3D absorbed dose distribution inside and outside 90Y, 177Lu and 131I sources of spherical shape by Monte Carlo simulation with the PENELOPE algorithm

F.BOTTA, M.VALENTE, A.DI DIA, M.CREMONESI, M. FERRARI, G.PAGANELLI, G.PEDROLI

Munchen, Germany

Congreso; Congress of the European Association of Nuclear Medicine; 2008

European Association of Nuclear Medicine (EANM )

Anti-cancer therapies with radiolabelled compounds require tumor dosimetry as key information. The aim of this study was to provide dose conversion factors (DF or S values according to RADAR or MIRD) and dose distribution for lesions typically treated by radionuclide therapy. MC simulations were performed considering spherical sources of 90Y, 177Lu, 131I, which are most used in both locoregional and systemic clinical practice. The dose distribution inside spheres and in surrounding volumes was investigated to represent dose in tumours, radiosensitive bordering tissues and/or disease infiltrated tissues. Methods: Six spheres (radius (r) ranging 6-36 mm), water-equivalent, homogeneously filled with 90Y, 177Lu, 131I, were considered as sources. The dose distribution was tallied in 1 mm3 cubical voxels from the spheres? centre to 20 mm out of them. The Monte Carlo code PENELOPE was used, simulating photon and electron transport including Bremsstrahlung. 107 particle histories run for every simulation. Results: Sphere r=6mm ? Average DF values (mGy/MBq/s) in the spheres were 1.04E-01(90Y), 2.25E-02(177Lu), 3.07E-02(131I). The maximum DF values, DFmax (mGy/MBq/s) were at the sphere centre: 1.43E-01(90Y), 2.34E-02(177Lu), 3.24E-02(131I); 90%-20% of Dmax were at 3.3-7.0 mm (90Y), 5.7-6.5 mm (177Lu),  5.7-6.5 mm (131I) from the sphere centre. Sphere r=15mm ? Average DF values (mGy/MBq/s) in the spheres were 9.3E-03 (90Y ), 1.63E0-3 (177Lu), 2.24E-03 (131I). DFmax (mGy/MBq/s) were at the sphere centre: 1.09E-02(90Y), 1.69E-03(177Lu),  2.29E-03(131I); 90%-20% of Dmax were at  12-16 mm (90Y), 14.5-15.3 mm (177Lu), 14.3-15.8 mm(131I) from the sphere centre. Sphere r=36mm ? Average DF values (mGy/MBq/s) in the spheres were 7.16E-04 (90Y ), 1.16E-04 (177Lu), 1.6E-4 (131I).  DFmax (mGy/MBq/s) were at the sphere centre: 7.61E-04(90Y), 1.17E-04(177Lu),  1.62E-04(131I); 90%-20% of Dmax were at 33.8-37.5 mm (90Y), 35.9-36.6 mm (177Lu), 35.7-36.6 mm(131I) from the sphere centre. Conclusion: The model offers suitable information for dosimetry in several radionuclide therapies. This represents an important tool towards the use of tailored treatments with improved safety  and efficacy. The comparison of the results can suggest the choice of a most appropriate radionuclide depending of the clinical situation.   Key words: radionuclide therapy; radiation dosimetry; 90Y; 177Lu; 131I.