Monte Carlo Simulations of Novel Scintillator Detectors and Dosimetry Calculations

Monte Carlo (MC) simulation techniques are becoming very common in the Medical Physicists community. MC can be used for modeling Single Photon Emission Computed Tomography (SPECT) and for dosimetry calculations. 188Re, is a promising candidate for radiotherapeutic production and understanding the mechanisms of the radioresponse of tumor cells "in vitro" is of crucial importance as a first step before "in vivo" studies. The dosimetry of 188Re, used to target different lines of cancer cells, has been evaluated by the MC code GEANT4. The simulations estimate the average energy deposition/per event in the biological samples. The development of prototypes for medical imaging, based on LaBr3:Ce scintillation crystals coupled with a position sensitive photomultiplier, have been studied using GEANT4 simulations. Having tested, in the simulation, surface treatments different from the one applied to the crystal used in our experimental measurements, we found out that the Energy Resolution (ER) and the Spatial Resolution (SR) could be improved, in principle, by machining in a different way the lateral surfaces of the crystal. We have then studied a system able to acquire both echographic and scintigraphic images to let the medical operator obtain the complete anatomic and functional information for tumor diagnosis. The scintigraphic part of the detector is simulated by GEANT4 and first attempts to reconstruct tomographic images have been made using as method of reconstruction a back-projection standard algorithm. The proposed camera is based on slant collimators and LaBr3:Ce crystals. Within the Field of View (FOV) of the camera, it possible to distinguish point sources located in air at a distance of about 2 cm from each other. In particular conditions of uptake, tumor depth and dimension, the preliminary results show that the Signal to Noise Ratio (SNR) values obtained are higher than the standard detection limit.

The role of mitochondria in the regulation of gamma rays induced mTOR-dependent senescence

The first aims of this study were to demonstrate if mitochondrial biogenesis and senescence can be induced simultaneously in cell lines upon exposure to a genotoxic stress, and if the presence of mtDNA mutations which impair the functionality of respiratory complexes can influence the ability of a cell to activate senescence. The data obtained on the oncocytic model XTC.UC1 demonstrated that the presence of mitochondrial dysfunction is involved in the maintenance of a senescent phenotype induced by γ-rays treatment. The involvement of mTORC1 in the regulation of senescence has been shown in this cell line. On the other hand, in cells which do not present mitochondrial dysfunction it has been verified that genotoxic stress determines the activation of both mitochondrial biogenesis and senescence. Further studies are necessary in order to verify if mitochondrial biogenesis sustains the activation of senescence. The second aim of this thesis was to determine the involvement of mTORC1 in the regulation of PGC-1α expression, in order to verify what is the cause of the development of oncocytoma in patients affected by two hereditary cancer syndromes; Cowden and Birt-hogg-Dubé . The study of oncocytic tumors developed by patients affected by these syndromes suggested that the double heterozigosity of the two causative genes, PTEN and FLCN respectively, induce the activation of mTORC1 and therefore the activation of PGC-1α expression. On XTC.UC1 cell line, the most suitable in vitro model, experiments of complementation of PTEN and FLCN were conducted. To date, these results demonstrated that mTORC1 is not involved in the regulation of PGC-1α expression, and PTEN and FLCN seem to have opposite effect on PGC-1α expression.

HIF1α regulates Mitochondrial biogenesis and Cellular senescence induced by Gamma Radiation

Cellular response to γ-rays is mediated by ATM-p53 axis. When p53 is phosphorylated, it can transactivate several genes to induce permanent cell cycle arrest (senescence) or apoptosis. Epithelial and mesenchymal cells are more resistant to radiation-induced apoptosis and respond mainly by activating senescence. Hence, tumor cells in a senescent state might remain as “dormant” malignant in fact through disruption of p53 function, cells may overcome growth arrest. Oncocytic features were acquired in the recurring neoplasia after radiation therapy in patient with colonrectal cancer. Oncocytic tumors are characterized by aberrant biogenesis and are mainly non-aggressive neoplasms. Their low proliferation degree can be explained by chronic destabilization of HIF1α, which presides to adaptation to hypoxia and also plays a pivotal role in hypoxia-related radio-resistance. The aim of the present thesis was to verify whether mitochondrial biogenesis can be induced following radiation treatment, in relation of HIF1α status and whether is predictive of a senescence response. In this study was demonstrate that mitochondrial biogenesis parameters like mitochondrial DNA copy number could be used for the prediction of hypoxic status of tissue after radiation treatment. γ-rays induce an increase of mitochondrial mass and function, in response to a genotoxic stress that pushes cells into senescence. Mitochondrial biogenesis is only indirectly regulated by p53, whose activation triggers a MDM2-mediated HIF1α degradation, leading to the release of PGC-1β inhibition by HIF1α. On the other hand, this protein blunts the mitochondrial response to γ-rays as well as the induction of p21-mediated cell senescence, indicating prevalence of the hypoxic over the genotoxic response. Finally in vivo, post-radiotherapy mtDNA copy number increase well correlates with lack of HIF1α increase in the tissue, concluding this may be a useful molecular tool to infer the trigger of a hypoxic response during radiotherapy, which may lead to failure of activation of senescence.