Name = Neuroendocrine tumors
ICDO = M8013/3, M8041/3, M8246/3, M8247/3, M8574/3
MeshID = D018358
Neuroendocrine tumors, or more properly gastro-entero-pancreatic or gastroenteropancreatic neuroendocrine
tumors(GEP-NETs), are cancers of the interface between the endocrine (hormonal) system and the nervous system.
The neuroendocrine system
The endocrine system is a communication system in which
hormonesact as biochemical messengers to regulate physiological events in living organisms. The nervous system performs the same functions using electrical impulses as messengers. The neuroendocrine systemis the combination of those two systems, or more specifically, the various interfaces between the two systems. A GEP-NET is a tumor of any such interface.
More specifically, the endocrine system is primarily a network of glands that produce and secrete hormones, usually into the bloodstream. It also includes cells that are not part of glands: the diffuse neuroendocrine system, scattered throughout other organs.
A hormone is a chemical that delivers a particular message to a particular organ, typically remote from the hormone's origin. For example, the hormone
insulin, secreted by the pancreas, acts primarily to allow glucose to enter the body's cells for use as fuel. The hormone gastrinis secreted by the stomachto tell the stomach to produce acids to digest food.
Hormones can be divided into subtypes such as
peptides, steroids, and neuroamines. For some researchers, there is no clear distinction between peptide hormones and peptides; the hormones are simply longer than other peptides. In the context of GEP-NETs, the terms "hormone" and "peptide" are often used interchangeably.
Classification of GEP-NETs by site of origin and by symptom
The vast majority of GEP-NETs fall into two nearly distinct categories:
carcinoids, and pancreatic endocrine tumors (PETs). Despite great behavioral differences between the two, they are grouped together as GEP-NETs because of similarities in cell structure. ["The main two groups of neuroendocrine GEP tumours are so-called carcinoid tumours and endocrine pancreatic tumours" (Öberg 2005a, 90, ).
"Less than 1% of carcinoids arise in the pancreas" (Warner 2005, 9).
Arnold "et alia" in effect define carcinoids as "extra-pancreatic endocrine gastronintestinal tumors" (Arnold "et al." 2004, 196).
Some doctors believe that there "is" significant overlap between PETs and carcinoids. For example, endocrine surgeon Rodney Pommier says that "there are pancreatic carcinoids" (Pommier 2003, [http://www.carcinoid.org/pcf/lectures/docs/Pommier.htm] ). However, Pommier made his statement in a talk at a conference on carcinoids, not in a peer-reviewed journal; and in his talk he did not define the word "carcinoid".
Another way to classify GEP-NETs is to separate those that begin in the glandular neuroendocrine system from those that begin in the diffuse neuroendocrine system. "Neuroendocrine tumors generally may be classified into two categories. The first category is an organ-specific group arising from neuroendocrine organs such as pituitary gland, thyroid, pancreas, and adrenal gland. The second group arises from the diffuse neuroendocrine cells/Kulchitsky cells that are widely distributed throughout the body and are highly concentrated in the pulmonary and gastrointestinal systems" (Liu "et al." 2001, [http://www.nature.com/modpathol/journal/v14/n9/full/3880406a.html] ).]
Siegfried Oberndorfer, in 1907, was the first person to distinguish clearly what we now call GEP-NETs from other forms of cancer. He gave the term "carcinoid" to these tumors, because they were so slow-growing that he considered them to be "cancer-like" rather than truly cancerous.In 1929, he reported that some such tumors were not so indolent – these he distinguished as what we now call PETs from what most authorities call carcinoids.Despite the differences between the two categories, some doctors, including oncologists, persist in calling all GEP-NETs "carcinoid", even into the twenty-first century. ["The term 'carcinoid' was introduced by S [iegfried] Oberndorfer in 1907 to distinguish carcinoids as less rapidly growing and well-differentiated epithelial tumors of the small intestine from the more aggressively growing adenocarcinoma of the gut.... Strictly speaking ... the term 'carcinoid' [is] reserved for endocrine tumors of the gastrointestinal tract ... and not for those of the pancreas.... The vagueness of the term 'carcinoid' results from its histological features which are almost identical with those of endocrine pancreatic tumors" (Arnold "et al." 2004, 195).
"During his tenure at the Pathological Institute of the University of Munich, Oberndorfer noted in 1907 that the lesions were distinct clinical entities and named them 'karzinoide' ('carcinoma-like'), emphasizing in particular their benign features. In 1929 he amended his classification to include the possibility that these small bowel tumors could be malignant and also metastasize" (Modlin 2004).
