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.

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

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.

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.

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.

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.

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

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.

People who have been treated with radiotherapy for pelvic and other abdominal cancers frequently develop gastrointestinal symptoms. These include:

- rectal bleeding

- diarrhea and steatorrhea

- other defecation disorders including fecal urgency and incontinence.

- nutritional deficiencies and weight loss

- abdominal pain and bloating

- nausea, vomiting and fatigue

Gastrointestinal symptoms are often found together with those in other systems including genitourinary disorders and sexual dysfunction. The burden of symptoms substantially impairs the patients' quality of life.

Nausea, vomiting, fatigue and diarrhea may happen early during the course of radiotherapy. Radiation enteropathy represents the longer-term, chronic effects which may be found after a latent period most commonly of 6 months to 3 years after the end of treatment. In some cases, it does not become a problem for 20-30 years after successful curative therapy.

A large number of people receive abdominal and or pelvic radiotherapy as part of their cancer treatment with 60–80% experiencing gastrointestinal symptoms. This is used in standard therapeutic regimens for cervical cancer, prostate cancer, rectal cancer, anal cancer, lymphoma and other abdominal malignancies. Symptoms can be made worse by the effects of surgery, chemotherapy or other drugs given to treat the cancer. Improved methods of radiotherapy have reduced the exposure of non-involved tissues to radiation, concentrating the effects on the cancer. However, as the parts of the intestine such as the ileum and the rectum are immediately adjacent to the cancers, it is impossible to avoid some radiation effects. Previous intestinal surgery, obesity, diabetes, tobacco smoking and vascular disorders increase the chances of developing enteropathy.

Radiation proctitis can occur at two times after treatment:

- "Acute radiation proctitis" — symptoms occur in the first few weeks after therapy. These symptoms include diarrhea and the urgent need to defecate, often with pain while doing so (tenesmus). Acute radiation proctitis usually resolves without treatment after several months, but symptoms may improve with butyrate enemas. This acute phase is due to direct damage of the lining (epithelium) of the colon.

- "Chronic radiation proctitis" — is a widely used term, but is not correct as in long term after radiotherapy, inflammation in the bowel is minimal and instead the term chronic radiation proctopathy — or better, Pelvic Radiation Disease — should be used. In the chronic setting the pathological changes are characterised by ischaemia and fibrosis. Symptoms may begin as early as several months after therapy but occasionally not until several years later. These symptoms include diarrhea, rectal bleeding, painful defecation, incontinence, excess flatulence and intestinal blockage. Intestinal blockage is a result of narrowing of the bowel which blocks the flow of feces. Connections (fistulae) may also develop between the colon and other parts of the body such as the skin or urinary system. Chronic radiation proctopathy occurs in part because of damage to the blood vessels which supply the colon. The colon is therefore deprived of oxygen and necessary nutrients.

Individuals with HNPCC have about an 80% lifetime risk for colon cancer. Two-thirds of these cancers occur in the proximal colon. The mean age of colorectal cancer diagnosis is 44 for members of families that meet the Amsterdam criteria. Also, women with HNPCC have an 80% lifetime risk of endometrial cancer. The average age of diagnosis of endometrial cancer is about 46 years. Among women with HNPCC who have both colon and endometrial cancer, about half present first with endometrial cancer, making endometrial cancer the most common sentinel cancer in Lynch syndrome. In HNPCC, the mean age of diagnosis of gastric cancer is 56 years of age with intestinal-type adenocarcinoma being the most commonly reported pathology. HNPCC-associated ovarian cancers have an average age of diagnosis of 42.5 years-old; approximately 30% are diagnosed before age 40. Other HNPCC-related cancers have been reported with specific features: the urinary tract cancers are transitional carcinoma of the ureter and renal pelvis; small bowel cancers occur most commonly in the duodenum and jejunum; the central nervous system tumor most often seen is glioblastoma.

