Neuroendocrine tumors (NETs): everything there is to know!

What is a neuroendocrine tumor (NET)? 

NETs originate in the cells of the neuroendocrine system. The neuroendocrine system is made up of cells called neuroendocrines, which are scattered throughout the body. They are similar to nerve cells (neurons) but they also produce hormones (endocrine cells). They receive signals from the nervous system and respond by producing and secreting hormones that control different body functions, such as digestion and breathing.

Neuroendocrine cells may sometimes undergo changes that make their growth pattern or behavior abnormal. These changes can lead to the formation of neuroendocrine tumors (NETs).

NETs can be detected before they spread. However, sometimes these changes can cause cancer and therefore spread to other parts of the body. This type of cancer is known as neuroendocrine carcinoma.

Neuroendocrine tumors are named and classified according to the location of the original tumor. For example, they can appear in the organs of the digestive tract (small intestine, rectum, stomach, colon, esophagus, appendix), in which case they are known as gastrointestinal neuroendocrine tumors (GINETs).

But they can also develop in the lungs (lung NETs) or in the pancreatic islet cells present in the pancreas (pancreatic NETs or p-NEETs).

Other types of neuroendocrine tumors exist, such as medullary thyroid carcinoma, pheochromocytoma (adrenal tumor) and Merkel cell carcinoma (a rare type of non-melanoma skin cancer).

What are the symptoms of a neuroendocrine tumor (NET)? 

Given the variety of NETs, the symptoms they cause are diverse. This is why they can go undetected until the disease reaches advanced stages.

The most common neuroendocrine tumors are gastroenteropancreatic neuroendocrine tumors.

When the tumor is ‘functional’, it causes excessive secretion of hormones, which may be the reason for its discovery.

It is at this point that the symptoms may appear, which makes the patient look for medical advice. Symptoms vary depending on the location of the tumor, but generally include painful abdominal cramps, diarrhea, sudden flushing of the face or hypoglycemia.

However, in the vast majority of cases, the tumor is said to be ‘non-functional’ and may only be noticed by the patient when it is large enough to affect or compress the neighboring organs. At this stage, symptoms such as stomach ache, jaundice, intestinal obstruction or weight loss may develop.

Sometimes the tumor is discovered by chance, during surgery or a routine examination, such as a scan or a colonoscopy, carried out for a completely different reason.

The symptoms of a NET are generally not suggestive of the disease, as they are similar to those of many other common diseases, making diagnosis difficult.

How can a neuroendocrine tumor (NET) be treated? 

Treatment of NETs has two aims: to remove the tumor or limit its spread, and to control hormonal secretions. Treatment plan usually includes a combination of different therapies and is tailored to each patient.

Monitoring without treatment 

Monitoring of NETs is suggested as initial treatment or after one or more courses of treatment. The choice of this kind of disease management is justified by the slow rate of progression of certain tumors, but also by the toxicity of treatments, which requires rational prescription, or the need for a certain recovery period for the body. This is known as ‘active’ monitoring, and should not be seen as the absence of treatment.

Surgery 

Surgery is the most effective way of managing NETs when they are localized. The aim of surgery is the complete resection of the tumor and removal of the surrounding lymph nodes.

If there are liver metastases, these can also be removed during the operation.

Surgery may also be suggested to treat local or metastatic relapses.

Drug treatments and other therapies 

Other therapeutic options are available for the management of NETs:

•  Somatostatin analogues administered in the form of injections, 
• Targeted therapies administered orally,
  
• Chemotherapy, administered orally or intravenously,
  
• Internal vectorized radiotherapy administered intravenously,
  
• Hepatic chemoembolisation,
  
• Radiofrequency, microwaves or cryotherapy.
  

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