Regarding the persistence of the term "carcinoid": on a PET news forum, a patient says "for over 4 yrs I was referred to as carcinoid. In fact even when it was discovered I was actually islet cell and the pathologist was questioned by my doctor (specialist in NET) about this the pathology said we classify all islet cell as carcinoid. Since then no matter how many times I say islet cell to a local doctor - oncologist, gastroendocrineologist, radiologist - you get the idea - they ALL continue to refer to me as carcinoid" (NET_IsletCell 2006).]
Pancreatic endocrine tumors (PETs) are also known as endocrine pancreatic tumors (EPTs) or islet cell tumors. PETs are assumed to originate generally in the
islets of Langerhanswithin the pancreas – or, Arnold "et alia" suggest, from endocrine pancreatic precursor cells (Arnold "et al." 2004, 199) – though they may originate outside of the pancreas. (The term " pancreatic cancer" almost always refers to "adenopancreatic cancer", also known as "exocrine pancreatic cancer". Adenopancreatic cancers are generally very aggressive, and are not neuroendocrine cancers. About 95 percent of pancreatic tumors are adenopancreatic; about 1 or 2 percent are GEP-NETs.) [According to Arnold "et alia", "endocrine pancreatic tumors are also called 'islet cell tumors'" (Arnold "et al." 2004, 199).
Many web sites state that 95 percent of pancreatic tumors are adenopancreatic, and that the remainder are nearly all neuroendocrine. But Warner states: "Clinically significant PETs ... account for only 1% - 2% of all pancreatic tumors" (Warner 2005, 3). ]
PETs may secrete hormones (as a result, perhaps, of impaired storage ability), and those hormones can wreak symptomatic havoc on the body. Those PETs that do not secrete hormones are called "nonsecretory" or "nonfunctioning" or "nonfunctional" tumors. Secretory tumors are classified by the hormone most strongly secreted – for example, insulinoma, which produces excessive insulin, and gastrinoma, which produces excessive gastrin (see more detail in the summary below).
Carcinoid tumors are further classified, depending on the point of origin, as foregut (
lung, thymus, stomach, and duodenum) or midgut (distal ileumand proximal colon) or hindgut (distal colon and rectum). Less than one percent of carcinoid tumors originate in the pancreas. But for many tumors, the point of origin is unknown.
Carcinoid tumors tend to grow much more slowly than PETs. A carcinoid tumor may produce serotonin (
5-HT), a biogenic aminethat causes a specific set of symptoms including
* diarrhea or increase in number of bowel movements
* weight loss
* weight gain
* heart palpitations
congestive heart failure(CHF)
This set of symptoms is called "carcinoid syndrome". Although this serotonin secretion is entirely different from a secretory PET's hormone secretion, carcinoid tumors with carcinoid syndrome are nevertheless sometimes called "functioning", adding to the frequent confusion of carcinoids with PETs. Carcinoid syndrome is primarily associated with midgut carcinoids. A severe episode of carcinoid syndrome is called "carcinoid crisis"; it can be triggered by surgery or chemotherapy, among other factors. [Larry Kvols, of the
Moffitt Cancer Center and Research Institutein Tampa, Florida, lists flushing, diarrhea, CHF, and asthma as the four critical characteristics of carcinoid syndrome (Kvols 2002, [http://www.carcinoid.org/pcf/lectures/docs/KVOLS.htm] ). ]
The mildest of the carcinoids are discovered only upon surgery for unrelated causes. These "coincidental carcinoids" are common; one study found that one person in ten has them. [" [In] 800 autopsy cases, ... incidence of tumor was 10% (6/60) in individuals having histological studies of all sections of the pancreas" (Kimura, Kuroda, and Morioka 1991, [http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&dopt=Abstract&list_uids=2070707] ).
Small tumors are not necessarily harmless: Rodney Pommier tells of a "chick pea-sized tumor causing [so much] hormonal effect" that the patient was wheelchair-bound, unable to walk (Pommier 2003, [http://www.carcinoid.org/pcf/lectures/docs/Pommier.htm] ). ]
Neuroendocrine tumors other than coincidental carcinoids are rare. Incidence of PETs is estimated at one new case per 100,000 people per year; incidence of clinically significant carcinoids is twice that. Thus the total incidence of GEP-NETs in the United States would be about 9,000 new cases per year. But researchers differ widely in their estimates of incidence, especially at the level of the secretory subtypes (the various "-omas"). ["The incidence of all noncarcinoid NETs is approximately one half that of all carcinoids. Noncarcinoid NETs have been reported to occur in .4 to 1.5/100,000 of the population.... Clinically significant PETs have been reported to occur in approximately 1 per 100,000 people per year" (Warner 2005, 1). One per 100,000 per year implies 3,000 new PET cases per year for the US population of roughly 300 million. And that, with Warner's other numbers, implies 6,000 new carcinoid cases for a total of 9,000 new GEP-NET cases per year. ]
In addition to the two main categories, there are even rarer forms of GEP-NETs. At least one form – neuroendocrine lung tumors – arises from the respiratory rather than the gastro-entero-pancreatic system.
