In oncology, small intestine cancer, also small bowel cancer and cancer of the small bowel, is a cancer of the small intestine. It is relatively rare compared to other gastrointestinal malignancies such as gastric cancer (stomach cancer) and colorectal cancer.

Small intestine cancer can be subdivided into duodenal cancer (the first part of the small intestine) and cancer of the jejunum and ileum (the later two parts of the small intestine). Duodenal cancer has more in common with stomach cancer, while cancer of the jejunum and ileum have more in common with colorectal cancer. Five year survival rates are 65%.

Several different subtypes of small intestine cancer exist. These include:

- adenocarcinoma

- gastrointestinal stromal tumor

- lymphoma

- ileal carcinoid tumor

Stomach cancer is often either asymptomatic (producing no noticeable symptoms) or it may cause only nonspecific symptoms (symptoms that may also be present in other related or unrelated disorders) in its early stages. By the time symptoms occur, the cancer has often reached an advanced stage (see below) and may have metastasized (spread to other, perhaps distant, parts of the body), which is one of the main reasons for its relatively poor prognosis. Stomach cancer can cause the following signs and symptoms:

Early cancers may be associated with indigestion or a burning sensation (heartburn). However, less than 1 in every 50 people referred for endoscopy due to indigestion has cancer. Abdominal discomfort and loss of appetite, especially for meat, can occur.

Gastric cancers that have enlarged and invaded normal tissue can cause weakness, fatigue, bloating of the stomach after meals, abdominal pain in the upper abdomen, nausea and occasional vomiting, diarrhea or constipation. Further enlargement may cause weight loss or bleeding with vomiting blood or having blood in the stool, the latter apparent as black discolouration (melena) and sometimes leading to anemia. Dysphagia suggests a tumour in the cardia or extension of the gastric tumour into the esophagus.

These can be symptoms of other problems such as a stomach virus, gastric ulcer, or tropical sprue.

Risk factors for small intestine cancer include:

- Crohn's disease

- Celiac disease

- Radiation exposure

- Hereditary gastrointestinal cancer syndromes: familial adenomatous polyposis, hereditary nonpolyposis colorectal cancer, Peutz-Jeghers syndrome

- Males are 25% more likely to develop the disease

Benign tumours and conditions that may be mistaken for cancer of the small bowel:

- Hamartoma

- Tuberculosis

Gastrointestinal cancer refers to malignant conditions of the gastrointestinal tract (GI tract) and accessory organs of digestion, including the esophagus, stomach, biliary system, pancreas, small intestine, large intestine, rectum and anus. The symptoms relate to the organ affected and can include obstruction (leading to difficulty swallowing or defecating), abnormal bleeding or other associated problems. The diagnosis often requires endoscopy, followed by biopsy of suspicious tissue. The treatment depends on the location of the tumor, as well as the type of cancer cell and whether it has invaded other tissues or spread elsewhere. These factors also determine the prognosis.

Overall, the GI tract and the accessory organs of digestion (pancreas, liver, gall bladder) are responsible for more cancers and more deaths from cancer than any other system in the body. There is significant geographic variation in the rates of different gastrointestinal cancers.

Symptoms of anal cancer can include pain or pressure in the anus or rectum, a change in bowel habits, a lump near the anus, rectal bleeding, itching or discharge. Bleeding may be severe.

The signs and symptoms of colorectal cancer depend on the location of the tumor in the bowel, and whether it has spread elsewhere in the body (metastasis). The classic warning signs include: worsening constipation, blood in the stool, decrease in stool caliber (thickness), loss of appetite, loss of weight, and nausea or vomiting in someone over 50 years old. While rectal bleeding or anemia are high-risk features in those over the age of 50, other commonly described symptoms including weight loss and change in bowel habit are typically only concerning if associated with bleeding.

A gastrointestinal carcinoid tumor is a rare, slow-growing form of cancer that affects certain cells in the lining of the stomach and intestines. The cells it affects make hormones that regulate the production of digestive juices and muscles that move food through the stomach and intestines. This kind of cancer usually occurs in the appendix, small intestine, or rectum. Its presence is associated with an increased risk of cancers affecting the other parts of the digestive system. It is usually treated with surgery.

