Determination of internal dosimetry in yttrium-90 micro sphere therapy by using Monte Carlo method

Yttrium-90 (90Y) is used in the treatment of non-resectable liver cancer diseases. Since the treatment is in the radionuclide group, internal radiation dosimetry has gained importance. In this study, Monte Carlo (MC) method was used to accurately calculate the amount of absorbed radiation dose involved in liver tissue with 90Y microsphere treatment. For this reason, the 3-D (3D) Liver Phantom (KF) is designed with AutoCAD program which simulates liver volume geometry to make absorbed dose calculations. Designed KF geometry is converted to STL file format for use in simulation world. The same geometric structure is made of 5 pieces of plexi material by carving the same volüme space with KF into the plexi material. This study was planned in two stages. In the first stage, Technetium-99m (99mTc) radionuclide was used which is used in more than %80 of the diagnostic procedures in nuclear medicine applications, emitting gamma with 140 keV energy, having a half-life of 6 hours and activity of 1900 mCi (70.3 MBq ).At this stage, tumor imitation with r = 15 mm and h = 60 mm was filled with 99mTc activity in KF, and 24 thermoluminescence dosimetry chips (TLD-100) were placed at 8 different locations to calculate the dose values. The volume of KF was filled with water as tissue equivalent, irradiated for 11 half-life covered by Pb bricks and then TLD-100 readings were performed in CNAEM. The designed KF measurement system was defined using MC simulation GATE program version 8.1, which contains the same geometry and physical parameters, and the dose values were taken by defining the DoseActor library at 24 points where TLD was placed. GATE simulation using 99mTc radionuclide was followed for 7.5 days for 70300000 particles and instant absorbed dose values were calculated. GATE MC simulation and TLD measurement results were compared and the results were consistent. In the second stage; In the GATE simulation program, 90Y radionuclide with an average energy of 934 keV, an activity of 1mCi ( 37 MBq ) with an intra-spherical half-life of r = 15 mm and 30 DoseActor libraries were placed in the KF geometry. GATE simulation using 90Y radionuclide was followed for 37001684 particles for 6 hours and instant absorbed dose values were calculated. In the first step, the GATE MC simulation and TLD measurement results were compared and the correlation coefficient was calculated R2 = 0.994 which was found that the two systems had a linear correlation. When the GATE simulation results and TLD measurement results were compared, the% error values were the highest as 21% and the additional low 2% and the average 12%. In the GATE MC simulation, the instant absorbed dose values were found to be maximum of 3.908x〖10〗^(-7)  5.817x〖10〗^(-9) Gy/s and a minimum of 5.102x〖10〗^(-9)5.161x〖10〗^(-10) Gy/s. The maximum absorbed dose from readings in the TLD measurement system were found 4.533x〖10〗^(-7)8.394x〖10〗^(-9) Gy/s and the lowest 6.473x〖10〗^(-9)4.193x〖10〗^(-10) Gy/s. The cumulative dose values obtained by the GATE simulation results with 90Y radionuclide for one half-life were the highest in the tumor tissue for 58.6500.277 Gy and the lowest 27.0300.189 Gy for an average of 37:6370:215 Gy. The cumulative dose values obtained by the GATE simulation results with 90Y for one half-life were calculated as the highest 5.7470.087 Gy, lowest 0.0 Gy, and average 0.5170.015 Gy in liver tissue, respectively. The results were found to be consistent with the literature data. It has been shown that more realistic and precise internal dosimetry calculations are made with GATE program.