Non-human animals also suffer from GEP-NETs; for example, neuroendocrine cancer of the liver is a disease of dogs, and
Devil facial tumor diseaseis a neuroendocrine tumor of Tasmanian Devils. ["The tumors [of Devil facial tumor disease] have been characterized as a neuroendocrine cancer" (Bostanci 2005).
Owen and Pemberton, in their book on Tasmanian Devils, discuss Devil facial tumor disease (DFTD) extensively, describing it as a cancer with possible viral or toxic causes or triggers. They never use the term "neuroendocrine". "Although cancer is a major cause of devil mortality, it's usually internal", they report (Owen and Pemberton 2005, 171). They discuss the first "official" case of DFTD, in 1995 (Owen and Pemberton 2005, 187), and quote Loh's description thereof: "subcutaneous lymph nodes enlarged and also scattered dermal and subcutaneous swellings, ... necrotic reactionary lesion in the masseter muscle, ... multifocal dermal leukosis with lymphosarcomatous infiltration of lymph nodes and in the periportal tissues of the liver and interstitially in the adrenal and also through skeletal muscle tissue, ... lymphosarcomatous infiltration of ... the masseter muscle, ... widespread lymphosarcomatous neoplasia, ... [and] occasional, but consistent findings of azurophilic intracytoplasmic material in some cells. Artefact or viral inclusions?" (Loh 2003). ]
Rufini "et alia" summarize: "Neuroendocrine tumors (NETs) are a heterogeneous group of neoplasms originating from endocrine cells, which are characterized by the presence of secretory granules as well as the ability to produce biogenic amines and polypeptide hormones. These tumors originate from endocrine glands such as the adrenal medulla, the pituitary, and the parathyroids, as well as endocrine islets within the thyroid or the pancreas, and dispersed endocrine cells in the respiratory and gastrointestinal tract. The clinical behavior of NETs is extremely variable; they may be functioning or not functioning, ranging from very slow-growing tumors (well-differentiated NETs), which are the majority, to highly aggressive and very malignant tumors (poorly differentiated NETs).... Classically, NETs of the gastrointestinal tract are classified into 2 main groups: (1) carcinoids, ... and (2) endocrine pancreatic tumors (EPTs)" (Rufini, Calcagni, and Baum 2006). (Note that the definition of "well-differentiated" may be counterintuitive: a tumor is well-differentiated if its cells are similar to normal cells, which have a well-differentiated structure of nucleus, cytoplasm, membrane, etc.)
Ramage "et alia" provide a summary that differs somewhat from that of Rufini "et alia": "NETs ... originate from pancreatic islet cells, gastroenteric tissue (from diffuse neuroendocrine cells distributed throughout the gut), neuroendocrine cells within the respiratory epithelium, and parafollicullar cells distributed within the thyroid (the tumours being referred to as medullary carcinomas of the thyroid). Pituitary, parathyroid, and adrenomedullary neoplasms have certain common characteristics with these tumours but are considered separately" (Ramage "et al." 2005, [http://gut.bmj.com/cgi/content/full/54/suppl_4/iv1] ).
ummary of human GEP-NETs by site of origin and by symptom
* carcinoids (about two thirds of GEP-NETs)
** with carcinoid syndrome (about 10 percent of carcinoids)
** without carcinoid syndrome (about 90 percent of carcinoids)
* PETs (about one third of GEP-NETs)
** nonfunctioning (15 to 30 percent of PETs)
** functioning (70 to 85 percent of PETs)
gastrinoma, producing excessive gastrinand causing Zollinger-Ellison Syndrome(ZES)
insulinoma, producing excessive insulin
glucagonoma, producing excessive glucagon
*** vasoactive intestinal peptideoma (VIPoma), producing excessive
vasoactive intestinal peptide(VIP)
*** PPoma, producing excessive
pancreatic polypeptide(often classed with nonfunctioning PETs)
somatostatinoma, producing excessive somatostatin
*** watery diarrhea, hypokalemia-achlorhydria (WDHA)
*** CRHoma, producing excessive corticotropin-releasing hormonse (CRH)
*** calcitoninoma, producing excessive
*** GHRHoma, producing excessive growth-hormone-releasing hormone (GHRH)
*** neurotensinoma, producing excessive
*** ACTHoma, producing excessive
*** GRFoma, producing excessive
Growth hormone-releasing factor(GRF)
parathyroidhormone–related peptide tumor
* rare GEP-NETs
** medullary carcinoma of the
Merkel cell cancer(trabecular cancer)
** small-cell lung cancer (SCLC)
** large-cell neuroendocrine carcinoma (of the lung)
** neuroendocrine carcinoma of the
** Multiple Endocrine Neoplasia type 1 (MEN-1 or MEN1) (usually nonfunctioning) (also causing ZES)
** Multiple Endocrine Neoplasia type 2 (MEN-2 or MEN2)
** neurofibromatosis type 1
** tuberous sclerosis
** von Hippel-Lindau (VHL) disease
** neuroendocrine tumor of the anterior
** Carney's complex
Classification of GEP-NETs by cell characteristics
The diverse and amorphous nature of GEP-NETs has led to a confused, overlapping, and changing terminology. In general, aggressiveness (malignancy), secretion (of hormones), and anaplasia (dissimilarity between tumor cells and normal cells) tend to go together, but there are many exceptions, which have contributed to the confusion in terminology. For example, the term "atypical carcinoid" is sometimes used to indicate an aggressive tumor without secretions, whether anaplastic or well-differentiated.
