Neuroendocrine Tumor

Neuroendocrine tumors arise from cells of the diffuse neuroendocrine system – a network of specialized cells distributed throughout the body that produces hormones and neurotransmitter-like substances. These cells are primarily found in the gastrointestinal tract, the pancreas, the lungs, and the adrenal glands, which explains why NETs can develop in so many different locations.

NETs are rare overall, but they are being diagnosed with increasing frequency – presumably also because imaging and laboratory diagnostics have become more precise. In Germany, several thousand new cases are recorded annually. Men and women are affected with roughly equal frequency, and the peak age is between 50 and 70 years, though NETs can fundamentally occur at any age.

What distinguishes neuroendocrine tumors from other types of cancer is their biological diversity. Some hardly grow at all over decades, while others are aggressive from the outset. Some actively produce hormones, thereby causing characteristic symptoms, while others remain hormonally silent and only become noticeable due to their size or metastases.

In the context of neuroendocrine tumors, patients frequently encounter various abbreviations that designate important differences and should not be confused.

NET stands for neuroendocrine tumor and is the umbrella term for well- to moderately differentiated neuroendocrine neoplasms. They typically grow more slowly and carry a better prognosis. NEC stands for neuroendocrine carcinoma – a poorly differentiated, aggressive form that grows more rapidly, metastasizes earlier, and has a significantly less favorable prognosis. The distinction between NET and NEC is crucial for treatment planning.

PNET stands in the oncological context for pancreatic neuroendocrine tumor – meaning a NET that develops in the pancreas. This term should not be confused with primitive neuroectodermal tumor, which is also abbreviated as PNET but represents a completely different disease of the nervous system.

Neuroendocrine tumors can develop anywhere neuroendocrine cells are present – and this is the case in many organs of the body. The location largely determines which symptoms occur and which therapy options are available.

The gastrointestinal tract is the most common location for neuroendocrine tumors. The small intestine, the appendix, the stomach, and the duodenum are particularly frequently affected. Small bowel NETs are often functionally active and can cause the so-called carcinoid syndrome. The term carcinoid is a historical name for certain neuroendocrine tumors of the gastrointestinal tract; while it is increasingly being replaced by more precise designations in modern classification, it remains common in clinical everyday usage.

Pancreatic neuroendocrine tumors (PNETs) arise in the islet cells of the pancreas. They can be functionally active and produce specific hormones – the most common example is the Insulinoma, which leads to dangerous hypoglycemia due to excessive insulin production. Non-functional pancreatic NETs are often detected late, as they do not cause specific symptoms for a long time.

In the lungs, neuroendocrine tumors develop as typical or atypical carcinoids, which usually grow slowly, as well as large-cell or small-cell neuroendocrine carcinomas, which take a significantly more aggressive course. Lung NETs frequently become noticeable through coughing, shortness of breath, or recurrent pneumonia.

Neuroendocrine tumors can also develop in the thyroid gland, the adrenal gland, and other sites. The Pheochromocytoma is a neuroendocrine tumor of the adrenal gland that can lead to episodic high blood pressure, racing heart, and sweating fits due to the release of stress hormones such as adrenaline.

One of the most important distinctions in neuroendocrine tumors is that between functional and non-functional tumors. Functional NETs actively produce hormones or hormone-like substances that directly cause symptoms. Non-functional NETs either produce no hormones or produce those that do not trigger clinical symptoms.

Functional NETs often get noticed earlier because their hormonal symptoms are characteristic – such as hypoglycemia in insulinomas or flushing in carcinoid syndrome. Non-functional NETs, on the other hand, often grow unnoticed for years and are frequently discovered only as an incidental finding during imaging for another reason, or when they cause complaints due to their size or metastases. This explains why many patients already have liver metastases at the time of diagnosis.

The symptoms of neuroendocrine tumors are exceptionally diverse and depend heavily on whether the tumor is functionally active, where it is located, and how large it is. Many affected individuals report that they went from doctor to doctor for years with non-specific symptoms before the diagnosis was established.

Functional NETs cause symptoms through the excessive production of hormones or hormone-like substances. Carcinoid syndrome, which occurs primarily in small bowel NETs with liver metastases, manifests through episodic flushing – a sudden reddening of the face and upper body – as well as diarrhea, abdominal cramps, and, in advanced cases, involvement of the heart. Insulinomas cause symptoms such as shaking, sweating fits, confusion, or loss of consciousness due to low blood sugar. Pheochromocytoma becomes noticeable through episodic high blood pressure, a racing heart, headaches, and sweating fits.

Non-functional NETs do not cause specific symptoms for a long time. When symptoms do appear, they are often non-specific: abdominal pain, a feeling of pressure in the upper abdomen, weight loss, persistent fatigue, or a palpable mass. In lung NETs, coughing or recurrent respiratory infections can be initial clues. Frequently, a non-functional NET is an incidental finding during an ultrasound or CT examination performed for another reason.

The diagnosis of a neuroendocrine tumor requires a combination of laboratory tests, imaging, and tissue analysis. Since NETs are rare and can cause a variety of symptoms, several years often pass before a diagnosis is made.

Important tumor markers in the blood are chromogranin A, which is elevated in most NETs, as well as neuron-specific enolase (NSE). Depending on the tumor type, specific hormones are additionally determined – such as insulin and C-peptide for an insulinoma, or catecholamines and their metabolites for a pheochromocytoma. In the urine, the excretion of 5-hydroxyindoleacetic acid (5-HIAA) can indicate carcinoid syndrome.

