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Dosimetry and Biodistribution of an Iodine-123-Labeled Somatostatin Analog in Patients with Neuroendocrine Tumors

Departments of Radiology, Oncology and Radiation Safety, Mayo Clinic, Rochester, Minnesota

Correspondence: For reprints contact: Dr. M.K. O'Connor, Section of Nuclear Medicine, Charlton Building, Mayo Clinic, Rochester, MN 55905.

ABSTRACT

A modified method for the preparation of a radiolabeled analog of somatostatin (¹²³I-octreotide) is described. The pharmacokinetics and dosimetry of this analog were evaluated in patients with neuroendocrine tumors. Thirty patients had multiple blood and urine samples and sequential anterior and posterior whole-body scintigraphy up to 40 hr postinjection of ¹²³I-octreotide. Region of interest analysis of the whole body images was used to determine organ and tumor doses. The ¹²³I-octreotide was rapidly cleared from the blood with a T of 10 min by the hepatobiliary system. By 40 hr, approximately 55% was eliminated in the feces. The gallbladder wall received the highest dose (0.48 rad/mCi), with other organs receiving doses of 0.12 rad/mCi or less. Tumors were identified in 25 of 28 satisfactory studies. Tumor doses ranged from 0.1 to 0.6 rad/mCi. Calculations with ¹³¹I instead of ¹²³I indicated that the gallbladder wall would receive 2 rad/mCi, while average tumor doses would range from 0.9 to 5.0 rad/mCi. Iodine-123-octreotide is a useful agent for the visualization of neuroendocrine tumors. The rapid washout of this agent from tumors precludes the possibility of radiotherapy with ¹³¹I-octreotide in these patients.

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