In 2000, the World Health Organization (WHO) revised the classification of GEP-NETs, abandoning the term "carcinoid" in favor of "neuroendocrine tumor" (NET) and abandoning "islet cell tumor" or "pancreatic endocrine tumor" for "neuroendocrine carcinoma" (NEC). Judging from papers published into 2006, the medical community is accepting this new terminology with great sluggishness. (Perhaps one reason for the resistance is that the WHO chose to label the least aggressive subclass of neuroendocrine neoplasm with the term – "neuroendocrine tumor" – widely used previously either for the superclass or for the generally aggressive noncarcinoid subclass.)
Klöppel "et alia" have written an overview that clarifies the WHO classification and bridges the gap to the old terminology (Klöppel, Perren, and Heitz 2004). In this article we conform to the old terminology.
ummary of classification by cell characteristics (the WHO classification)
** Öberg, WHO, Klöppel "et alia": gastro-entero-pancreatic neuroendocrine tumor (GEP-NET)
* Subclass 1 (less malignant)
** Öberg: carcinoid
** WHO: neuroendocrine tumor (NET)
** Klöppel "et alia": well-differentiated neuroendocrine tumor (NET) (carcinoid)
** this article: carcinoid
* Subclass 2 (more malignant)
** Öberg: endocrine pancreatic tumor
** WHO: neuroendocrine carcinoma (NEC)
** Klöppel "et alia": well-differentiated neuroendocrine carcinoma (NEC) (malignant carcinoid)
** this article: pancreatic endocrine tumor (PET) or endocrine pancreatic tumor (EPT) or islet cell tumor or noncarcinoid GEP-NET
* Subclass 3 (most malignant)
** WHO: poorly-differentiated neuroendocrine carcinoma
** Klöppel "et alia": poorly-differentiated neuroendocrine carcinoma (high-grade malignant carcinoid)
* Subclass 4 (mixed)
** WHO: mixed endocrine/exocrine tumor
* Subclass 5 (miscellaneous)
** WHO: rare neuroendocrine-like lesions
GEP-NETs are also sometimes called "APUDomas", but that term is now considered to be misleading, since it is based on a discredited theory of the development of the tumors. ["The APUD concept led to the belief that these cells arise from the embryologic neural crest. This hypothesis eventually was found to be incorrect" (Warner 2005, 2).
"The APUD-concept is currently abandoned" (Öberg 1998, 2, [http://theoncologist.alphamedpress.org/cgi/reprint/3/5/339.pdf] ).]
Metastases and malignancy
In the context of GEP-NETs, the terms "
metastatic" and " malignant" are often used interchangeably.
GEP-NETs are often malignant, since the primary site often eludes detection for years, sometimes decades – during which time the tumor has the opportunity to metastasize. Researchers differ widely in their estimates of malignancy rates, especially at the level of the secretory subtypes (the various "-omas").
The most common metastatic sites are the liver, the lymph nodes, and the bones. Liver metastases are so frequent and have such prominent blood supply that for many patients, they dominate the course of the cancer. For a patient with a nonsecretory PET, for example, the primary threat to life may be the sheer bulk of the tumor load in the liver.
CT-scans, MRIs, sonography(ultrasound), and endoscopy(including endoscopic ultrasound) are common diagnostic tools. Symptoms from hormone secretions or from carcinoid syndrome, or measures of the corresponding hormones in the blood, can aid in diagnosis. CT-scans using contrast medium can detect 95 percent of tumors over 3 cm in size, and no tumors under 1 cm (University of Michigan Medical School n. d., [http://www.med.umich.edu/lrc/presentation/endo/islet.htm] ).
