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

Purpose: Therapy of tumors expressing somatostatin receptors, sstr, has recently been clinically tested using somatostatin analogues labeled with In-111 and Y-90. Several other radionuclides, i.e., I-131, Tb-161, Cu-64, Re-188, 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. Dosimetric calculations were performed with the assumption that biokinetics were similar for all radionuclides bound to somatostatin analogues. The cumulated tumor: normal-tissue activity concentration, TN (C) over tilde, was calculated for the various physical half-lives of the radionuclides. Using mathematical models, the tumor: normal-tissue mean absorbed dose rate ratio, TN D, and tumor: normal-tissue mean absorbed dose ratio, TND, were calculated for various tumor sizes in mice and humans. Results: TN (C) over tilde of radionuclide-labeled octreotide increased with physical half-life for most organs, both in mice and in humans. TN D showed that radionuclides emitting electrons with too high energy are not suitable for therapy of small tumors. Furthermore, radionuclides with a higher frequency of photon emissions relative to electron emissions will yield lower TN D and are thus less suitable for therapy than radionuclides with a lower frequency of photon emissions. The TND was highest for Tb-161 in both mice and humans. Conclusions: The results demonstrate that long-lived radionuclides, which emit electrons with rather low energy and which have low frequency of photon emissions, should be the preferred therapy for disseminated small sstr-expressing tumors. (C) 2001 Elsevier Science Inc.