Die kliniese dosismeting by megavolt lineere versnellers

[摘要] This study deals with the characteristics, dosimetry andapplications of linear accelerators. Firstly the theoryand operation of a linear accelerator is discussed, fora detailed knowledge of its operation is essential foran understanding of the parameters which may influencethe electron and X-ray beam characteristics.The modern linear accelerator uses a specially designedwaveguide. Electrons generated by an electron gun, areaccelerated by means of a pulsed radio frequency electromagnetic wave. This RF-wave of the TM 01 mode may be generated by various devices like magnetrons and klystrons.The frequency of the R.F. generator has to be regulated towithin close limits of the tuned waveguide frequency to ensure that the electrons always have the same energy.Another important component of a linear accelerator isthe bending magnet. The most common type of magnet in usetoday is a 270° bending magnet which seems to be superiorto the 900bending magnet. In the case of the 90° magnet,the beam deviation due to small variations in magnet current, is much more pronounced than in the case of the 270°magnet.Further, the linear accelerator produces electrons andX-rays at energies which are much higher than the y-rays ofCobalt units. Therefore the main essentials of thetheory of X-ray and electron interaction with matter,as well as dosimetrie techniques are described.The results obtained for the Mevatron 6 and Mevatron 8linear accelerators in present use at the Institute ofRadiation and Isotopes, Bloemfontein are presented. Avery important observation was made regarding the determination of electron energies. The size of the dose chamber is extremely important and the diameter of the chamber has to be as small as possible. The most suitableinstrument for this purpose was found to be an extrapolation chamber, such as the product manufactured by S.H.M.Nuclear. To comply with the international definition ofthe roentgen and rad, the Baldwin Farmer 0,6 cc chamberis still used to measure the absorbed dose with the appropriate CE factors.A new method for the treatment of Mycosis Fungoides hasbeen developed and described. The accelerator had to bemodified to be able to deliver a beam suitable for wholebody irradiation. Previous to this method, the patienthad to be treated by thirty or more fields to cover thewhole body. Various problems were experienced where thefields were joined. The new method uses the well knownStanford technique of two fields angled at plus and minusfifteen degrees to the horisontal direction. In this waya flat field (± 2,5%) could be generated. It was also foundthat a sixfield technique with the fields spaced equallyaround the body, gave the best results. The various aspects of the dosimetry is described, as well as the safety measures employed to protect a patient from accidentalover exposure. This is a very real problem since the linearaccelerator has to produce a dose-rate of approximately 6500Rads per minute at one metre from the focus to deliver adose-rate of 100 Rads per minute at five metres where thewhole body irradiation is done. It was found that the doseratefor electrons in air varies with distance according tor-2,59, where r is the distance from the source.The whole body irradiation procedure led to a study ofthe physical properties of electron beams. The beams employed in radiotherapy can be classified as broad beams.No mention of broad beam electron scattering could be foundin the literature. A detailed study was therefore made ofthe scattering parameters for broad beams. The stoppingpower for electrons in air was determined for the electronenergies in normal use at this institute. These resultswere compared to the values predicted from the theory ofelectron pencil beam scattering through thin foils. Although the results indicated that there is a reasonablecorrelation between the theory and experiment, it is recommended that a more thorough theoretical study be conducted and the theory of pencil beam scattering extendedto cover broad beam scattering. The well known Monte Carlo technique could be a useful procedure to employ.A problem which has been bothering radiotherapists, isthe reduction in depth dose when the focal to skin distance is reduced. This led to the use of the longestpractical focal to skin distance that could deliver satisfactory dose-rates. The isocentric technique, which involves an effective reduction in the f.s.d. by 15 cm, hastherefore not commonly been used on deep X-ray- and shortf.s.d. cobalt machines. This technique is however feasiblewith a linear accelerator.Accelerators are capable of delivering dose-rates of upto five hundred rads per minute at an isocentric distanceof one metre. Furthermore, calculations and experimental evidence indicate that at an isocentric distance ofone metre there is no significant difference in the depthdose or accuracy of the isocentric- or constant f.s.d.techniques.By utilising the isocentric technique the treatment fieldscould be altered from the control console. This would subsequently reduce setting up times and the patient would behandled less often. Due to the reduction of patient handling the isocentric technique would be less suspectable toerror, although one error could be more serious than withthe constant f.s.d. technique. A project is envisaged wherethe PDP 8 computer, which is at present being used in a DECRad 8, Radiotherapy planning system, will be interfacedwith the existing linear accelerators, to act as a vigilantto minimize human error.It is concluded that on linear accelerators, the isocentric technique is superior to the constant f.s.d. technique.

[发布机构] University of the Free State