According to Arnold "et alia", "many tumors are asymptomatic even in the presence of metastases" (Arnold "et al." 2004, 197).
Cells that receive hormonal messages do so through receptors on the surface of the cells. For reasons that are not understood, many neuroendocrine tumor cells possess especially strong receptors; for example, PETs often have strong receptors for
somatostatin, a very common hormone in the body. We say that such tumor cells "overexpress" the somatostatin receptors (SSTRs) and are thus "avid for" the hormone; their "uptake" of the hormone is strong. This avidity for somatostatin is a key for diagnosis – and it makes the tumors vulnerable to certain "targeted therapies", described below.
However, the half-life of somatostatin in circulation is under three minutes, making it useless for diagnosis and targeted therapies. For this reason, synthetic forms of somatostatin are used instead; the earliest was
octreotide, first marketed, by Sandozas Sandostatin, in 1988. The synthetic forms are typically called "somatostatin analogs" ("somatostatin analogues"), but according to the US Food and Drug Administration(FDA), the proper term is "somatostatin congeners". (In this article we conform to the old terminology, as the medical community has been slow to adopt the term " congener".) The analogs have a much longer half-life than somatostatin, and other properties that make them more suitable for diagnosis and therapy.
The diagnostic procedure that utilizes a somatostatin analog is the OctreoScan, also called somatostatin receptor scintigraphy (SRS or SSRS): a patient is injected with octreotide chemically bound to a radioactive substance, often
indium-111; for those patients whose tumor cells are avid for octreotide, a radiation-sensitive scan can then indicate the locations of the larger lesions.
An OctreoScan is a relatively crude test that generates subjective results. A
gallium-68 receptor PET-CT, integrating a PET image with a CT image, is much more senstitive than an OctreoScan, and it generates objective (quantified) results in the form of a standardized uptake value (SUV).
Fluorodeoxyglucose (FDG)-PET is not useful in diagnosis of GEP-NETs. As Warner says, "Most GEP NETs do not image with fluorodeoxyglucose PET" (Warner 2005, 7). According to Öberg, new PET "tracers such as C-5-hydroxy-L-tryptophan show very high sensitivity for detection of tumors, higher than for somatostatin receptor scintigraphy" (Öberg 2005b, [http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&dopt=Abstract&list_uids=15933475] ).
The list of potential markers for GEP-NETs is long. Aside from the hormones of secretory tumors, the most important markers are
* achaete-scute homolog 1 (ASCL1)
* urine 5-hydroxy indole acetic acid (5-HIAA) (grade C)
* neuron-specific enolase (NSE, gamma-gamma dimer)
* synaptophysin (P38)
and other markers include
* synaptobrevin (VAMP-1)
* synapsin (1A, 1B, 2A, 2B)
* protein P65
* protein S-100
* protein gene product (PGP) 9.5
* intermediate filaments (cytokeratins, vimentin, neurofilaments)
* protein 7B2
* chromogranin B (secretogranin I)
* chromogranin C (secretogranin II)
* cytochrome b561
* leu-7 (HNK-1)
* human chorionic gonadotropin-alpha (HCG-α)
* human chorionic gonadotropin-beta (HCG-β)
* thyroid function tests (TFTs)
* parathyroid hormone (PTH)
* carcinoembryonic antigen (CEA)
* β-human chorionic gonadotrophin (β-HCG) (grade D)
* N Peptide K
* neurokinin A
* substance P
* various rarer peptide hormones
and newer (as of 2005) markers include
* N-terminally truncated variant of heat shock protein 70 (Hsp70)
* CDX-2, a homeobox gene product
* neuroendocrine secretory protein-55
Aside from their use in diagnosis, some markers can track the progress of therapy while the patient avoids the detrimental side-effects of CT-scan contrast.
According to Warner, the best care, at least for noncarcinoid GEP-NETs, is provided by "an active [as opposed to wait-and-see] approach using sequential multimodality treatment" delivered by a "multidisciplinary team, which also may include a surgeon, endocrinologist, oncologist, interventional radiologist, and other specialists". This recommendation is based on his view that, except for most insulinomas, "almost all" PETs "have long-term malignant potential" – and in sixty percent of cases, that potential is already manifest. "Indeed, the most common cause of death from PETs is hepatic [that is, liver] failure" (Warner 2005, 4).
