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.

Prominent symptoms usually do not appear until the cancer has infiltrated over 60% of the circumference of the esophageal tube, by which time the tumor is already in an advanced stage. Onset of symptoms is usually caused by narrowing of the tube due to the physical presence of the tumor.

The first and the most common symptom is usually difficulty in swallowing, which is often experienced first with solid foods and later with softer foods and liquids. Pain when swallowing is less usual at first. Weight loss is often an initial symptom in cases of squamous-cell carcinoma, though not usually in cases of adenocarcinoma. Eventual weight loss due to reduced appetite and undernutrition is common. Pain behind the breastbone or in the region around the stomach often feels like heartburn. The pain can frequently be severe, worsening when food of any sort is swallowed. Another sign may be an unusually husky, raspy, or hoarse-sounding cough, a result of the tumor affecting the recurrent laryngeal nerve.

The presence of the tumor may disrupt the normal contractions of the esophagus when swallowing. This can lead to nausea and vomiting, regurgitation of food and coughing. There is also an increased risk of aspiration pneumonia due to food entering the airways through the abnormal connections (fistulas) that may develop between the esophagus and the trachea (windpipe). Early signs of this serious complication may be coughing on drinking or eating. The tumor surface may be fragile and bleed, causing vomiting of blood. Compression of local structures occurs in advanced disease, leading to such problems as upper airway obstruction and superior vena cava syndrome. Hypercalcemia (excess calcium in the blood) may occur.

If the cancer has spread elsewhere, symptoms related to metastatic disease may appear. Common sites of spread include nearby lymph nodes, the liver, lungs and bone. Liver metastasis can cause jaundice and abdominal swelling (ascites). Lung metastasis can cause, among other symptoms, impaired breathing due to excess fluid around the lungs (pleural effusion), and dyspnea (the feelings often associated with impaired breathing).

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.

Duodenal cancer is a cancer in the beginning section of the small intestine. It is relatively rare compared to gastric cancer and colorectal cancer. Its histology is usually adenocarcinoma.

Familial adenomatous polyposis (FAP), Gardner syndrome, Lynch syndrome, Muir–Torre syndrome, celiac disease, Peutz–Jeghers syndrome, Crohn's disease and juvenile polyposis syndrome are risk factors for developing this cancer.

The duodenum is the first part of the small intestine. It is located between the stomach and the jejunum. After foods combine with stomach acid, they descend into the duodenum where they mix with bile from the gallbladder and digestive juices from the pancreas.

Colorectal polyps are not usually associated with symptoms. When they occur, symptoms include rectal bleeding, bloody stools, abdominal pain and fatigue. A change in bowel habits may occur including constipation and diarrhoea. Occasionally, if a polyp is big enough to cause a bowel obstruction, there may be nausea, vomiting and severe constipation.

Oesophagogastric junctional adenocarcinoma is a cancer of the lower part of the oesophagus, often linked to a Barrett's oesophagus.

The incidence of oesophagogastric junctional adenocarcinoma is rising rapidly in western countries, in contrast to the declining frequency of distal gastric adenocarcinoma. Treatment options for adenocarcinomas involving the oesophagogastric junction are limited and the overall prognosis is extremely poor.

Pancreatic cancer is the fifth-most-common cause of cancer deaths in the United States, and the seventh most common in Europe. In 2008, globally there were 280,000 new cases of pancreatic cancer reported and 265,000 deaths. These cancers are classified as endocrine or nonendocrine tumors. The most common is ductal adenocarcinoma. The most significant risk factors for pancreatic cancer are advanced age (over 60) and smoking. Chronic pancreatitis, diabetes or other conditions may also be involved in their development. Early pancreatic cancer does not tend to result in any symptom, but when a tumor is advanced, a patient may experience severe pain in the upper abdomen, possibly radiating to the back. Another symptom might be jaundice, a yellowing of the skin and eyes.