Stomach cancer, also known as gastric cancer, is cancer developing from the lining of the stomach. Early symptoms may include heartburn, upper abdominal pain, nausea and loss of appetite. Later signs and symptoms may include weight loss, yellowing of the skin and whites of the eyes, vomiting, difficulty swallowing, and blood in the stool among others. The cancer may spread from the stomach to other parts of the body, particularly the liver, lungs, bones, lining of the abdomen and lymph nodes.

The most common cause is infection by the bacterium "Helicobacter pylori", which accounts for more than 60% of cases. Certain types of "H. pylori" have greater risks than others. Smoking, dietary factors such as pickled vegetables, and obesity are other risk factors. About 10% of cases run in families and between 1% and 3% of cases are due to genetic syndromes inherited from a person's parents such as hereditary diffuse gastric cancer. Most cases of stomach cancers are gastric carcinomas. This type can be divided into a number of subtypes. Lymphomas and mesenchymal tumors may also develop in the stomach. Most of the time, stomach cancer develops in stages over years. Diagnosis is usually by biopsy done during endoscopy. This is followed by medical imaging to determine if the disease has spread to other parts of the body. Japan and South Korea, two countries that have high rates of the disease, screen for stomach cancer.

A Mediterranean diet lowers the risk of cancer as does the stopping of smoking. There is tentative evidence that treating "H. pylori" decreases the future risk. If cancer is treated early, many cases can be cured. Treatments may include some combination of surgery, chemotherapy, radiation therapy, and targeted therapy. If treated late, palliative care may be advised. Outcomes are often poor with a less than 10% five-year survival rate globally. This is largely because most people with the condition present with advanced disease. In the United States five-year survival is 28% while in South Korea it is over 65% partly due to screening efforts.

Globally, stomach cancer is the fifth leading cause of cancer and the third leading cause of death from cancer making up 7% of cases and 9% of deaths. In 2012, it newly occurred in 950,000 people and caused 723,000 deaths. Before the 1930s, in much of the world, including most Western developed countries, it was the most common cause of death from cancer. Rates of death have been decreasing in many areas of the world since then. This is believed to be due to the eating of less salted and pickled foods as a result of the development of refrigeration as a method of keeping food fresh. Stomach cancer occurs most commonly in East Asia and Eastern Europe. It occurs twice as often in males as in females.

Anal cancer is a cancer (malignant tumor) which arises from the anus, the distal opening of the gastrointestinal tract. It is a distinct entity from the more common colorectal cancer.

Anal cancer is typically an anal squamous cell carcinoma that arises near the squamocolumnar junction, often linked to human papillomavirus (HPV) infection. It may be keratinizing (basaloid) or non-keratinizing (cloacogenic). Other types of anal cancer are adenocarcinoma, lymphoma, sarcoma or melanoma. From data collected 2004-2010, the relative five year survival rate in the United States is 65.5%, though individual rates may vary depending upon the stage of cancer at diagnosis and the response to treatment.

Colorectal cancer (CRC), also known as bowel cancer and colon cancer, is the development of cancer from the colon or rectum (parts of the large intestine). A cancer is the abnormal growth of cells that have the ability to invade or spread to other parts of the body. Signs and symptoms may include blood in the stool, a change in bowel movements, weight loss, and feeling tired all the time.

Most colorectal cancers are due to old age and lifestyle factors with only a small number of cases due to underlying genetic disorders. Some risk factors include diet, obesity, smoking, and lack of physical activity. Dietary factors that increase the risk include red and processed meat as well as alcohol. Another risk factor is inflammatory bowel disease, which includes Crohn's disease and ulcerative colitis. Some of the inherited genetic disorders that can cause colorectal cancer include familial adenomatous polyposis and hereditary non-polyposis colon cancer; however, these represent less than 5% of cases. It typically starts as a benign tumor, often in the form of a polyp, which over time becomes cancerous.

Bowel cancer may be diagnosed by obtaining a sample of the colon during a sigmoidoscopy or colonoscopy. This is then followed by medical imaging to determine if the disease has spread. Screening is effective for preventing and decreasing deaths from colorectal cancer. Screening, by one of a number of methods, is recommended starting from the age of 50 to 75. During colonoscopy, small polyps may be removed if found. If a large polyp or tumor is found, a biopsy may be performed to check if it is cancerous. Aspirin and other non-steroidal anti-inflammatory drugs decrease the risk. Their general use is not recommended for this purpose, however, due to side effects.