Two tricky issues in evaluating therapies are durability (is the therapy long-lasting?) and stasis (are the tumors neither growing nor shrinking?). For example, one therapy might give good initial results – but within months the benefit evaporates. And another therapy might be disparaged by some for causing very little tumor shrinkage, but be championed by others for causing significant tumoristasis.
urgery and chemotherapy
Surgery is the only therapy that can cure GEP-NETs. However, the typical delay in diagnosis, giving the tumor the opportunity to metastasize, makes most GEP-NETs ineligible for surgery (non-resectable).
There is "no established standard therapy for the liver metastasis of pancreatic endocrine tumors" (Sato "et al." 2000, [http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&dopt=Abstract&list_uids=10791232] ). The most common nonsurgical therapy for all GEP-NETs is chemotherapy, although chemotherapy is reported to be largely ineffective for carcinoids, not particularly durable (long-lasting) for PETs, and inappropriate for PETs of nonpancreatic origin. [Ramage "et alia" say that "response to chemotherapy in patients with strongly positive carcinoid tumours was of the order of only 10% whereas patients with SSRS negative tumours had a response rate in excess of 70%. The highest response rates with chemotherapy are seen in the poorly differentiated and anaplastic NETs: response rates of 70% or more have been seen with cisplatin and etoposide based combinations. These responses may be relatively short lasting in the order of only 8–10 months. Response rates for pancreatic islet cell tumours vary between 40% and 70% and usually involve combinations of streptozotocin (or lomustine), dacarbazine, 5-fluorouracil, and adriamycin. However, the best results have been seen from the Mayo clinic where up to 70% response rates with remissions lasting several years have been seen by combining chemoembolisation of the hepatic artery with chemotherapy. The use of chemotherapy for midgut carcinoids has a much lower response rate, with 15–30% of patients deriving benefit, which may only last 6–8 months (Ramage "et al." 2005, [http://gut.bmj.com/cgi/content/full/54/suppl_4/iv1] ).
For 125 patients with histologically proven unresectable islet-cell carcinomas, "median duration of regression was 18 months for the doxorubicin combination and 14 months for the 5-FU combination" (Arnold "et al." 2004, 230).]
When chemotherapy fails, the most common therapy, in the United States, is more chemotherapy, with a different set of agents. Some studies have shown that the benefit from one agent is not highly predictive of the benefit from another agent, except that the long-term benefit of any agent is likely to be low.
Strong uptake of somatostatin analogs is a negative indication for chemo.
There are two major somatostatin-analog-based targeted therapies. The first of the two therapies provides symptomatic relief for patients with secretory tumors. In effect, somatostatin given subcutaneously or intramuscularly "clogs up" the receptors, blocking the secretion of hormones from the tumor cells. Thus a patient who might otherwise die from severe diarrhea caused by a secretory tumor can gain additional years of life.
Specific counter-hormones or other hormone-blocking medications are sometimes also used to provide symptomatic relief.
Hormone-delivered radiotherapy – PRRT
The second of the two major somatostatin-analog-based targeted therapies is called "peptide receptor radionuclide therapy" (PRRT), though we might simply call it "hormone-delivered
radiotherapy". In this form of "radioisotope therapy" (RIT), radioactive substances (called " radionuclides" or "radioligands") are chemically "conjugated" with hormones (peptides or neuroamines); the combination is given intravenously to a patient who has good uptake of the chosen hormone. The radioactive labelled hormones enter the tumor cells, and the attached radiation damages the tumor- and nearby cells. Not all cells are immediately killed this way. The process of tumor cells dying as result of this therapy can go on for several months, even up to two years. In patients with strongly overexpressing tumor cells, nearly all the radiation either gets into the tumors or is excreted in urine. As Rufini "et alia" say, GEP-NETs "are characterized by the presence of neuroamine uptake mechanisms and/or peptide receptors at the cell membrane, and these features constitute the basis of the clinical use of specific radiolabeled ligands, both for imaging and therapy" (Rufini, Calcagni, and Baum 2006, [http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&dopt=Abstract&list_uids=16762613] ).
The use of PRRT for GEP-NETs is similar to the use of
iodine-131 as a standard therapy (in use since 1943) for nonmedullary thyroid tumors (which are not GEP-NETs). Thyroid cells (whether normal or neoplastic) tend to be avid for iodine, and nearby cells are killed when iodine-131 is infused into the bloodstream and is soon attracted to thyroid cells. Similarly, overexpressing GEP-NET cells (neoplastic cells only) are avid for somatostatin analogs, and nearby cells are killed when radionuclides attached to somatostatin analogs are infused into the bloodstream and are soon attracted to the tumor cells. In both therapies, hormonal targeting delivers a much higher dose of radiation than external beam radiation could safely deliver.
