Monte Carlo Comparison between four beta-emitting Radionuclides of Interest for Targeted Radionuclide Therapy of Small Tumors: 177Lu, 67Cu, 47Sc and 161Tb

C. CHAMPION; P. ZANOTTI-FREGONARA; M. A. QUINTO; C. MORGAT; E. HIDIE

Barcelona

Conferencia; European Association of Nuclear Medicine; 2016

ABSTRACT PURPOSE: Radionuclide therapy is increasingly seen as a promising option to target micrometastases or minimal residual disease. Copper-67, scandium-47 and terbium-161 have a medium-energy - emission which is similar to that of lutetium-177. They offer the advantage of having diagnostic partner isotopes suitable for pretreatment imaging. The aim of this study was to compare the effectiveness of these isotopes at irradiating small tumor volumes. METHODS: Electron dose from uniform isotope distributions was assessed with the Monte Carlo code CELLDOSE in spheres of various sizes (from 5 mm down to 10 µm diameter). All electron emissions, including - spectra, Auger and conversion electrons were included. As 177Lu, 67Cu, 47Sc and 161Tb differ in electron energy per decay, doses were compared assuming 1 MeV released per µm3, which would result in 160 Gy if totally absorbed. RESULTS: In a 5 mm sphere, the four isotopes yielded similar dose deposits per MeV/µm3 (145 ? 149 Gy). The absorbed doses decreased with decreasing sphere size, thus underscoring the difficulty of irradiating micrometastases. 161Tb, however, delivered a higher dose compared to the other isotopes. For instance, in a 100-µm metastasis, dose deposits were 24.5Gy with 177Lu, 24.1Gy with 67Cu, 14.8Gy with 47Sc and 44.5Gy with 161Tb. Auger and conversion electrons accounted for 71% of 161Tb dose in this 100-µm metastasis. The differences between radionuclides further increased in cell-sized spheres. In a 10-µm cell, the dose delivered by 161Tb was 3.6 times higher than that from 177Lu, 4.1 times that from 67Cu and 8.1 times that from 47Sc. CONCLUSION: Terbium-161 can effectively target micrometastases and single tumor cells thanks to its decay spectrum that combines medium-energy - emission and low-energy conversion and Auger electrons. These results are in agreement with some recent studies on cell cultures and tumor xenografts.