Treatments used for colorectal cancer may include some combination of surgery, radiation therapy, chemotherapy and targeted therapy. Cancers that are confined within the wall of the colon may be curable with surgery while cancer that has spread widely are usually not curable, with management being directed towards improving quality of life and symptoms. Five year survival rates in the United States are around 65%. This, however, depends on how advanced the cancer is, whether or not all the cancer can be removed with surgery, and the person's overall health. Globally, colorectal cancer is the third most common type of cancer, making up about 10% of all cases. In 2012, there were 1.4 million new cases and 694,000 deaths from the disease. It is more common in developed countries, where more than 65% of cases are found. It is less common in women than men.

The possible signs of oropharyngeal cancer are:

- A sore throat that persists

- Pain or difficulty with swallowing

- Unexplained weight loss

- Voice changes

- Ear pain

- A lump in the back of the throat or mouth

- A lump in the neck

- A dull pain behind the sternum

- Cough

Symptoms of Hypopharyngeal Cancer include:

- Swollen lymph nodes in the neck (first sign of a problem in half of all patients)

- Sore throat in one location that persists after treatment

- Pain that radiates from the throat to the ears

- Difficult or painful swallowing (often leads to malnutrition and weight loss because of a refusal to eat)

- Voice changes (late stage cancer)

Hypopharyngeal cancer is a disease in which malignant cells grow in the hypopharynx (the area where the larynx and esophagus meet).

It first forms in the outer layer (epithelium) of the hypopharynx (last part of the pharynx), which is split into three areas. Progression of the disease is defined by the spread of cancer into one or more areas and into deeper tissues.

This type of cancer is rare. Only about 2,500 cases are seen in the US each year. Because of this, Hypopharyngeal Cancer is difficult to catch in its earliest stages and has one of the highest mortality rates of any head and neck cancer.

Oropharyngeal cancer is a disease in which cancer form in the tissues of the throat (oropharynx). The oropharynx is the middle part of the throat that includes the base of the tongue, the tonsils, the soft palate, and the walls of the pharynx. Oropharyngeal cancers can be divided into two types, HPV-positive, which are related to human papillomavirus infection, and HPV-negative cancers, which are usually linked to alcohol or tobacco use.

While each type of cancer has its own distinctive symptoms, most of them can indicate their presence by the occurrence and the prolonged presence of any common symptom. Some of the general symptoms of cancer in cats are:

- Any lump that changes shape or size

- Any sore that does not heal

- Change in bowel or bladder habits

- Difficulty eating or swallowing

- Difficulty urinating or defecating

- Unexplained bleeding or discharge from body

- Loss of appetite

- Chronic weight loss

- Coughing or difficulty breathing

- Stiffness

- Oral odor

Note that ravenous hunger, while a sign of other diseases like hyperthyroid, can also be a sign of cancer.

Symptoms of cancer in dogs may include:

- Lumps (which are not always malignant, but should always be examined by a vet)

- Swelling

- Persistent sores

- Abnormal discharge from any part of the body

- Bad breath

- Listlessness/lethargy

- Rapid, often unexplained weight loss

- Sudden lameness

- Offensive odor

- Black, tarry stools (a symptom of ulcers, which can be caused by mast cell tumors)

- Decreased or loss of appetite

- Difficulty breathing, urinating or defecating

Lymphoma is the most common form of cancer in cats, which is often associated with feline leukemia virus, and accounts for 25 percent of all cases. Feline lymphoma usually strikes the digestive system, causing excessive vomiting and diarrhea. Other common symptoms of lymphoma in cats include swollen lymph nodes, loss of appetite, weight loss, and difficulty breathing.

In its early stages, it can go unnoticed. It can be painless with slight physical changes. But the precursor tissue changes, can be noticed by the doctors.

Early stage symptoms can include persistent red or white patches, a non-healing ulcer, progressive swelling or enlargement, unusual surface changes, sudden tooth mobility without apparent cause, unusual oral bleeding or epitaxis and prolonged hoarseness.

Late stage symptoms can include an indurated area, paresthesia or dysesthesia of the tongue or lips, airway obstruction, chronic serous otitis media, otalgia, trismus, dysphagia, cervical lymphadenopathy, persistent pain or referred pain and altered vision.