As of 2006, PRRT is available in at least dozen medical centers in Europe. In the USA it is FDA-approved, and available at Excel Diagnostics/The RITA Foundation at 9701 Richmond Avenue, Houston, Texas, but using a radionuclide,
indium-111, that is much weaker than the lutetium-177 and the even stronger yttrium-90 used on the European continent. In the UK, only the radionuclide metaiodobenzylguanidine (I-MIBG) is licensed (but GEP-NETs are rarely avid for MIBG). Most patients (from all over the world) are treated (with lutetium-177) in The Netherlands, at the Erasmus Medical Center. PRRT with lutetium or yttrium is nowhere an "approved" therapy, but the German health insurance system, for example, covers the cost for German citizens.
PRRT using yttrium or lutetium was first applied to humans about 1999. Practitioners continue to refine their choices of radionuclides to maximize damage to tumors, of somatostatin analogs to maximize delivery, of chelators to bind the radionuclides with the hormones (and chelators can also increase uptake), and of protective mechanisms to minimize damage to healthy tissues (especially the kidneys). [A search for "peptide receptor radionuclide therapy", in quotes, at http://pubmed.org, and an examination of the resulting abstracts, shows that PMID 10399029, published in 1999, was the first article stating that indium had been used on humans. Referring to yttrium-90, the article mentions that "the first PRRT trials with [90Y-DOTA0,Tyr3] octreotide started recently". PMID 15653653, published in January, 2005, contains the first results of the use of lutetium and yttrium on humans.]
Hepatic artery-delivered therapies
One therapy for liver metastases of GEP-NETs is hepatic artery embolization (HAE). Larry Kvols, of the
Moffitt Cancer Center and Research Institutein Tampa, Florida, says that "hepatic artery embolization has been quite successful. During that procedure a catheter is placed in the groin and then threaded up to the hepatic artery that supplies the tumors in the liver. We inject a material called embospheres [tiny spheres of glass or resin, also called "microspheres"] into the artery and it occludes the blood flow to the tumors, and in more than 80% of patients the tumors will show significant tumor shrinkage" (Kvols 2002, [http://www.carcinoid.org/pcf/lectures/docs/KVOLS.htm] ). HAE is based on the observation that tumor cells get nearly all their nutrients from the hepatic artery, while the normal cells of the liver get about 75 percent of their nutrients (and about half of their oxygen) from the portal vein, and thus can survive with the hepatic artery effectively blocked. ["The liver gets about 80% of its blood and half the oxygen from the portal vein, and only 20% of the blood and the other 50% of the oxygen from the artery.... The liver gets 80% of its blood from the portal vein and 20% from that little hepatic artery. But tumors get 100% of their blood off the hepatic artery, and this has been shown by multiple lines of evidence (Pommier 2003, [http://www.carcinoid.org/pcf/lectures/docs/Pommier.htm] ).
"The normal liver gets its blood supply from two sources; the portal vein (about 70%) and the hepatic artery (30%)" (Fong and Schoenfield n. d., [http://www.medicinenet.com/chemo_infusion_and_chemoembolization_of_liver/article.htm] ).]
Another therapy is hepatic artery chemoinfusion, the injection of chemotherapy agents into the hepatic artery. Compared with systemic chemotherapy, a higher proportion of the chemotherapy agents are (in theory) delivered to the lesions in the liver. ["The theoretical advantage is that higher concentrations of the agents can be delivered to the tumors without subjecting the patients to the systemic toxicity of the agents.... In reality, however, much of the chemotherapeutic agents does end up in the rest of the body" (Fong and Schoenfield, [http://www.medicinenet.com/chemo_infusion_and_chemoembolization_of_liver/article.htm] ).]
Hepatic artery chemoembolization (HACE), sometimes called transarterial chemoembolization (TACE), combines hepatic artery embolization with hepatic artery chemoinfusion: embospheres bound with chemotherapy agents, injected into the hepatic artery, lodge in downstream capillaries. The spheres not only block blood flow to the lesions, but by halting the chemotherapy agents in the neighborhood of the lesions, they provide a much better targeting leverage than chemoinfusion provides.
Radioactive microsphere therapy (RMT) combines hepatic artery embolization with radiation therapy – microspheres bound with radionuclides, injected into the hepatic artery, lodge (as with HAE and HACE) in downstream capillaries. This therapy is also called "selective internal radiation therapy", or "SIRT". In contrast with PRRT, the lesions need not overexpress peptide receptors. (But PRRT can attack all lesions in the body, not just liver metastases.) Due to the mechanical targeting, the yttrium-labeled microspheres "are selectively taken up by the tumors, thus preserving normal liver" (Salem "et al." 2002, [http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&dopt=Abstract&list_uids=12354840] ). [The "microspheres preferentially cluster around the periphery of tumor nodules with a high tumor:normal tissue ratio of up to 200:1". The SIRT-spheres therapy is not FDA-approved for GEP-NETs; "it is FDA approved for liver metastases secondary to colorectal carcinoma and is under investigation for treatment of other liver malignancies, such as hepatocellular carcinoma and neuroendocrine malignancies" (Welsh, Kennedy, and Thomadsen 2006, [http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&dopt=Abstract&list_uids=16979443] ). ]
Radiofrequency ablation (RFA) is used when a patient has relatively few metastases. In RFA, a needle is inserted into the center of the lesion and is vibrated at high frequency to generate heat; the tumor cells are killed by cooking.