Pancreatic cancer has a poor prognosis, with a five-year survival rate of less than 5%. By the time the cancer is diagnosed, it is usually at an advanced, inoperable stage. Only one in about fifteen to twenty patients is curative surgery attempted. Pancreatic cancer tends to be aggressive, and it resists radiotherapy and chemotherapy.

The most common general classification is:

- hyperplastic,

- neoplastic (adenomatous & malignant),

- hamartomatous and,

- inflammatory.

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

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.

Urachal cancer can exist for some years without any symptoms. The most frequent initial symptom is haematuria which occurs when the urachal tumour has penetrated the bladder wall, but mucinuria (mucin in the urine), local pain or swelling, recurrent local or urinary tract infections and umbilical discharge can (but is not always) be seen.

Esophageal cancer is cancer arising from the esophagus—the food pipe that runs between the throat and the stomach. Symptoms often include difficulty in swallowing and weight loss. Other symptoms may include pain when swallowing, a hoarse voice, enlarged lymph nodes ("glands") around the collarbone, a dry cough, and possibly coughing up or vomiting blood.

The two main sub-types of the disease are esophageal squamous-cell carcinoma (often abbreviated to ESCC), which is more common in the developing world, and esophageal adenocarcinoma (EAC), which is more common in the developed world. A number of less common types also occur. Squamous-cell carcinoma arises from the epithelial cells that line the esophagus. Adenocarcinoma arises from glandular cells present in the lower third of the esophagus, often where they have already transformed to intestinal cell type (a condition known as Barrett's esophagus). Causes of the squamous-cell type include tobacco, alcohol, very hot drinks, poor diet, and chewing betel nut. The most common causes of the adenocarcinoma type are smoking tobacco, obesity, and acid reflux.

The disease is diagnosed by biopsy done by an endoscope (a fiberoptic camera). Prevention includes stopping smoking and eating a healthy diet. Treatment is based on the cancer's stage and location, together with the person's general condition and individual preferences. Small localized squamous-cell cancers may be treated with surgery alone with the hope of a cure. In most other cases, chemotherapy with or without radiation therapy is used along with surgery. Larger tumors may have their growth slowed with chemotherapy and radiation therapy. In the presence of extensive disease or if the affected person is not fit enough to undergo surgery, palliative care is often recommended.

As of 2012, esophageal cancer was the eighth-most common cancer globally with 456,000 new cases during the year. It caused about 400,000 deaths that year, up from 345,000 in 1990. Rates vary widely among countries, with about half of all cases occurring in China. It is around three times more common in men than in women. Outcomes are related to the extent of the disease and other medical conditions, but generally tend to be fairly poor, as diagnosis is often late. Five-year survival rates are around 13% to 18%.

The change from normal to premalignant cells that indicate Barrett's esophagus does not cause any particular symptoms. Barrett's esophagus, however, is associated with these symptoms:

- frequent and longstanding heartburn

- trouble swallowing (dysphagia)

- vomiting blood (hematemesis)

- pain under the sternum where the esophagus meets the stomach

- unintentional weight loss because eating is painful (odynophagia)

The risk of developing Barrett's esophagus is increased by central obesity (vs. peripheral obesity). The exact mechanism is unclear. The difference in distribution of fat among men (more central) and women (more peripheral) may explain the increased risk in males.

Barrett's esophagus refers to an abnormal change (metaplasia) in the cells of the lower portion of the esophagus. It is characterized by the replacement of the normal stratified squamous epithelium lining of the esophagus by simple columnar epithelium with goblet cells (which are usually found lower in the gastrointestinal tract). The medical significance of Barrett's esophagus is its strong association (0.1 per 1 cm Prague C>M> total segment length per patient-year) with esophageal adenocarcinoma, a very often deadly cancer, because of which it is considered to be a premalignant condition.

