A neuroendocrine tumor is a tumor that arises from
the neuroendocrine system. The neuroendocrine system synthesizes chemical compounds
known as neurotransmitters, which act both as hormones and as messengers to
neurons (nerve cells), and exert their effects by regulating the functions of
different organs in the body. Neuroendocrine cells are spread throughout the
body in organs such as the stomach, bowels and lungs.

Although estimates vary, the annual
incidence (including diagnosed and undiagnosed cases) of neuroendocrine
tumors is approximately 2.5-5 per 100,000.The prevalence (only the diagnosed
cases) has been estimated as 35 per 100,000; this prevalence may actually be
significantly higher if clinically silent tumors are included. 

NETs can be of non-cancerous (benign) or cancerous
(malignant) nature.

NETs are classified according to where the cancer
started, meaning, based upon the location where the primary tumor arises from
in the body. So for example: 

·        
small bowel NETs

·        
large bowel NETs

·        
appendiceal NETs 

·        
pancreatic NETs

·        
gastric NETs

·        
lung NETs.

In
rare occasions, NETs arise from other organs, such as the liver, gallbladder,
bile ducts, kidneys, ovaries or testicles. Some NETs are referred to as
carcinoid tumors.

Unlike
other solid tumors, NETs are distinguishable in 2 important characteristics:

·        
NETs arise from cells of the
diffuse endocrine system rather than from cells specific to a certain organ or
tissue

·        
Just as neuroendocrine cells synthesize
and secrete hormones and bioactive substances the role of which is to control
critical functions of different or, NETs are similarly capable of over-secreting
these substances, and this can be behind the occurrence of a variety of
symptoms and clinical syndromes.

NETs are indolent tumors since they often grow
slowly, and it may take several years before symptoms become apparent, hence
rendering the presentation to medical care rather late in the disease process,
and as such delaying the diagnosis of the tumor. However, some NETs may be
fast-growing and more likely to spread to surrounding tissues, and to other
parts of the body.

According to the North American Neuroendocrine Tumor Society
(NANETS) and the World Health Organization (WHO) guidelines, neuroendocrine
neoplasms are classified into 3 grades:

-         
Low grade (G1)

-         
Intermediate grade (G2)
NETs

-         
High grade (G3)
neuroendocrine carcinomas.

Grade refers to the aggressiveness of the tumor. NETs are
also classified as well differentiated or poorly differentiated.
Differentiation refers to the extent to which neoplastic cells resemble their
non-neoplastic (normal or healthy) counterparts.

Prognosis

1- For well differentiated NETs (G1/G2):

Mean Survival Duration

-         
Local disease: median
survival duration is 223 months

-         
Regional disease: median
survival duration is 111 months

-         
Distant metastases: median
survival duration is 33 months

5-year Survival

-         
Local disease: 83%

-         
Regional disease: 68%

-         
Distant metastases: 35%

2- For poorly differentiated carcinomas (G3):

Mean Survival Duration

-         
Local disease: median
survival duration is 34 months

-         
Regional disease: median
survival duration is 14 months

-         
Distant metastases: median
survival duration is 5 months

5-year Survival

-         
Local disease: 38%

-         
Regional disease: 21%

-         
Distant metastases: 4%