Kapoor Mayank, Kasi Anup
All India Institute of Medical Sciences (AIIMS) Rishikesh
University of Kansas
An octreotide scan, also known as somatostatin receptor scintigraphy (SRS), is valuable for detecting carcinoid tumors and various neuroendocrine tumors (NETs). Neuroendocrine cells are found in multiple areas of the body, including the brain, thyroid, lungs, and gastrointestinal tract. The scan has a sensitivity ranging from 75% to 100% for detecting pancreatic NETs. Octreotide is a synthetic analog of somatostatin, an endogenous peptide released by neuroendocrine cells, activated immune cells, and various types of inflammatory cells. Octreotide is radiolabeled with Indium-111 (In) for imaging purposes. This radiolabeled tracer binds to tumor cells that express somatostatin receptors (SSTRs). Somatostatin is secreted in 2 forms in the body, consisting of 14 and 28 amino acids, which result from differential proteolytic processing of a single precursor. Both forms can activate SSTRs at nanomolar concentrations, as they both contain the peptide region critical for receptor interaction. Cortistatins are another class of endogenously secreted neuropeptides that bind to SSTRs with high affinity. Somatostatin exerts its antiproliferative and antisecretory effects by binding to 1 of the 5 SSTR subtypes (SSTR1, SSTR2, SSTR3, SSTR4, and SSTR5), which are G-protein–coupled receptors (GPCRs). These receptors are widely distributed in the brain, pituitary gland, pancreas, thyroid, spleen, kidneys, gastrointestinal tract, blood vessels, peripheral nervous system, and immune cells. SSTRs are most abundantly expressed in well-differentiated NETs, with SSTR2 showing the highest expression, followed by SSTR1, SSTR5, SSTR3, and SSTR4. However, expression patterns can vary by tumor type. Tumors such as pituitary adenomas, gastrointestinal tract NETs, lung NETs, pheochromocytomas, and paragangliomas typically show predominant expression of SSTR2. In contrast, adrenocorticotropic hormone (ACTH)-secreting pituitary adenomas (SSTR5), insulinomas (SSTR1 and SSTR5), and medullary thyroid carcinomas predominantly express other SSTR subtypes. Both normal and tumor cells can exhibit varying patterns of SSTR subtype expression. The 2 types of imaging are based on these receptors. The first and most common is the octreotide scan, which uses In-DTPA (diethylenetriamine pentaacetate)-D-Phe-1-octreotide and primarily binds to SSTR2 and SSTR5. This scan provides a planar whole-body image, which, in modern medicine, is fused with single-photon emission computed tomography (SPECT) and computed tomography (CT). The specificity of the octreotide scan and the anatomic detail from SPECT/CT are thus combined. An octreotide scan has been shown to localize 86% of carcinoids, 89% of neuroblastomas, 86% of pheochromocytomas, 94% of paragangliomas, and 80% of primitive NETs. The effectiveness of the scan in detecting medullary thyroid carcinomas and pituitary tumors is comparatively lower. The second and more recent SSTR-based imaging technique uses the positron emitter gallium (Ga) to label somatostatin analogs such as Ga-DOTATOC (DOTA-Tyr3-octreotide), Ga-DOTANOC (1-Nal3-octreotide), and Ga-DOTATATE (DOTA-(Tyr)-octreotate). Uptake of these agents is measured by positron emission tomography (PET) imaging. Gamma cameras detect the gamma emissions from the radiolabeled tracer, enabling precise localization of SSTR-positive tumor sites.
奥曲肽扫描，也称为生长抑素受体闪烁显像（SRS），对于检测类癌肿瘤和各种神经内分泌肿瘤（NETs）很有价值。神经内分泌细胞存在于身体的多个部位，包括大脑、甲状腺、肺和胃肠道。该扫描检测胰腺NETs的灵敏度范围为75%至100%。奥曲肽是生长抑素的合成类似物，生长抑素是一种由神经内分泌细胞、活化的免疫细胞和各种类型的炎症细胞释放的内源性肽。为了成像目的，奥曲肽用铟 - 111（In）进行放射性标记。这种放射性标记的示踪剂与表达生长抑素受体（SSTRs）的肿瘤细胞结合。生长抑素在体内以两种形式分泌，由14个和28个氨基酸组成，这是由单一前体的不同蛋白水解加工产生的。两种形式都可以在纳摩尔浓度下激活SSTRs，因为它们都含有对受体相互作用至关重要的肽区域。皮质抑素是另一类内源性分泌的神经肽，它们以高亲和力与SSTRs结合。生长抑素通过与5种SSTR亚型（SSTR1、SSTR2、SSTR3、SSTR4和SSTR5）中的一种结合发挥其抗增殖和抗分泌作用，这些亚型是G蛋白偶联受体（GPCRs）。这些受体广泛分布于大脑、垂体、胰腺、甲状腺、脾脏、肾脏、胃肠道、血管、外周神经系统和免疫细胞中。SSTRs在分化良好的NETs中表达最为丰富，其中SSTR2表达最高，其次是SSTR1、SSTR5、SSTR3和SSTR4。然而，表达模式可能因肿瘤类型而异。垂体腺瘤、胃肠道NETs、肺NETs、嗜铬细胞瘤和副神经节瘤等肿瘤通常显示SSTR2的主要表达。相比之下，分泌促肾上腺皮质激素（ACTH）的垂体腺瘤（SSTR5）、胰岛素瘤（SSTR1和SSTR5）和甲状腺髓样癌主要表达其他SSTR亚型。正常细胞和肿瘤细胞都可以表现出不同的SSTR亚型表达模式。两种成像类型都基于这些受体。第一种也是最常见的是奥曲肽扫描，它使用铟标记的二乙三胺五乙酸 - D - 苯丙氨酸 - 1 - 奥曲肽（In - DTPA - D - Phe - 1 - octreotide），主要与SSTR2和SSTR5结合。该扫描提供平面全身图像，在现代医学中，它与单光子发射计算机断层扫描（SPECT）和计算机断层扫描（CT）融合。因此，奥曲肽扫描的特异性与SPECT/CT的解剖细节相结合。已证明奥曲肽扫描可定位86%的类癌、89%的神经母细胞瘤、86%的嗜铬细胞瘤、94%的副神经节瘤和80%的原始NETs。该扫描在检测甲状腺髓样癌和垂体肿瘤方面的有效性相对较低。第二种也是较新的基于SSTR的成像技术使用正电子发射体镓（Ga）标记生长抑素类似物，如镓 - DOTATOC（DOTA - Tyr3 - octreotide）, 镓 - DOTANOC（1 - Nal3 - octreotide）和镓 - DOTATATE（DOTA - (Tyr) - octreotate）。这些药物的摄取通过正电子发射断层扫描（PET）成像来测量。伽马相机检测来自放射性标记示踪剂的伽马射线发射，从而能够精确定位SSTR阳性肿瘤部位。
