Evaluation of the contamination of a therapy photon beam by electrons

This dissertation describes the studies of the contamination of high energy photon beams with electrons and/or low energy photons. The main advantages that have made high energy x- and gamma rays used in the treatment of malignant diseases desirable in the medical field are: the low dose deposited through the first few millimetres of tissue under the epidermis and the high penetration that they exhibit. These characteristics can be badly degraded if the beam is excessively contaminated with secondary radiation. To demonstrate the presence of this contamination radiation, a suite of measurements has shown the deviations of these beams from the desirable characteristics and supports the contention of the existence of a secondary component which accompanies these beams. The major question of the nature of this radiation and its elimination is addressed in this work. Using an electromagnet, the charged particle component (electrons) can be separated from the uncharged component (photons). The results of this separation clearly show that the secondary radiation present is mainly electrons. It is also demonstrated that the removal of these electrons from the beam with the aid of the magnetic field, improves the characteristics of the beams significantly. The method of using a large magnet is not appropriate under clinical conditions however. Thus, metal foils acting as electron stoppers, are placed in the path of the beam in an attempt to improve the characteristics of the beam. The measurements show that an optimum electron filter system can be designed. In situations were a minimum dose at the epidermis and first few millimeters of tissue is desired, lead filters with predetermined characteristics in the path of the beam, improve the characteristics of the beam and allow the prescribed dose to be delivered to the tumour