Therapy using labelled somatostatin analogues: Comparison of the absorbed doses with 111In-DTPA-D-Phe1-octreotide and yttrium-labelled DOTA-D-Phe1-Tyr3-octreotide

OBJECTIVE: We estimated the absorbed doses for In-DTPA-D-Phe-octreotide and Y-DOTA-D-Phe-Tyr-octreotide in the same patients in order to compare the potential effectiveness (tumour dose) and safety (kidney and red marrow dose) of these drugs for peptide-targeted radiotherapy of somatostatin receptor positive tumours. METHODS: Six patients with neuroendocrine tumours underwent quantitative In-DTPA-D-Phe-octreotide SPECT and Y-DOTA-D-Phe-Tyr-octreotide PET scan at intervals of 1 week. All studies were performed with a co-infusion of amino acids for renal protection. PET and SPECT were reconstructed using iterative algorithms, incorporating attenuation and scatter corrections. Tissue uptakes (IA%) were measured and used to calculate residence times. Absorbed doses to tissues were estimated and the maximal allowed activity, defined as either the activity delivering 23 Gy to the kidneys (MAAK) or 2 Gy to the red marrow (MAARM), was calculated and the resulting tumour absorbed doses were computed. RESULTS: For the MAAK the mean absorbed dose to the red marrow was lower for Y-DOTA-D-Phe-Tyr-octreotide than for In-DTPA-D-Phe-octreotide (1.8±0.9 Gy vs. 6.4±1.6 Gy; P<0.001). The median absorbed dose to tumours for the MAAK was two-fold higher for Y-DOTA-D-Phe-Tyr-octreotide as compared to In-DTPA-D-Phe-octreotide (30.1 vs. 12.6 Gy; P<0.05). The median absorbed dose to tumours estimated for the MAARM was 10-fold higher for Y-DOTA-D-Phe-Tyr-octreotide than for In-DTPA-D-Phe-octreotide (35.1 Gy vs. 3.9 Gy; P<0.05). CONCLUSIONS: This direct intra-patient comparison confirms that the use of Y-DOTA-D-Phe-Tyr-octreotide is more appropriate for therapy of somatostatin receptor bearing tumours. When using In-DTPA-D-Phe-octreotide, the red marrow represents the major critical organ; this can result in significant toxicity if high activities have to be administered to obtain efficient tumour irradiation.

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