The main cause of Barrett's esophagus is thought to be an adaptation to chronic acid exposure from reflux esophagitis. The incidence of esophageal adenocarcinoma has increased substantially in the Western world in recent years. The condition is found in 5–15% of patients who seek medical care for heartburn (gastroesophageal reflux disease), although a large subgroup of patients with Barrett's esophagus do not have symptoms. Diagnosis requires endoscopy (more specifically, esophagogastroduodenoscopy, a procedure in which a fibreoptic cable is inserted through the mouth to examine the esophagus, stomach, and duodenum) and biopsy. The cells of Barrett's esophagus, after biopsy, are classified into four general categories: nondysplastic, low-grade dysplasia, high-grade dysplasia, and frank carcinoma. High-grade dysplasia and early stages of adenocarcinoma can be treated by endoscopic resection and new endoscopic therapies such as radiofrequency ablation, whereas advanced stages (submucosal) are generally advised to undergo surgical treatment. Nondysplastic and low-grade patients are generally advised to undergo annual observation with endoscopy, with radiofrequency ablation as a therapeutic option. In high-grade dysplasia, the risk of developing cancer might be at 10% per patient-year or greater.

The condition is named after the Australian-born British thoracic surgeon Norman Barrett (1903–1979), who described it in 1950.

Those with the eating disorder bulimia are more likely to develop Barrett’s esophagus because bulimia can cause severe acid reflux, and because purging also floods the esophagus with acid.

Adenocarcinoma (; plural adenocarcinomas or adenocarcinomata ) is a type of cancerous tumor that can occur in several parts of the body. It is defined as neoplasia of epithelial tissue that has glandular origin, glandular characteristics, or both. Adenocarcinomas are part of the larger grouping of carcinomas, but are also sometimes called by more precise terms omitting the word, where these exist. Thus invasive ductal carcinoma, the most common form of breast cancer, is adenocarcinoma but does not use the term in its name—however, esophageal adenocarcinoma does to distinguish it from the other common type of esophageal cancer, esophageal squamous cell carcinoma. Several of the most common forms of cancer are adenocarcinomas, and the various sorts of adenocarcinoma vary greatly in all their aspects, so that few useful generalizations can be made about them.

In the most specific usage (narrowest sense), the glandular origin or traits are exocrine; endocrine gland tumors, such as a VIPoma, an insulinoma, or a pheochromocytoma, are typically not referred to as adenocarcinomas but rather are often called neuroendocrine tumors. Epithelial tissue sometimes includes, but is not limited to, the surface layer of skin, glands, and a variety of other tissue that lines the cavities and organs of the body. Epithelial tissue can be derived embryologically from any of the germ layers (ectoderm, endoderm, or mesoderm). To be classified as adenocarcinoma, the cells do not necessarily need to be part of a gland, as long as they have secretory properties. Adenocarcinoma is the malignant counterpart to adenoma, which is the benign form of such tumors. Sometimes adenomas transform into adenocarcinomas, but most do not.

Well differentiated adenocarcinomas tend to resemble the glandular tissue that they are derived from, while poorly differentiated adenocarcinomas may not. By staining the cells from a biopsy, a pathologist can determine whether the tumor is an adenocarcinoma or some other type of cancer. Adenocarcinomas can arise in many tissues of the body owing to the ubiquitous nature of glands within the body, and, more fundamentally, to the potency of epithelial cells. While each gland may not be secreting the same substance, as long as there is an exocrine function to the cell, it is considered glandular and its malignant form is therefore named adenocarcinoma.

The cancerous mass tends to block food from getting to the small intestine. If food cannot get to the intestines, it will cause pain, acid reflux, and weight loss because the food cannot get to where it is supposed to be processed and absorbed by the body.

Patients with duodenal cancer may experience abdominal pain, weight loss, nausea, vomiting, and chronic GI bleeding.

Most patients with fundic gland polyps (FGPs) do not have any symptoms, and the diagnosis is made on gastroscopy done for other reasons. Retrospective analysis of patients with sporadic FGPs shows that a high percentage do have symptoms, but that this is more likely to be related to the underlying disease responsible for the polyposis. These symptoms include:

- epigastric pain

- nausea

- vomiting

- weight loss

The polyps on endoscopy are usually tiny, numerous and sessile, and usually scattered throughout the fundus of the stomach, where parietal cells are more numerous. They have the same colour as the gastric mucosa, and never have a stalk.

