Carcinoid tumors: iodine-131 MIBG scintigraphy

High-specific-activity 131iodine-metaiodobenzylguanidine for therapy of unresectable pheochromocytoma and paraganglioma

Pheochromocytomas and paragangliomas (PPG) are rare cancers arising from the adrenal medulla (pheochromocytoma) or autonomic ganglia (paraganglioma). They have highly variable biological behavior. Most PPG express high-affinity norepinephrine transporters, allowing active uptake of the norepinephrine analog, ¹³¹iodine-metaiodobenzylguanidine (¹³¹I-MIBG). Low-specific-activity forms of ¹³¹I-MIBG have been used since 1983 for therapy of PPG. High-specific-activity ¹³¹I-MIBG therapy improves hypertension management, induces partial radiological response or stable disease, decreases biochemical markers of disease activity and is well tolerated by patients. This drug, approved in the USA in July 2018, is the first approved agent for patients with unresectable, locally advanced or metastatic PPG and imaging evidence of metaiodobenzylguanidine uptake, who require systemic anticancer therapy.

Molecular imaging of norepinephrine transporter-expressing tumors: current status and future prospects

The human norepinephrine transporter (hNET) is a transmembrane protein responsible for reuptake of norepinephrine in presynaptic sympathetic nerve terminals and adrenal chromaffin cells. Neural crest tumors, such as neuroblastoma, paraganglioma and pheochromocytoma often show high hNET expression. Molecular imaging of these tumors can be done using radiolabeled norepinephrine analogs that target hNET. Currently, the most commonly used radiopharmaceutical for hNET imaging is meta-[123I]iodobenzylguanidine ([123I]MIBG) and this has been the case since its development several decades ago. The γ-emitter, iodine-123 only allows for planar scintigraphy and single photon emission computed tomography imaging. These modalities typically have a poorer spatial resolution and lower sensitivity than positron emission tomography (PET). Additional practical disadvantages include the fact that a two-day imaging protocol is required and the need for thyroid blockade. Therefore, several PET alternatives for hNET imaging are actively being explored. This review gives an in-depth overview of the current status and recent developments in clinical trials leading to the next generation of clinical PET ligands for imaging of hNET-expressing tumors.

Recent advances in radiotracers targeting norepinephrine transporter: structural development and radiolabeling improvements

The norepinephrine transporter (NET) is a major target for the evaluation of the cardiac sympathetic nerve system in patients with heart failure and Parkinson's disease. It is also used in the therapeutic applications against certain types of neuroendocrine tumors, as exemplified by the clinically used ^123/131I-MIBG as theranostic single-photon emission computed tomography (SPECT) agent. With the development of more advanced positron emission tomography (PET) technology, more radiotracers targeting NET have been reported, with superior temporal and spatial resolutions, along with the possibility of functional and kinetic analysis. More recently, fluorine-18-labelled NET tracers have drawn increasing attentions from researchers, due to their longer radiological half-life relative to carbon-11 (110 min vs. 20 min), reduced dependence on on-site cyclotrons, and flexibility in the design of novel tracer structures. In the heart, certain NET tracers provide integral diagnostic information on sympathetic innervation and the nerve status. In the central nervous system, such radiotracers can reveal NET distribution and density in pathological conditions. Most radiotracers targeting cardiac NET-function for the cardiac application consistent of derivatives of either norepinephrine or MIBG with its benzylguanidine core structure, e.g. ¹¹C-HED and ¹⁸F-LMI1195. In contrast, all NET tracers used in central nervous system applications are derived from clinically used antidepressants. Lastly, possible applications of NET as selective tracers over organic cation transporters (OCTs) in the kidneys and other organs controlled by sympathetic nervous system will also be discussed.