The main symptom is vomiting, which typically occurs after meals of undigested food, devoid of any bile. A history of previous peptic ulcers and loss of weight is not uncommon. In advanced cases, signs to look for on physical examination are wasting and dehydration. Visible peristalsis from left to right may be present. Succussion splash is a splash-like sound heard over the stomach in the left upper quadrant of the abdomen on shaking the patient, with or without the stethoscope. Bowel sound may be increased due to excessive peristaltic action of stomach. Fullness in left hypochondrium may also be present.

Even though anatomically part of the GI tract, diseases of the mouth are often not considered alongside other gastrointestinal diseases. By far the most common oral conditions are plaque-induced diseases (e.g. gingivitis, periodontitis, dental caries). Some diseases which involve other parts of the GI tract can manifest in the mouth, alone or in combination, including:

- Gastroesophageal reflux disease can cause acid erosion of the teeth and halitosis.

- Gardner's syndrome can be associated with failure of tooth eruption, supernumerary teeth, and dentigerous cysts.

- Peutz–Jeghers syndrome can cause dark spots on the oral mucosa or on the lips or the skin around the mouth.

- Several GI diseases, especially those associated with malabsorption, can cause recurrent mouth ulcers, atrophic glossitis, and angular cheilitis (e.g. Crohn's disease is sometimes termed orofacial granulomatosis when it involves the mouth alone).

- Sideropenic dysphagia can cause glossitis, angular cheilitis.

With Ménétrier disease, the stomach is characterized by large, tortuous gastric folds in the fundus and body of the stomach, with antrum generally spared, giving the mucosa a cobblestone or cerebriform (brain-like) appearance. Histologically, the most characteristic feature is massive foveolar hyperplasia (hyperplasia of surface and glandular mucous cells). The glands are elongated with a corkscrew-like appearance and cystic dilation is common. Inflammation is usually only modest, although some cases show marked intraepithelial lymphocytosis. Diffuse or patchy glandular atrophy, evident as hypoplasia of parietal and chief cells, is typical.

Although ICD-10 classifies it under "Other gastritis" (K29.6), and the lamina propria may contain mild chronic inflammatory infiltrate, Ménétrier disease is not considered a form of gastritis. It is rather considered as one of the two most well understood hypertrophic gastropathies; the other being Zollinger–Ellison syndrome.

Gastric atresia is the complete occlusion of the pyloric outlet of the stomach and is an extremely rare event.

Hereditary breast–ovarian cancer syndromes (HBOC) are cancer syndromes that produce higher than normal levels of breast cancer and ovarian cancer in genetically related families (either one individual had both, or several individuals in the pedigree had one or the other disease). The hereditary factors may be proven or suspected to cause the pattern of breast and ovarian cancer occurrences in the family.

Individuals with the disease present with upper abdominal pain (epigastric), at times accompanied by nausea, vomiting, loss of appetite, edema, and weight loss. A small amount of gastrointestinal bleeding may occur, which is typically due to superficial mucosal erosions; large volume bleeding is rare. 20% to 100% of patients, depending on time of presentation, develop a protein-losing gastropathy accompanied by low blood albumin and edema.

Symptoms and pathological features of Ménétrier disease in children are similar to those in adults, but disease in children is usually self-limited and often follows respiratory infection.

The small intestine consists of the duodenum, jejunum and ileum. Inflammation of the small intestine is called enteritis, which if localised to just part is called duodenitis, jejunitis and ileitis, respectively. Peptic ulcers are also common in the duodenum.

Chronic diseases of malabsorption may affect the small intestine, including the autoimmune coeliac disease, infective Tropical sprue, and congenital or surgical short bowel syndrome. Other rarer diseases affecting the small intestine include Curling's ulcer, blind loop syndrome, Milroy disease and Whipple's disease. Tumours of the small intestine include gastrointestinal stromal tumours, lipomas, hamartomas and carcinoid syndromes.

Diseases of the small intestine may present with symptoms such as diarrhoea, malnutrition, fatigue and weight loss. Investigations pursued may include blood tests to monitor nutrition, such as iron levels, folate and calcium, endoscopy and biopsy of the duodenum, and barium swallow. Treatments may include renutrition, and antibiotics for infections.