Gastrointestinal carcinoid tumors are slow-growing cancers that develop in the digestive system, often remaining silent for years before causing any noticeable symptoms. These rare tumors arise from specialized cells that normally produce hormones, and while they tend to grow slowly, understanding their nature and behavior is essential for anyone diagnosed with this condition.

Understanding Gastrointestinal Carcinoid Tumors

Gastrointestinal carcinoid tumors are a type of cancer that forms in the lining of the digestive tract. These tumors develop from neuroendocrine cells, which are specialized cells that have characteristics of both nerve cells and hormone-producing cells^[3]. These cells are scattered throughout the body, but the majority are found in the gastrointestinal tract, where they help control the release of digestive juices and the muscles that move food through the stomach and intestines^[3].

The digestive system contains more neuroendocrine cells than any other part of the body, which explains why carcinoid tumors most often start in this location^[4]. These tumors can develop anywhere along the gastrointestinal tract, including the stomach, small intestine, appendix, colon, and rectum. Sometimes, a person may develop more than one tumor at the same time^[3].

One of the defining features of these tumors is their slow growth. Most gastrointestinal carcinoid tumors are rare and grow very slowly over many years^[3]. Because of this slow progression, many people may have a carcinoid tumor for a long time without ever knowing it^[4]. This characteristic makes them different from many other types of cancer that tend to grow and spread more rapidly.

How Common Are These Tumors?

Gastrointestinal carcinoid tumors are relatively rare. The overall prevalence in the United States is estimated to be only one to two cases per 100,000 people^[19]. However, because many of these tumors remain silent and grow slowly, their true prevalence may actually be higher than reported^[19].

Research examining carcinoid tumors over several decades has revealed that the overall incidence has been steadily increasing over the past 30 years^[19]. This increase is likely due to better diagnostic methods and imaging techniques that can detect these tumors earlier, rather than an actual rise in how often they occur^[12].

Currently, the most common site for gastrointestinal carcinoid tumors is the small intestine, accounting for about 30 percent of cases, followed by the rectum at approximately 20 percent^[6]. Historically, the appendix was considered the most common location, but improved detection methods have shown that small intestine tumors are actually more frequent^[6]. When looking specifically at tumors in the appendix, carcinoid tumors remain the most common type of growth found there^[6].

The average age at diagnosis is around 61 years^[16], though these tumors can occur at various ages. There appears to be a slight female predominance, with studies showing more cases in women than men^[9].

What Causes Gastrointestinal Carcinoid Tumors?

The exact cause of gastrointestinal carcinoid tumors is not fully understood. However, these tumors form when neuroendocrine cells in the digestive tract undergo changes that cause them to grow and multiply in an uncontrolled manner. When cells change and grow out of control, they can form a lump or mass called a tumor^[28].

Although the cellular origin of these tumors remains uncertain, research has provided some clues. The consistent expression of certain proteins in carcinoid tumors and the presence of specific intestinal factors suggest that these tumors originate from an epithelial precursor cell, which is a type of cell that lines organs and tissues^[16].

Most neuroendocrine tumors of the small and large intestines occur sporadically, meaning they happen by chance without any clear inherited pattern^[16]. Unlike some other cancers that have well-defined environmental or lifestyle triggers, carcinoid tumors do not have such clearly identified external causes.

Who Is at Risk?

While anyone can develop a gastrointestinal carcinoid tumor, certain factors may increase a person’s likelihood of developing these tumors. A risk factor is anything that increases the chance of developing a disease. Having a risk factor does not guarantee that someone will get cancer, just as not having risk factors does not mean they will not develop it^[3].

One significant risk factor is family history. Having a family history of certain inherited syndromes can increase the risk of developing gastrointestinal carcinoid tumors^[3]. These syndromes include multiple endocrine neoplasia type 1 (MEN1) syndrome, a condition where tumors form in hormone-producing glands, and neurofibromatosis type 1 (NF1) syndrome, a genetic disorder that causes tumors to grow on nerves^[3].

Certain medical conditions that affect the stomach’s ability to make stomach acid also increase risk. These include atrophic gastritis, a condition where the stomach lining becomes thin and inflamed; pernicious anemia, a condition where the body cannot absorb vitamin B12 properly; and Zollinger-Ellison syndrome, a rare condition that causes the stomach to produce too much acid^[3].

Another important consideration is that gastrointestinal neuroendocrine tumors, especially those of the small intestine, are often associated with other cancers. Other cancers occurring at the same time or at different times have been reported in approximately 29 percent of patients with small intestinal neuroendocrine tumors^[16]. This connection may be partly due to the chance discovery of slow-growing carcinoid tumors during investigations for other conditions^[16].

What Are the Symptoms?

One of the challenges with gastrointestinal carcinoid tumors is that they often do not cause any signs or symptoms, especially in the early stages^[2][3]. When symptoms do occur, they are usually vague and depend on where the tumor is located^[2].

The most common symptom associated with carcinoid tumors is vague abdominal pain, reported in about 40 percent of patients^[6]. Other frequently reported symptoms include nausea and vomiting, which occur in approximately 29 percent of patients, weight loss in about 19 percent, and gastrointestinal blood loss in around 15 percent of cases^[6].

Because many carcinoid tumors do not produce symptoms, they are often discovered incidentally, meaning they are found by chance during tests or procedures being performed for other reasons. Incidental carcinoids occur in about 40 percent of patients and have been discovered at multiple sites throughout the gastrointestinal tract^[6]. For example, appendix tumors may be found during surgery for appendicitis, or during other pelvic procedures^[19].