Norepinephrine Transporter as a Target for Imaging and Therapy

The norepinephrine transporter (NET) is essential for norepinephrine uptake at the synaptic terminals and adrenal chromaffin cells. In neuroendocrine tumors, NET can be targeted for imaging as well as therapy. One of the most widely used theranostic agents targeting NET is metaiodobenzylguanidine (MIBG), a guanethidine analog of norepinephrine. ¹²³I/¹³¹I-MIBG theranostics have been applied in the clinical evaluation and management of neuroendocrine tumors, especially in neuroblastoma, paraganglioma, and pheochromocytoma. ¹²³I-MIBG imaging is a mainstay in the evaluation of neuroblastoma, and ¹³¹I-MIBG has been used for the treatment of relapsed high-risk neuroblastoma for several years, however, the outcome remains suboptimal. ¹³¹I-MIBG has essentially been only palliative in paraganglioma/pheochromocytoma patients. Various techniques of improving therapeutic outcomes, such as dosimetric estimations, high-dose therapies, multiple fractionated administration and combination therapy with radiation sensitizers, chemotherapy, and other radionuclide therapies, are being evaluated. PET tracers targeting NET appear promising and may be more convenient options for the imaging and assessment after treatment. Here, we present an overview of NET as a target for theranostics; review its current role in some neuroendocrine tumors, such as neuroblastoma, paraganglioma/pheochromocytoma, and carcinoids; and discuss approaches to improving targeting and theranostic outcomes.

Metastatic Gastrointestinal Carcinoid Tumor with Unknown Primary Site

Carcinoid tumors are rare and slow growing malignancies derived from enterochromaffin cells. Two-thirds of carcinoid tumors arise in the gastrointestinal tract, and in 3% of these cases the primary site cannot be determined. Presenting symptoms depend on the location of the primary tumor but may be nonspecific, and in 13% of patients distant metastases are discovered on diagnosis. The classic carcinoid syndrome occurs in less than 10% of cases and only after metastasis to the liver. We present a case of a young woman with a gastrointestinal carcinoid tumor of unknown site that had metastasized to the liver. We also provide a review of the current diagnostic modalities. Familiarity with the signs and symptoms of carcinoid tumors and the diagnostic techniques thereof may facilitate early detection and improved outcome.

Management of sub-5 mm rectal carcinoids with lymph node metastases

Minute (<5 mm) and small (5–10 mm) rectal carcinoids discovered during colonoscopy are generally considered to be non-aggressive, and the management and surveillance of patients with this entity are usually limited. We present the case of a 61-year-old Chinese female with multiple sub-5 mm carcinoid tumours in the rectum without any computed tomography (CT) evidence of lymph node or distant metastases. She underwent an ultra-low anterior resection for a sessile rectal polyp with the histological appearance of a moderately differentiated adenocarcinoma. Seven foci of minute carcinoids in the rectum and perirectal lymph node metastastic spread from the carcinoid tumours were also discovered on histopathology. There were no lymph node metastases originating from adenocarcinoma. This case report and review of the literature suggests that minute rectal carcinoids are at risk of metastasizing and that these patients should be investigated for lymph node and distant metastatic spread with CT and somatostatin receptor scintigraphy or its equivalent, as this would influence prognosis and surgical management of these patients. Findings relating to lymphovascular invasion, perineural invasion, high Ki-67, mitotic rate, depth of tumour invasion, central ulceration, multifocal tumours and size are useful in predicting metastases and may be used in scoring tools. Size alone is not a good predictor of metastastic spread.

Pancreatic neuroendocrine tumors

Pancreatic neuroendocrine tumors (pancreatic NETs) are rare, low- to intermediate-grade neoplasms thought to arise from the pancreatic islets. Recent advances in pathology and our understanding of the biological behavior of this group of tumors has resulted in changes in their nomenclature and how we treat them. This review puts into perspective our current understanding of pancreatic NETs in terms of their incidence, pathology, and management.

Theranostics: evolution of the radiopharmaceutical meta-iodobenzylguanidine in endocrine tumors

Since 1981, meta-iodobenzylguanidine (MIBG), labeled with (131)I and later (123)I, has become a valuable agent in the diagnosis and therapy of a number of endocrine tumors. Initially, the agent located pheochromocytomas and paragangliomas (PGLs), both sporadic and familial, in multiple anatomic sites; surgeons were thereby guided to excisional therapies, which were previously difficult and sometimes impossible. The specificity in diagnosis has remained above 95%, but sensitivity has varied with the nature of the tumor: close to 90% for intra-adrenal pheochromocytomas but 70% or less for PGLs. For patients with neuroblastoma, carcinoid tumors, and medullary thyroid carcinoma, imaging with radiolabeled MIBG portrays important diagnostic evidence, but for these neoplasms, use has been primarily as an adjunct to therapy. Although diagnosis by radiolabeled MIBG has been supplemented and sometimes surpassed by newer scintigraphic agents, searches by this radiopharmaceutical remain indispensable for optimal care of some patients. The radiation imparted by concentrations of (131)I-MIBG in malignant pheochromocytomas, PGLs, carcinoid tumors, and medullary thyroid carcinoma has reduced tumor volumes and lessened excretions of symptom-inflicting hormones, but its value as a therapeutic agent is being fulfilled primarily in attacks on neuroblastomas, which are scourges of children. Much promise has been found in tumor disappearance and prolonged survival of treated patients. The experiences with therapeutic (131)I-MIBG have led to development of new tactics and strategies and to well-founded hopes for elimination of cancers. Radiolabeled MIBG is an exemplar of theranostics and remains a worthy agent for both diagnosis and therapy of endocrine tumors.

