Radiopharmaceutical therapy, traditionally limited to refractory metastatic cancer, is being increasingly used at earlier stages, such as for treating minimal residual disease. The aim of this study was to compare the effectiveness of (90)Y, (177)Lu, (111)In, and (161)Tb at irradiating micrometastases. (90)Y and (177)Lu are widely used β(-)-emitting radionuclides. (161)Tb is a medium-energy β(-) radionuclide that is similar to (177)Lu but emits a higher percentage of conversion and Auger electrons. (111)In emits γ-photons and conversion and Auger electrons.
Methods: We used the Monte Carlo code CELLDOSE to assess electron doses from a uniform distribution of (90)Y, (177)Lu, (111)In, or (161)Tb in spheres with diameters ranging from 10 mm to 10 μm. Because these isotopes differ in electron energy per decay, the doses were compared assuming that 1 MeV was released per μm(3), which would result in 160 Gy if totally absorbed.
Results: In a 10-mm sphere, the doses delivered by (90)Y, (177)Lu, (111)In, and (161)Tb were 96.5, 152, 153, and 152 Gy, respectively. The doses decreased along with the decrease in sphere size, and more abruptly so for (90)Y. In a 100-μm metastasis, the dose delivered by (90)Y was only 1.36 Gy, compared with 24.5 Gy for (177)Lu, 38.9 Gy for (111)In, and 44.5 Gy for (161)Tb. In cell-sized spheres, the dose delivered by (111)In and (161)Tb was higher than that of (177)Lu. For instance, in a 10-μm cell, (177)Lu delivered 3.92 Gy, compared with 22.8 Gy for (111)In and 14.1 Gy for (161)Tb.
Conclusion: (177)Lu, (111)In, and (161)Tb might be more appropriate than (90)Y for treating minimal residual disease. (161)Tb is a promising radionuclide because it combines the advantages of a medium-energy β(-) emission with those of Auger electrons and emits fewer photons than (111)In.
Keywords: 111In; 161Tb; 177Lu; 90Y; radiopharmaceutical therapy.
© 2016 by the Society of Nuclear Medicine and Molecular Imaging, Inc.