The specific symptoms can vary depending on the location of the tumor. In the duodenum, the first part of the small intestine that connects to the stomach, symptoms may include abdominal pain, constipation, diarrhea, changes in stool color, nausea, vomiting, yellowing of the skin and whites of the eyes (a condition called jaundice), and heartburn^[3].

In the jejunum and ileum, the middle and last parts of the small intestine, symptoms may include pain in the abdomen that may come and go, weight loss, feeling tired, discomfort in the abdomen, and sometimes nausea and vomiting^[14]. In the rectum, people might experience rectal bleeding, pain, or constipation^[19].

Carcinoid Syndrome

A special set of symptoms called carcinoid syndrome can occur when the tumor spreads to the liver or other parts of the body^[3]. This syndrome is relatively rare but is most commonly associated with tumors in the small intestine. Although rare overall, carcinoid syndrome is seen in up to 20 percent of patients diagnosed with carcinoid tumors anywhere in the body^[6].

Carcinoid syndrome happens because the tumor produces and releases hormones and other chemicals into the bloodstream. The most common chemical involved is serotonin, a hormone that affects many body functions^[18]. When carcinoid syndrome is present, it most commonly causes watery diarrhea, which occurs in all affected patients, and flushing of the face and neck, a feeling of warmth and redness that occurs in about 70 percent of patients^[6].

Other symptoms of carcinoid syndrome may include wheezing and difficulty breathing similar to asthma, which affects about 38 percent of patients, and heart valve problems, particularly affecting the tricuspid valve, which occurs in about 23 percent of cases^[6]. Carcinoid syndrome diarrhea occurs in approximately 80 percent of all patients with carcinoid syndrome and poses a substantial burden on daily life^[18].

Interestingly, of the patients who present with carcinoid syndrome, 95 percent have liver metastases, meaning the cancer has spread to the liver, at the time of presentation^[6]. This is because when tumors are located in the intestines and have not spread, the hormones they produce are broken down by the liver before they can cause widespread effects. However, when the tumor spreads to the liver itself, these hormones can enter the general circulation and cause symptoms throughout the body.

Can Gastrointestinal Carcinoid Tumors Be Prevented?

There are no specific prevention strategies known for gastrointestinal carcinoid tumors. Unlike some cancers that can be prevented through lifestyle changes, vaccination, or avoiding certain exposures, carcinoid tumors do not have well-established preventable risk factors that can be modified.

Because most gastrointestinal carcinoid tumors occur sporadically without clear external causes, and because they grow so slowly, prevention efforts focus more on early detection rather than prevention of their formation. For individuals with known genetic syndromes that increase risk, such as MEN1 or NF1 syndrome, regular medical monitoring may help detect tumors at earlier, more treatable stages.

General health practices that support digestive system health, such as maintaining a balanced diet and addressing stomach conditions promptly, may be beneficial. However, these measures have not been proven to prevent the development of carcinoid tumors specifically. The most important step for individuals at higher risk is to maintain regular communication with their healthcare provider and follow recommended screening or monitoring protocols if they have conditions that increase their risk.

How the Body Changes: Understanding the Disease Process

To understand how gastrointestinal carcinoid tumors affect the body, it helps to know what happens when these tumors grow and produce substances that alter normal bodily functions. The disease process involves both mechanical effects from the tumor itself and biochemical effects from the hormones and chemicals the tumor may produce.

Neuroendocrine cells normally make hormones that help control digestive juices and the muscles used in moving food through the stomach and intestines^[3]. When these cells become cancerous and form a tumor, they may continue to produce these hormones, but often in excessive amounts or in an uncontrolled manner. A gastrointestinal carcinoid tumor may make hormones and release them into the body^[3].

The physical presence of the tumor can cause problems depending on its size and location. As the tumor grows, it can block the passage of food through the intestines, leading to bowel obstruction. This mechanical effect explains symptoms such as abdominal pain, nausea, and vomiting. In some cases, the tumor may cause bleeding, which can result in blood appearing in the stool or leading to anemia over time.

The biochemical changes are particularly important in understanding carcinoid syndrome. When tumors produce excess serotonin and other bioactive substances, these chemicals affect various body systems. In the digestive system, excess serotonin increases the secretion of fluid and electrolytes into the intestines while decreasing their absorption^[24]. This imbalance is what causes the diarrhea characteristic of carcinoid syndrome.

The process of diarrhea in carcinoid syndrome involves the intestinal lining increasing fluid secretion while reducing its ability to absorb water and nutrients. This creates an osmotic effect, where fluid is pulled into the intestinal space and propelled through the digestive tract more rapidly than normal^[24]. The body normally processes about nine liters of fluid daily through the digestive system, but when this balance is disrupted by tumor hormones, it can lead to significant fluid loss^[24].

The flushing episodes associated with carcinoid syndrome occur when hormones released by the tumor cause blood vessels near the skin surface to dilate or widen. This brings more blood to the surface of the skin, creating the characteristic redness and feeling of warmth, particularly in the face and upper body.

When carcinoid tumors spread beyond their original location, they most commonly go to nearby lymph nodes or to the liver. The liver is a frequent site of spread because blood from the intestines flows directly to the liver through the portal vein. Once in the liver, tumor cells can grow and form new tumors. This spread to the liver is particularly significant because it allows tumor hormones to bypass the liver’s filtering system and enter the general blood circulation, which is why carcinoid syndrome typically only occurs when liver metastases are present.