Molecular imaging of neuroendocrine tumors

Neuroendocrine tumors (NET) are a heterogeneous group of tumors that arise from neuroendocrine cells. These tumors may arise from various organs, including lung, thymus, thyroid, stomach, duodenum, small bowel, large bowel, appendix, pancreas, adrenal, and skin. Most are well differentiated and have the ability to produce biogenic amines and various hormones. NET usually occur sporadically but they also be associated with various familial syndromes. For the vast majority of NET, surgical resection is the treatment of choice whenever feasible. Localization of NET prior to surgery and for staging and follow-up relies on both anatomic and functional imaging modalities. In fact, the unique secretory characteristics of these tumors lend themselves to imaging by molecular imaging modalities, which can target specific metabolic pathways or receptors. Neuroendocrine cells have a variety of such target receptors and pathways for which radiopharmaceuticals have been developed, including [(123)I/(131)I]-metaiodobenzylguanidine (MIBG), [(111)In]pentetreotide, [(68)Ga] somatostatin analogs, [(18)F] fluorodeoxyglucose (FDG), [(11)C/(18)F] dihydroxyphenylalanine (DOPA), [(11)C] 5-hydroxytryptophan (5-HTP) (99m)Tc pentavalent dimercaptosuccinic acid ([(99m)Tc] (V) DMSA, and [(18)F] fluorodopamine (FDA). Here, we review the molecular imaging approaches for NET using various radiopharmaceuticals.

Gastrointestinal Carcinoids: An Increasing Incidence of Rectal Distribution

Carcinoid tumors are slow-growing and usually become symptomatic late in the course of the disease. We evaluated our 10-year experience in the management of GI carcinoid tumors. The records of 133 patients with GI carcinoids were reviewed. The rectum was the most common site for carcinoid tumors with an incidence of 30 per cent followed by jejunoileal at 29.3 per cent. Other sites of carcinoid tumors were the appendix (8.3%), colon (8.3%), and duodenum (3.8%). Endoscopy was the most helpful modality in diagnosing GI carcinoids. CT was not helpful in preoperative diagnosis of carcinoid tumor. Fifteen patients died in follow-up with eight deaths related to carcinoid tumors, in the small bowel (6), rectum (1), and colon (1). Overall survival was 68.7 per cent and mortality rate was 19.5 per cent from carcinoid tumors. Most of the deaths occurred in patients with carcinoid syndrome, synchronous malignancy, and malignant carcinoid tumors. The mean disease-free survival was 51 months (range, 15 to 138 months). Screening colonoscopy, in addition to decreasing colorectal adenocarcinoma mortality, is useful in diagnosing carcinoid tumors at an earlier stage and in decreasing mortality from malignant colorectal carcinoid tumors.

Scintigraphic imaging of body neuroendocrine tumors

Radionuclide imaging is often used in the diagnosis and work-up of a wide range of neoplasms, on the basis of the biologic behavior of the tumor. Neuroendocrine tumors are a subgroup of neoplasms that are generally small and slow growing, and consequently their identification with conventional anatomic imaging can be difficult. Depending on the physiologic properties of the tumor, functional images obtained with radionuclides are often complementary to anatomic images, not only in the localization of the tumor and its metastases, but also in the assessment of prognosis and response to therapy. Familiarity with the choice of the appropriate radiopharmaceutical, proper imaging protocols, and the wide range of imaging patterns will enable the radiologist to guide the clinician in case management.

