Monte Carlo simulation of positron following-up in biological matter: applications in nuclear medicine

The subject of my PhD work is to outline the problem linked to threshold used to visualize PET images, in order to obtain more accurate tumoral volumes which could be treated more efficiently in radiotherapy. To do that, we have proposed to use the Monte Carlo code we have already developed to study the following-up of electrons and ions in water. During this PhD study, we have also tried to describe with the best accuracy (at a cellular scale) the deposited energy induced by positrons in the biological matter thanks to the Monte Carlo simulation previously cited that we have adapted to describe the following-up of positrons, especially in adding the capture process which has recently been calculated by our team. In one hand, the goal of this work is to measure the risks taken by the patient during a medical exam (such as a PET acquisition). In another hand, the simulation of the following-up of positrons in the biological matter coupled to the simulation of the following-up of photons in water, has allowed the detection of photons after the Positronium (electron-positron pair formed during the positron slowing-down in the matter) annihilation. Then a simple modelling of the geometrical and physical parameters of the medical device has allowed a quantification of the number of events detected on the ring. More precisely, we have access to the number of true, scattered and random pairs. Thanks to these data, we are thus able to reconstruct a sinogram as well as the initial radiative volume. Then a comparison with some experimental results gives access to a semi-empirical relation between the volume and the threshold. (author)