Dosimetric comparison of radionuclides for therapy of somatostatin receptor-expressing tumors

Purpose: Therapy of tumors expressing somatostatin receptors, sstr, has rec ently been clinically tested using somatostatin analogues labeled with In-1 11 and Y-90. Several other radionuclides, i.e., I-131, Tb-161, Cu-64, Re-18 8, Lu-177, and Ga-67, have also been proposed for this type of therapy. The aim of this work was to investigate the usefulness of the above-mentioned radionuclides bound to somatostatin analogues for tumor therapy. Methods: Biokinetic data of In-111-labeled octreotide in mice and man were used, primarily from our studies but sometimes from the literature. Dosimet ric calculations were performed with the assumption that biokinetics were s imilar for all radionuclides bound to somatostatin analogues. The cumulated tumor: normal-tissue activity concentration, TN (C) over tilde, was calcul ated for the various physical half-lives of the radionuclides. Using mathem atical models, the tumor: normal-tissue mean absorbed dose rate ratio, TN D , and tumor: normal-tissue mean absorbed dose ratio, TND, were calculated f or various tumor sizes in mice and humans. Results: TN (C) over tilde of radionuclide-labeled octreotide increased wit h physical half-life for most organs, both in mice and in humans. TN D show ed that radionuclides emitting electrons with too high energy are not suita ble for therapy of small tumors. Furthermore, radionuclides with a higher f requency of photon emissions relative to electron emissions will yield lowe r TN D and are thus less suitable for therapy than radionuclides with a low er frequency of photon emissions. The TND was highest for Tb-161 in both mi ce and humans. Conclusions: The results demonstrate that long-lived radionuclides, which e mit electrons with rather low energy and which have low frequency of photon emissions, should be the preferred therapy for disseminated small sstr-exp ressing tumors. (C) 2001 Elsevier Science Inc.