L-3-[I-123]iodo-alpha-methyltyrosine scintigraphy in carcinoid tumors: Correlation with biochemical activity and comparison with [In-111-DTPA-D-Phe(1)]-octreotide imaging

Carcinoid tumors can produce serotonin (5-hydroxytryptamine) and catecholam ines from the precursors tryptophan and tyrosine. Our aim was to evaluate t he tyrosine analog L-3-[I-123]iodo-alpha -methyltyrosine (IMT) in the detec tion and the determination of biochemical activity of these tumors in compa rison with In-111-labeled [diethylenetriaminepentaacetic acid (DTPA)-D-Phe( 1)]-octreotide (In-111-octreotide) scintigraphy. Methods: SPECT and planar whole-body imaging were performed 15 min after administration of 300 MBq IM T in 22 patients with metastatic carcinoid tumors. The number of lesions de tected was compared with the number detected by In-111-octreotide scintigra phy. The size and intensity of uptake of all lesions were graded using a si mple scoring system, yielding a total body uptake score for both tracers. T hese scores were compared (nonparametric correlation) with biochemical mark ers of serotonin and catecholamine metabolism. Results: IMT SPECT detected only 63 of 145 lesions detected by In-111-octreotide imaging (43%). IMT SPE CT performance was best in the liver (60% detection rate). Both IMT uptake and In-111-octreotide uptake scores correlated with markers of serotonin me tabolism (respective Values for urinary 5-hydroxyindoleacetic acid: r = 0.6 7 and 0.48, P < 0.001 and 0.05; for urinary serotonin: r = 0.56 and 0.40, P = 0.002 and 0.05; and for platelet serotonin: r = 0.57 and 0.45, P < 0.01 and 0.05). No correlation with adrenaline or noradrenaline metabolites was found, However, IMT uptake, but not In-111-octreotide uptake, correlated wi th dopamine metabolite excretion (homovanillic acid: r = 0.60, P < 0.05; an d dopamine relative sum: r = 0.61, P < 0.05), IMT uptake was higher in pati ents with increased dopamine metabolite excretion (P = 0.05). Conclusion: I MT uptake can be demonstrated in carcinoid lesions, but the method detected only 43% of carcinoid lesions that were positive on In-111-octreotide scin tigraphy. Uptake of both tracers is related to the serotonin secretory acti vity. However, IMT uptake, but not In-111-octreotide uptake, was related to tumor dopamine metabolism. These findings may be of interest in the metabo lic targeting of carcinoids.