Nuclear medicine applications for neuroendocrine tumors

Sensitive, specific radiopharmaceuticals are available for scintigraphic diagnosis and internal radiotherapy of neuroendocrine tumors. 123I-MIBG (metaiodobenzylguanidine) scintigraphy is the examination of choice for visualizing tumor sites of pheochromocytoma. In the event of malignant pheochromocytoma or carcinoid tumor, this examination allows assessment of the presence or absence of tumor uptake and can guide radiotherapy with 131I-MIBG. The peptides secreted by neuroendocrine tumors can be radiolabeled for targeting of their specific receptors. Scintigraphy using a 111In-labeled somatostatin analog (octreotide) is the examination of choice for diagnosis of the spread of gastroenteropancreatic and carcinoid tumors, as it is more sensitive than morphologic imaging techniques. It can also guide radiotherapy performed with the same pharmaceutical vector. These same two agents (MIBG and octreotide) can be used therapeutically by replacing 123I with 131I and 111In by 90Y. A transient palliative effect is obtained for a variable number of tumors (most often large ones) that take up the radiopharmaceutic agent well. There is general consensus that, for relatively radioresistant solid tumors, this type of radiotherapy is efficient only in the event of small tumor targets (a few millimeters in diameter) whose uptake is maximal, allowing more homogeneous distribution than that achieved with large tumors. Thus for optimal control of the disease it is recommended first to use scintigraphic imaging to confirm that the tumor takes up the radiopharmaceutical agent in question (123I-MIBG or 111In-octreotide) and then reduce the tumor burden surgically before injecting high therapeutic activity (possibly with reinjection of peripheral stem cells). This treatment can be repeated three times every 3 months before evaluating the response. In these conditions, internal radiotherapy can be beneficial or even determinant for controlling disease progression.