Imaging techniques include ultrasound, CT, and MRI. Particularly characteristic of NETs is somatostatin receptor imaging, today mostly performed as a DOTATATE PET-CT. Since many NETs carry somatostatin receptors on their surface, they can be visualized very precisely using this method – including small metastases that remain invisible in conventional imaging.

The final diagnosis is established through histological examination of tumor tissue and determination of the Ki-67 index, which is crucial for grading.

Grading describes how biologically aggressive a neuroendocrine tumor is. It is based on the Ki-67 index, which indicates how many tumor cells are actively dividing. G1 tumors have a Ki-67 index below three percent and grow very slowly – many patients live stably with this for many years. G2 tumors lie between three and 20 percent and grow at a moderate pace. G3 tumors have a Ki-67 index above 20 percent. This group includes highly aggressive neuroendocrine carcinomas (NECs) on one hand, but also so-called well-differentiated G3 NETs, which grow somewhat slower than carcinomas and can respond to different therapies. This distinction is clinically significant as it directly influences the choice of chemotherapy.

Staging describes the spread of the disease. Stages I and II designate localized tumors with no or limited lymph node involvement. Stage III encompasses locally advanced tumors, while Stage IV designates the presence of distant metastases – most commonly in the liver. Even in Stage IV, G1 and G2 NETs can remain stable for many years and be treated effectively, which distinguishes them from most other types of cancer in Stage IV.

The treatment of neuroendocrine tumors is guided by location, grading, staging, functionality, and the patient’s general health condition. It is planned within an interdisciplinary team and should ideally take place in a specialized NET center.

Surgical removal of the tumor is the primary treatment measure for localized NETs and can be curative in early stages. Even in the case of liver metastases, a surgical resection can make sense in certain instances.

Somatostatin analogs such as octreotide or lanreotide are a central therapeutic pillar for NETs. They inhibit hormone production, alleviate symptoms of carcinoid syndrome, and can slow down tumor growth. For G3 NECs, chemotherapy is typically deployed, as these tumors grow more aggressively and respond less to somatostatin analogs.

PRRT – Peptide Receptor Radionuclide Therapy – is a nuclear medicine treatment option specifically developed for NETs with adequate somatostatin receptor expression. In this therapy, a radioactively labeled somatostatin analog is used, which binds targetedly to the tumor cells and irradiates them from the inside. PRRT has shown good results in studies for advanced NETs and is now an established procedure.

Neuroendocrine tumors (NETs) present a unique challenge for integrative support, as they are frequently treated over many years. Long-term therapy with somatostatin analogs brings its own burdens: diarrhea, fatty stools, gallstones, and deficiencies due to impaired nutrient absorption are among the most common side effects. Complementary medical support targets these chronic complaints specifically and can complement conventional therapy meaningfully throughout the entire course of the disease.

Since many NETs originate in the gastrointestinal tract and both the disease itself and the therapy directly affect intestinal health, an intestinal cleanse occupies a particularly important position in neuroendocrine tumors. It can improve nutrient absorption, stabilize the intestinal mucosa, and strengthen the immune system. A targeted infusion therapy with micronutrients is also highly relevant for NETs, as impaired absorption in the intestine frequently leads to clinically significant deficiencies that can hardly be compensated for orally.

The acupuncture can have a supportive effect on fatigue that often persists for years, on abdominal cramps, as well as on typical accompanying symptoms of carcinoid syndrome (such as episodic hot flashes/flushes). The mitochondrial therapy can be used for the chronic exhaustion typical of NETs to support cellular energy metabolism. An individually dosed mistletoe therapy as well as specifically oriented measures of phytotherapy can contribute to promoting general resilience.

The local hyperthermia can be considered individually in certain constellations, such as for liver metastases – the decision is always made in close consultation with the treating team.

Particular caution is warranted for NETs treated with PRRT (Peptide Receptor Radionuclide Therapy): since PRRT delivers targeted radioactive radiation to tumor cells and partly destroys them through oxidative stress, high-dose antioxidant measures should be avoided close in time to the treatment. Regenerative procedures are therefore utilized during therapy breaks so as not to impair the efficacy of the nuclear medicine treatment.

The prognosis for neuroendocrine tumors varies extraordinarily widely and depends primarily on grading and location. A generalized statement regarding life expectancy is not possible for this heterogeneous tumor group – and would also be misleading.

G1 NETs often carry a very good prognosis even in the metastatic stage. Many affected individuals live with them for ten years or longer without the disease progressing significantly. G2 NETs require closer monitoring but are likewise frequently controllable over many years. G3 NECs, on the other hand, grow rapidly and require intensive therapy – their prognosis is significantly less favorable and resembles that of other aggressive cancers.

Liver metastases are common in NETs and significantly influence the prognosis. Nonetheless, even patients with extensive liver metastases can remain stable for many years with G1 or G2 tumors. In the terminal stage, liver failure, malnutrition due to malabsorption, and the consequences of hormonal excess are paramount. In this phase, palliative support is central, with the goal of alleviating symptoms and preserving quality of life for as long as possible.

Neuroendocrine tumors are complex and raise many questions – about their development, their course, and the treatment options. We have answered the most common questions here in an objective and easy-to-understand manner.

The information listed contains relevant topics and serves to improve understanding.