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Direct Monte Carlo (MC) simulation is considered the gold standard approach for internal dosimetry in nuclear medicine, and it is increasingly used in planning Trans-Arterial Radio-Embolization (TARE) of HepatoCellular Carcinoma (HCC) {and hepatic metastases}. However its computational times, longer with respect to other simplified approaches, constitute a limiting factor, especially when dealing with {large size and finely discretized voxelized volumes}. Aim of this work was the investigation of the influence of cuts on the production of secondary particles and of input CT images resamplings on dosimetric accuracy and computational time in patient-specific voxel-level MC simulations of 90Y-labelled glass microspheres TARE treatment, to find optimal combinations of settings for speeding up such simulations. GATE GEANT4 interface was used to perform simulations employing CT and 99mTc SPECT as input data, examining multiple CT resolutions (via CT resamplings characterized by voxel volume factors 2, 8, and 64 with respect to native one, and a {CT} resampling with SPECT resolution) and production cuts (0.01 mm, 0.05 mm, 0.1 mm, 0.5 mm and some more, specific for each resampling). Increasing cut length and reducing CT resolution produces an early rapid decrease followed by a late slow decrease of simulation time as a function of this two parameters. {A} reduction up to 30% with respect to reference simulation time, while preserving acceptable dosimetric accuracy, {was obtained}. The best combination of settings among the examined ones resulted the choice of CT resampling with 8 times the native voxel volume and of 0.1-0.5 mm cut, ensuring dosimetric agreement within 1% in liver-related VOIs, while reducing simulation time to 45%.

Monte Carlo simulation of radiation transport is the most accurate and patient-specific technique available for internal dosimetry in nuclear medicine. Taking morphological and functional information from CT and PET 3D scans, it is possible to perform MC dose calculations at voxel level. GATE and GAMOS are among the most used and validated toolkits developed for performing such medical physics calculations. Aim of this work was the comparison of dose rates evaluated by means of GATE and GAMOS simulations for a case of PET/CT diagnostic exam conducted with 18F-choline radiopharmaceutical. The results obtained with the two toolkits showed a good agreement, with just some minor differences imputable to their different procedure of voxel density assignment. A further investigation on spurious decays generated in air was conducted through simulations employing a PET-filtering technique. In this way it was highlighted the effect of PET background on the evaluation of dose rates imparted to air-rich organs, in particular lungs.

The production of 18F, 11C, 13N, and 15O positron emitting radionuclides for PET imaging is usually accomplished in Nuclear Medicine Departments through direct nuclear reactions induced by protons accelerated by compact medical cyclotrons on liquid or gaseous targets. Messina University has funded the construction of a PET-cyclotron-radio-chemistry plant at the Messina University Hospital, equipped with a 11 MeV self-shielded cyclotron. We estimated the expected production yields of these nuclides, accounting for target thickness, production of other radioactive nuclides, and time effects on the irradiated target purity. To this aim, both TALYS code (v. 1.8) and an analytical approach based on EXFOR experimental data were used. The general agreement between the two approaches, and with the available literature data, allows to assess the expected yields at the End of Bombardment, and relative target purities, to be used for further radiopharmaceutical preparation steps.