Carcinoid syndrome: diagnosis and medical management

Gastro-intestinal carcinoids are slow growing tumors arising from enterochromaffin or Kulchitsky cells. Their clinical presentation depends on what combination of bioactive substances is secreted. Midgut carcinoid can present with the carcinoid syndrome in the presence of liver metastases. Its most typical clinical manifestations include cutaneous flushing and diarrhea. A nonspecific biochemical tumor marker for carcinoid tumors is serum chromogranin A and a specific marker for the carcinoid syndrome is the increased urinary excretion of 5-hydroxy indole acetic acid (5-HIAA). Localizing studies in carcinoid tumors/syndrome are: transabdominal ultrasonography (US), endoscopy, endoscopic US, videocapsule endoscopy, computerized tomography, magnetic resonance imaging, selective abdominal angiography, 111In-pentetreotide scintigraphy (and intraoperative radionuclide probe), 123I (131I)-metaiodobenzylguanidine (MIBG) scintigraphy, bone scintigraphy and 11C-5-HT positron emission tomography (PET). Therapies for carcinoid tumors/syndrome are: surgery, somatostatin analogs, interferon-alpha, radiotherapy, liver dearterialization, liver (chemo, or radio)-embolization, alcohol sclerotherapy of liver metastases, radiofrequency ablation of liver metastases, cryosurgery of liver metastases, occasionally liver transplantation, radiotherapy-coupled somatostatin analogs, 131I-MIBG and occasionally chemotherapy.

Current status of gastrointestinal carcinoids

Gastrointestinal (GI) carcinoids are ill-understood, enigmatic malignancies, which, although slow growing compared with adenocarcinomas, can behave aggressively. Carcinoids are classified based on organ site and cell of origin and occur most frequently in the GI (67%) where they are most common in small intestine (25%), appendix (12%), and rectum (14%). Local manifestations--mass, bleeding, obstruction, or perforation--reflect invasion or tumor-induced fibrosis and often result in incidental detection at emergency surgery. Symptoms are protean (flushing, sweating, diarrhea, bronchospasm), usually misdiagnosed, and reflect secretion of diverse amines and peptides. Biochemical diagnosis is established by elevation of plasma chromogranin A (CgA), serotonin, or urinary 5-hydroxyindoleacetic acid (5-HIAA), while topographic localization is by Octreoscan, computerized axial tomography (CAT) scan, or endoscopy/ultrasound. Histological identification is confirmed by CgA and synaptophysin immunohistochemistry. Primary therapy is surgical excision to avert local manifestations and decrease hormone secretion. Hepatic metastases may be amenable to cytoreduction, radiofrequency ablation, embolization alone, or with cytotoxics. Hepatic transplantation may rarely be beneficial. Chemotherapy and radiotherapy have minimal efficacy and substantially decrease quality of life. Intravenously administered receptor-targeted radiolabeled somatostatin analogs are of use in disseminated disease. Local endoscopic excision for gastric (type I and II) and rectal carcinoids may be adequate. Somatostatin analogues provide the most effective symptomatic therapy, although interferon has some utility. Overall 5-year survival for carcinoids of the appendix is 98%, gastric (types I/II) is 81%, rectum is 87%, small intestinal is 60%, colonic carcinoids is 62%, and gastric type III/IV is 33%.

Intraoperative Detection of Carcinoid Tumor of the Ovary by Gamma Probe

PURPOSE

To demonstrate the detection of carcinoid tumor of the ovary by gamma probe during surgery in a patient in whom the exact location of a metastatic tumor was uncertain.

METHODS

Twenty-four hours after injection of 350 MBq (9.5 mCi) I-123, an anterior image of the abdomen was acquired before surgery to demonstrate MIBG accumulation. Holding the ovary in the hand during the operation, a surgical gamma probe indicated the exact location of highest radioactivity.

RESULTS/CONCLUSIONS

Intraoperative MIBG scanning is a useful noninvasive detection method to localize metastases of carcinoid in the ovary. This method should be used only when it is not clear where the area of increased activity is located in the body because there are certain costs involved.

Facial flushes and diarrhoea

Images

Biology and management of the midgut carcinoid

Midgut carcinoid tumors derive from gut entoderm. These tumors may cause a complex of symptoms comprising the carcinoid syndrome by secreting a wide variety of bioactive agents in addition to serotonin. Such symptoms generally follow metastases to the liver but may also occur in primary ovarian or retroperitoneal tumors. After localization and biochemical characterization, the bioactivity of these tumors should be blocked by octreotide, sometimes in combination with other pharmacologic antagonists, so that primary resection may be performed safely. If curative resection is impossible, then a cytoreductive management scheme should be employed that includes surgical debulking and hepatic arterial embolization, followed by palliation with octreotide.