When the polyps are biopsied, the pathology typically shows shortened gastric pits, and both superficial and deep cystic lesions in the fundic glands, lined by all three types of cells of acid-producing mucosa: mucous, parietal and chief cells. As sometimes parietal cell hyperplasia may develop deep dilations of gland, one should be really strict in the diagnosis of FGPs (i.e. the presence of deep and superficial dilations). Infrequently, the two lesions may coexist. Foci of dysplasia can sometimes be seen.

Apocrine gland anal sac adenocarcinomas first appear as small lumps associated with one of the anal sacs (rarely bilateral), but they can grow to a large size. Smaller tumors are undetectable without a rectal examination, while larger tumors can cause pain and straining to defecate. Between 25 and 40 percent of dogs with these tumors will also develop hypercalcaemia through secretion of parathyroid hormone-related protein by the tumor. Symptoms of hypercalcaemia include increased drinking and urination, vomiting, loss of appetite, weight loss, and bradycardia (slow heart rate). Apocrine gland anal sac adenocarcinomas also have a tendency to metastasize to the regional lymph nodes, spleen, and eventually lungs and, less commonly, bones. The sublumbar (iliac) lymph nodes are the most common site of metastasis and can become larger than the original tumor.

Urachal cancer is a very rare type of cancer arising from the urachus or its remnants. The disease might arise from metaplasic glandular epithelium or embryonic epithelial remnants originating from the cloaca region.

It occurs in roughly about one person per 1 million people per year varying on the geographical region. Men are affected slightly more often than women mostly in the 5th decade of life but the disease can occur in also in other age groups.

It can involve the urinary bladder, but is not bladder cancer in the usual sense. Urachal cancer can occur at any site along the urachal tract.

Urachal cancer was mentioned by Hue and Jacquin in 1863 followed by an elaborate work by T. Cullen in 1916 about diseases of the umbilicus, while C. Begg further characterized urachal cancer in the 1930s. Detailed diagnostic and staging schemes were proposed by Sheldon et al in 1984, which remain widely used today.

Signet ring cell carcinoma (SRCC) is a rare form of highly malignant adenocarcinoma that produces mucin. It is an epithelial malignancy characterized by the histologic appearance of signet ring cells.

Primary SRCC tumors are most often found in the glandular cells of the stomach (SRCC originates in the stomach in 90 percent of patients), and less frequently in the breast, gallbladder, urinary bladder, and pancreas. SRCCs do not normally form in the lungs, though a few incidences have been reported.

Among colorectal cancers, the prevalence of SRCC is less than one percent. Though incidence and mortality of gastric cancer has declined in many countries over the past 50 years, there has been an increase in occurrences of gastric SRCC-type cancers.

SRCC tumors grow in characteristic sheets, which makes diagnosis using standard imaging techniques, like CT and PET scans, less effective.

Examples of cancers where adenocarcinomas are a common form:

- esophageal cancer; most cases in the developed world are adenocarcinomas.

- pancreas; over 80% of pancreatic cancers are ductal adenocarcinomas.

- prostate cancer is nearly always adenocarcinoma

- cervical cancer: most is squamous cell cancer, but 10–15% of cervical cancers are adenocarcinomas

- stomach cancer

Signs and symptoms of pseudomyxoma peritonei may include abdominal or pelvic pain and/or bloating, distension, digestive disorders, weight changes, increased girth, and infertility.

An anal sac adenocarcinoma is an uncommon and aggressive malignant tumor found in dogs that arises from the apocrine glandular tissue of anal sac. The disease exists in cats as well, but is much less common in that species. They are the second most common cancerous cause of hypercalcaemia (high serum calcium) in dogs, following T-cell lymphoma.

Other common manifestations of the disease include: weakness and tiring easily; dry mouth; sleep problems; and a palpable abdominal mass."