Cryoablation is similar to RFA; an endothermic substance is injected into the tumors to kill by freezing. Cryoablation has been considerably less successful for GEP-NETs than RFA.
Interferon is sometimes used to treat GEP-NETs; its use was pioneered by Dr. Kjell Öberg at Uppsala. For GEP-NETs, Interferon is often used at low doses and in combination with other agents (especially somatostatin analogs such as octreotide). But some researchers claim that Interferon provides little value aside from symptom control.
As described above, somatostatin analogs have been used for about two decades to alleviate symptoms by blocking the production of hormones from secretory tumors. They are also integral to PRRT. In addition, some doctors claim that, even without radiolabeling, even patients with nonsecretory tumors can benefit from somatostatin analogs, which purportedly can shrink or stabilize GEP-NETs. But some researchers claim that this "cold" octreotide provides little value aside from symptom control.
Finally, therapies based on growth factor inhibitors are in the experimental stage. These inhibitors of
epidermal growth factor receptors(EGFRs), of vascular endothelial growth factor receptors(VEGFRs), and of angiopoietin-related growth factor(AGF) include imatinib, sunitinib, temozolide, thalidomide, sorafenib, and panitumumab.
Fong, Tse-Ling Fong and Leslie J. Schoenfield. N. d. Arterial Chemotherapy Infusion of the Liver Chemoembolization of the Liver (TACE), http://www.medicinenet.com/chemo_infusion_and_chemoembolization_of_liver/article.htm.
Kvols LK. 2002. Carcinoid Tumors and the Carcinoid Syndrome: What's New in the Therapeutic Pipeline. "The Carcinoid Cancer Foundation: Carcinoid Symposium 2002", http://www.carcinoid.org/pcf/lectures/docs/KVOLS.htm.
Loh, Richmond. 2003. Tasmanian Devil ("Sarcophilus harrisii") facial tumour (DFT). Paper prepared for the Devil Facial Tumour Disease Workshop, Sir Raymond Ferral Centre, University of Tasmania, Newnham, 14 October 2003: 2.
Öberg K. 2005a. Neuroendocrine Gastroenteropancreatic Tumours: Current Views on Diagnosis and Treatment. "Business Briefing: European Oncology Review 2005" 90-2.
Pommier R. 2003. The role of surgery and chemoembolization in the management of carcinoid. California Carcinoid Fighters Conference. October 25, http://www.carcinoid.org/pcf/lectures/docs/Pommier.htm.
cite journal |author=Rufini V, Calcagni ML, Baum RP |title=Imaging of neuroendocrine tumors |journal=Semin Nucl Med. |volume=36 |issue=3 |pages=228–47 |year=2006 |month=Jul |pmid=16762613 |doi=10.1053/j.semnuclmed.2006.03.007 |url==
University of Michigan Medical School. N. d. Tumoral secretion of hormones normally produced in islets, http://www.med.umich.edu/lrc/presentation/endo/islet.htm.
If you can't open the .ppt pages listed below, download and install (free) OpenOffice from http://openoffice.org, and the page will then be readable. If you prefer, you can use any other PowerPoint reader.
* http://www.nanets.net North American Neuroendocrine Tumor Society
* [http://www.caringforcarcinoid.org Caring for Carcinoid Foundation]
* [http://www.rare-cancer.org Rare Cancer Organization]
* http://www.carcinoid.org (with information on noncarcinoid GEP-NETs as well)
** [http://www.carcinoid.org/medpro/docs/WolteringVinikISIbook1.pdf Neuroendocrine Tumors: A Comprehensive Guide to Diagnosis and Management]
* http://www.netpatientfoundation.com (UK)
* [http://uc.jinr.ru/3SummerSchool/Beyer4.pdf#search=%22prrt%20geneva%20switzerland%22 Radioisotopes in Medicine]
* http://www.med.umich.edu/lrc/presentation/endo/islet.htm (overview in tabular format)
* [http://www.prrt.nl/index.php?lang=en Erasmus MC Lutetium Treatment] About peptide receptor radionuclide therapy (PRRT)
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