Comparison between Three Promising β-emitting Radionuclides, 67Cu, 47Sc and 161Tb, with Emphasis on Doses Delivered to Minimal Residual Disease

PURPOSE: Radionuclide therapy is increasingly seen as a promising option to target minimal residual disease. Copper-67, scandium-47 and terbium-161 have a medium-energy beta-emission which is similar to that of lutetium-177, but offer the advantage of having diagnostic partner isotopes suitable for pretreatment imaging. The aim of this study was to compare the efficacy of Cu-67, Sc-47 and Tb-161 to irradiate small tumors. METHODS: The absorbed dose deriving from a homogeneous distribution of Cu-67, Sc-47 or Tb-161 in water-density spheres was calculated with the Monte Carlo code CELLDOSE. The diameters of the spheres ranged from 5 mm to 10 mu m, thus simulating micrometastases or single tumor cells. All electron emissions, including beta-spectra, Auger and conversion electrons were taken into account. Because these radionuclides differ in electron energy per decay, the simulations were run assuming that 1 MeV was released per mu m(3), which would result in a dose of 160 Gy if totally absorbed. RESULTS: The absorbed dose was similar for the three radionuclides in the 5-mm sphere (146-149 Gy), but decreased differently in smaller spheres. In particular, Tb-161 delivered higher doses compared to the other radionuclides. For instance, in the 100-mu m sphere, the absorbed dose was 24.1 Gy with Cu-67, 14.8 Gy with Sc-47 and 44.5 Gy with Tb-161. Auger and conversion electrons accounted for 71% of Tb-161 dose. The largest dose differences were found in cell-sized spheres. In the 10-mu m sphere, the dose delivered by Tb-161 was 4.1 times higher than that from Cu-67 and 8.1 times that from Sc-47. CONCLUSION: Tb-161 can effectively irradiate small tumors thanks to its decay spectrum that combines medium-energy beta-emission and low-energy conversion and Auger electrons. Therefore Tb-161 might be a better candidate than Cu-67 and Sc-47 for treating minimal residual disease in a clinical setting.