Dose deposits from⁹⁰Y,¹⁷⁷Lu,¹¹¹In, and¹⁶¹Tb in micrometastases of various sizes: Implications for radiopharmaceutical therapy

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 of⁹⁰Y,¹⁷⁷Lu,¹¹¹In, and¹⁶¹Tb at irradiating micrometastases.⁹⁰Y and¹⁷⁷Lu are widely used β^--emitting radionuclides.¹⁶¹Tb is a medium-energy β^-radionuclide that is similar to¹⁷⁷Lu but emits a higher percentage of conversion and Auger electrons.¹¹¹In emits -photons and conversion and Auger electrons. Methods: We used the Monte Carlo code CELLDOSE to assess electron doses from a uniform distribution of⁹⁰Y,¹⁷⁷Lu,¹¹¹In, or¹⁶¹Tb in spheres with diameters ranging from 10 mm to 10 m. Because these isotopes differ in electron energy per decay, the doses were compared assuming that 1 MeV was released per m3, which would result in 160 Gy if totally absorbed. Results: In a 10-mm sphere, the doses delivered by⁹⁰Y,¹⁷⁷Lu,¹¹¹In, and¹⁶¹Tb were 96.5, 152, 153, and 152 Gy, respectively. The doses decreased along with the decrease in sphere size, and more abruptly so for⁹⁰Y. In a 100- m metastasis, the dose delivered by⁹⁰Y was only 1.36 Gy, compared with 24.5 Gy for¹⁷⁷Lu, 38.9 Gy for¹¹¹In, and 44.5 Gy for¹⁶¹Tb. In cell-sized spheres, the dose delivered by¹¹¹In and¹⁶¹Tb was higher than that of¹⁷⁷Lu. For instance, in a 10- m cell,¹⁷⁷Lu delivered 3.92 Gy, compared with 22.8 Gy for¹¹¹In and 14.1 Gy for¹⁶¹Tb. Conclusion:¹⁷⁷Lu,¹¹¹In, and¹⁶¹Tb might be more appropriate than⁹⁰Y for treating minimal residual disease.¹⁶¹Tb is a promising radionuclide because it combines the advantages of a medium-energy β^-emission with those of Auger electrons and emits fewer photons than¹¹¹In. 2016 by the Society of Nuclear Medicine and Molecular Imaging, Inc.