Validation of SIMIND Monte Carlo for 99mTc and 177Lu

Purpose : Monte Carlo (MC) simulation in Nuclear Medicine is a powerful tool for modelling many physical phenomena which are difficult to track or to measure directly. MC simulation in SPECT/CT imaging is particularly suitable for optimizing the quantification of activity in a patient, and, consequently, the absorbed dose to each organ. To do so, it is mandatory validate MC results with real data acquired with gamma camera. The aim of this study was the validation of SIMIND Monte Carlo code for modelling a Siemens Symbia Intevo Excel SPECT-CT gamma camera both for 99m Tc and 177 Lu. Methods : Phantom experiments using 99m Tc and 177 Lu have been performed with the purpose to measure spatial resolution, sensitivity and evaluate the calibration factor (CF) and recovery coefficients (RC) from acquired data. The geometries used for 2D planar imaging were (1) Petri dish and (2) capillary source while for 3D volumetric imaging were (3) a uniform filled cylinder phantom and (4) a Jaszczack phantom with spheres with different volumes. The experimental results have been compared with the results obtained from Monte Carlo simulations performed in the same geometries. Results : Comparison shows good accordance between simulated and experimental data. The measured planar spatial resolution was 8.3±0.8mm for 99m Tc and 12.2±0.7mm for 177 Lu. The corresponding data obtained by SIMIND for 99mTc was 7.4±0.1 mm, while for 177Lu was 11.7±0.1 mm. The CF was 110.1±5.5 cps/MBq for Technetium and 18.3±1.0 cps/MBq for Lutetium. The corresponding CF obtained by SIMIND for 99mTc was 100.1±0.3 cps/MBq, while for 177 Lu 20.4±0.7 cps/MBq. Moreover, a complete curve RCs vs Volume (ml) both for Technetium and Lutetium was determined to correct the PVE for all volumes of clinical interest. In none of the cases a RC factor equal to 100 was found. Conclusions : The results of validation process show that SIMIND can be used for simulating both gamma camera planar and SPECT images of Siemens Symbia Intevo using 99mTc and 177Lu radionuclides for different medical purposes and treatments.