose lines with values between 10 and 90% is under 0.3 cm. The difference between calculated and measured output factors does not exceed 2%.

The variable energy cyclotron of the Catholic University of Louvain is used to produce intense neutron beams for neutron therapy purposes. As a first step, neutrons were produced by bombarding a Be target with 50 MeV deuterons; at present they are produced by 65 MeV protons. This paper describes the improvements to the target system. A new (17 mm) Be target with the old (10 mm) Be target are inserted in a movable support which allows the production of neutrons either by 65 MeV protons or by 50 MeV deuterons. Both targets can be removed for proton beam therapy. The dosimetric characteristics of the p(65)+Be and d(50)+Be neutron beams are compared: dose rate, gamma -contribution, depth dose and room activation.

The radiation treatment of brain tumours with ¹²⁵I seeds requires a precise knowledge of the entire three-dimensional dose distribution of the seed. Assuming a point source distribution modified by an anisotropic factor, the absorbed energy is calculated and compared with measurements obtained using ⁷LiF thermoluminescence dosemeters. The measurements are carried out in air and in Mix-D. A mean relative difference of 5% is found between calculation and measurement of the dose perpendicular to the seed axis. It is concluded that the entire three-dimensional of absorbed dose in tissue can be determined by a simple formula, which enables the calculation of absorbed dose at any point of interest with an uncertainty of 6%.

Central-axis depth dose data for diagnostic qualities (1-4 mm Al HVL) have been used to calculate the total energy imparted to patients of phantoms during X-ray examinations. A 'saturated scatter' method has been used and allowance is made for the finite dimensions of the patient by using PPD data for large but finite field sizes. For the purpose of integration, PDD data have been represented as a function of depth by two exponential components. Data in the form of energy imparted per unit central-axis surface dose and per unit surface X-ray field are tabulated as functions of kV_p, first HVL, phantom thickness and the field size of the original PDD data. Comparison with energy fluence calculations of total energy imparted made by other workers indicates good agreement under similar phantom and X-ray quality conditions, although it is noted that anatomical variations between patients will in practice lead to uncertainties in the total energy imparted.

The bone mineral content (BMC) of the spine can be measured from the attenuation of two photon beams. An equation was derived for the theoretical precision of BMC measurement and validated experimentally. Though ¹⁵³Gd was found to possess the better energy combination, the precision due to the photon energy levels using ²⁴¹Am, ¹³⁷Cs was only 1.5 times the value obtained from ¹⁵³Gd. The available photon outputs from ²⁴¹Am and ¹⁵³Gd were investigated and with a 12 mm diameter disc source a theoretical precision of 1% could be obtained with source strengths 20 GBq ²⁴¹Am or 5 GBq ¹⁵³Gd in a 40 min and 20 min patient scan respectively. There was no great advantage in either energy combination when the problems of fat or patient dose were considered. Given the disadvantages of ¹⁵³Gd, its cost and availability, ²⁴¹Am, ¹³⁷Cs can be used as a practical alternative to measure spine BMC from a reconstructed bone mineral image.

The diagnostic utility of proton (¹H) nuclear magnetic resonance (NMR) imaging is greatly enhanced when the image is weighted with proton relaxation time information. The authors indicate factors affecting the accuracy of such measurements (especially with respect to T₁ determination) and, specifically, the accuracy of one approach which has been used in their laboratory is assessed. A brief discussion pertaining to the validity of data obtained from biological material is presented.

In functional imaging (Kaihara et al. 1969, MacIntyre et al. 1970) the temporal variation in count-rate associated with each image element of a dynamic study is described by a few indices. By displaying the index value at the same spatial location as the count-rate information from which it was derived, data from a large number of image frames forming a dynamic study can be compressed into a small number of images. At the present time preliminary studies suggest that the indices the authors calculate provide an accurate measure of hepatogram shape and that one or more will show appreciable change in different pathological conditions thus providing a method for distinguishing between them. An extended study is required to define those ares in which it may improve the diagnostic accuracy of cholescintigraphy.

An increasing number of radiotherapy departments are being equipped with high-energy accelerators which have a facility for electron beam therapy, usually with a range of electron energies available. The aim of the device described is to enable the radiographers using the treatment unit to carry out a simple check on the electron energy, whilst making the necessary trial run after changing energies.

A paper by Y.S. Horowitz and A. Dubi (see ibid., vol.27, p.867, 1982) which proposed a modification of Burlin's general cavity theory for photons is commented on. The author feels that Horowitz and Dubi's paper contradicts some well established principles in cavity theory.

A report by M.A. Smith and P. Tothill (see ibid., vol.27, p.1515-21, 1982) is commented on. That report suggested, without evidence, that intra-laboratory variation in measurements of the bone mineral of the spine may be due to a technical problem termed 'crossover'. The authors feel that Smith and Tothill are to be commended for drawing attention to the matter of crossover, but also that they have misrepresented the procedures of others and have alluded to intra-laboratory differences where none have been shown to exist.

A letter by R.B. Mazers et al. (see ibid., vol.28, no.6, p.747-8, 1983) is commented on. That letter commented on a scientific note by the authors (see ibid., vol.27, p.1515-21, 1982) in which the authors stated that there were intra-laboratory differences between cross-sectional studies of vertebral bone mineral content in normal women as measured by dual-photon absorptiometry. The authors say that the letter by Mazers et al. provides further justification for their belief that technical problems in dual-photon absorptiometry need to be explored.