Tissue substitutes for particulate radiations and their use in radiation dosimetry and radiotherapy

Abstract

Most of the tissue substitute materials currently used inclinical radiation dosimetry are designed to simulate muscle or bone when irradiated with photons A few materials have been developed for neutron dosimetry, but substitutes speci fically designed for beams of high energy charged particles are not to be found in the literature. This thesis deals with the formulation and manufacture of, tissue substitutes for particulate radiations and the subsequent application of these substitutes in dlectron, pion, proton and neutron dosimetry. The method of "elemental equivalence" was used and over 80 solid, gel and liquid substitutes have been produced2 which simulate the most important tissues (adipose, blood, bone, muscle, etc), body organs (brain, lungj etc) and tissue components (fat, protein, water). Most of these materials are "tissue equivalent" and are useful for all types of radiations. The compilation of selected chemical compounds (compound library) used for the formulatign; and the computer programs written for the theoretical evaluation of the new materials are described and discussed. The experimental comparison of some selected substitutes with the corresponding real tissues, using fast neutrons, high energy protons, cobalt-60 gamma rays and 120 kVP X-rays., verified the high precision of the simulation procedures. The results of depth dose measurements in various tissue substitutes ý as well as water, using 7.5 MeV neutrons 150 MeV protons, 70 MeV negative pions , 10 MeV electrons and cobalt-60 gamma rays are presented. The effect of tissue heterogeneities on the dose distributions from thesý radiations was investigated. Isodose shift factors for air, lung, fat and bone were derived for all the above radiation modalities and detailed lung correction factors were measured for 7.5 MeV neutrons and cobalt-60 gamma rays. In view of the proposed use of the 160 MeV proton beam of the Atomic Energy Research Establishment (Harwell, JJ. K) for patient treatment, a complete series of pre-therapeutic measurements was performed with this proton beam facility using the new materials, and the results are presented and discussed in detail. Finally, the applications of the new substitutes in other practical clinical aspects are described and some examples of such applications given.