Octreotide Imaging

Background

In-111 pentetreotide imaging (octreoscan) is used for the detection and localization of neuroendocrine tumors.

Somatostatin attaches to receptors Neuroendocrine tumors and acts as an inhibitor. Somatostatin has a very short plasma half life (a few minutes), so somatostatin analogs are used for imaging and treatment. Octreotide (Sandostatin) is the the somatostatin analog used in nuclear medicine and has a half life of about 2 hours. Chelation of octreotide to In-111 is achieved using DTPA, resulting in In-111 pentetreotide.

While somatostatin binds to all subtypes of somatostatin receptors, octreotide only attaches to subtypes 2 and 5. 80% of neuroendocrine tumors have type 2 somatostatin receptors, which means that a negative In-111 pentetreotide scan doesn't mean a neuroendocrine tumor is not present.

Indications

Somatostatin receptors can be found on neuroendocrine and some non-neuroendocrine tumors. These include:

Tumor	Sensitivity	comments
carcinoid	80-90%
paraganglioma	94%
gastrinoma	75-93%
VIPoma	88%
insulinoma	40-50%
glucagonoma	unknown
medullary thyroid cancer	41%	PET is much more sensitive, but octeroscan should be first line
benign pheochromocytoma	58%
malignant pheochromocytoma	87%
neuroblastoma		I-123 MIBG is preferred
small-cell lung cancer	92%	Sensitivity for mediastinal lymph nodes is about 80%, but low for extrathoracic metastatic lesions
pituitary adenoma
meningiomas
well-differentiated gliomas
lymphoma
breast carcinoma

Thyroid uptake can also be seen in patients with endemic goiter with or without thyroid autonomy.

Patient Preparation

Patients with neuroendocrine tumors may receive somatostatin analogs (e.g., octreotide) for treatment. This bound somatostatin may compete with In-111 pentetreotide and result in a false negative study. Therefore, it is recommended that patients stop any somatostatin analog treatment 72 hours before imaging.

Another issue is bowel activity. Overnight bowel prep with a laxative is sometimes recommended. Patients with carcinoid syndrome who have diarrhea may skip the bowel preparation.

Normal biodistribution

The normal biodistribution of In-111 pentetreotide is shown. The thyroid, liver, spleen, kidneys, and bladder are always seen with variable uptake in the gastrointestinal tract on delayed images.

False Positive

Somatostatin receptors may be expressed in chronic granulomatous processes such as sarcoidosis, tuberculosis and in chronic inflammatory diseases like Crohn disease, ulcerative colitis, and rheumatoid arthritis, leading to false positive results.

References

• Becker W, Schrell U, Buchfelder M, Hensen J, Wendler J, Gramatzki M, Wolf F. Somatostatin receptor expression in the thyroid demonstrated with 111In-octreotide scintigraphy. Nuklearmedizin. 1995 Jun;34(3):100-3.
• Intenzo CM, Jabbour S, Lin HC, Miller JL, Kim SM, Capuzzi DM, Mitchell EP. Scintigraphic imaging of body neuroendocrine tumors. Radiographics. 2007 Sep-Oct;27(5):1355-69.
• Tzannou IA et al. The use of radiolabeled somatostatin analog scintigraphy in the staging of small cell lung cancer patients. Am J Clin Oncol. 2007 Oct;30(5):503-6.