Replacing Lu-177 with Tb-161 in DOTA-TATE and PSMA-617 therapy: potential dosimetric implications for activity selection

Aim To explore the dosimetric effect of substituting Lu-177 with Tb-161 in targeted radionuclide therapy (TRT) using the registered tracers DOTA-TATE and PSMA-617. Methods Using established kinetic data for [ 177 Lu]Lu-DOTA-TATE and [ 177 Lu]Lu-PSMA-617, radiation absorbed doses to typical tumour lesion as well as non-target tissues ([ 177 Lu]Lu-DOTA-TATE: kidneys, spleen and liver, [ 177 Lu]Lu-PSMA-617: kidneys, liver and salivary glands) were calculated for Lu-177 and Tb-161. Results For both DOTA-TATE and PSMA-617, the substitution of Lu-177 with Tb-161 results in an increase in the delivered dose per unit of activity to tumour tissue by 40%. If an equivalent non-target delivered dose is strived for in order not to increase toxicity, based on kidney absorbed dose, 7400 MBq Lu-177 per cycle should be substituted with 5400 MBq Tb-161 for DOTA-TATE and 5300 MBq of Tb-161 for PSMA-617. Conclusion When substituting Lu-177 with Tb-161, activity conversion is necessary in order not to exceed non-target dose limits.