A large follow up study (3119 patients; average follow up 24 years) has found significant variation in the cancer rates depending on the mutation involved. Up to the age of 75 years the risks of colorectal cancer, endometrial cancer, ovarian cancer, upper gastrointestinal (gastric, duodenal, bile duct or pancreatic), urinary tract cancers, prostate cancer and brain tumours were as follows: for MLH1 mutations the risk was - 46%, 43%, 10%, 21%, 8%, 17% and 1% respectively: for MSH2 mutations the risks were 57%, 17%, 10%, 25%, 32%, and 5% respectively: for MSH6 mutations the risks were 15%, 46%, 13%, 7%, 11%, 18% and 1% respectively.

Radiation proctitis (and the related radiation colitis) is inflammation and damage to the lower parts of the colon after exposure to x-rays or other ionizing radiation as a part of radiation therapy. Radiation proctitis most commonly occurs after treatment for cancers such as cervical cancer, prostate cancer, and colon cancer. Radiation proctitis involves the lower intestine, primarily the sigmoid colon and the rectum and is part of the conditions known as pelvic radiation disease and radiation enteropathy.

Hereditary nonpolyposis colorectal cancer (HNPCC) or Lynch syndrome is an autosomal dominant genetic condition that has a high risk of colon cancer as well as other cancers including endometrial cancer (second most common), ovary, stomach, small intestine, hepatobiliary tract, upper urinary tract, brain, and skin. The increased risk for these cancers is due to inherited mutations that impair DNA mismatch repair. It is a type of cancer syndrome.

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

Crohn's disease is an inflammatory bowel disease that can affect any part of the digestive tract, even the stomach, although it's a rare presentation. Its main feature is inflammatory ulcers that can affect the total thickness of the stomach wall and can bleed but rarely perforate.

Symptoms include abdominal pain, loss of appetite, and weight loss. Diarrhea is also a symptom that can develop, so checking stools for the appearance of blood is important. It is possible for symptoms of Crohn's disease to remain with a person for weeks or go away on their own. Reporting the symptoms to a doctor is recommended to prevent further complications.

In the stomach there is a slight balance between acid and the wall lining which is protected by mucus. When this mucus lining is disrupted for whatever reason, signs and symptoms of acidity result. This may result in upper abdominal pain, indigestion, loss of appetite, nausea, vomiting and heartburn. When the condition is allowed to progress, the pain may become continuous; blood may start to leak and be seen in the stools. If the bleeding is rapid and of adequate volume it may even result in vomiting of bright red blood (hematemesis). When the acidity is uncontrolled, it can even cause severe blood loss (anemia) or lead to perforation (hole) in the stomach which is a surgical emergency. In many individuals, the progressive bleeding from an ulcer mixes with the feces and presents as black stools. Presence of blood in stools is often the first sign that there is a problem in the stomach.

Medullary carcinoma may refer to one of several different tumors of epithelial origin. As the term "" is a generic anatomic descriptor for the mid-layer of various organ tissues, a medullary tumor usually arises from the "mid-layer tissues" of the relevant organ.

Medullary carcinoma most commonly refers to:

- Medullary thyroid cancer

- Medullary carcinoma of the breast

Medullary carcinoma may also refer to tumors of:

- Pancreas

- Ampulla of Vater

- Gallbladder

- Stomach

- Large intestine

- Kidney — Renal medullary carcinoma

Krukenberg tumors often come to the attention when they cause abdominal or pelvic pain, bloating, ascites, or pain during sexual intercourse. Krukenberg tumors can occasionally provoke a reaction of the ovarian stroma which leads to hormone production, that results in vaginal bleeding, a change in menstrual habits, or hirsutism, or occasionally virilization as a main symptom.

All these symptoms are non-specific and can also arise with a range of problems other than cancer, and a diagnosis can only be made following confirmatory investigations such as computed tomography (CT) scans, laparotomy and/or a biopsy of the ovary.

There is substantial debate regarding histopathologic classification of pseudomyxoma peritonei. In 1995, Ronnett et al. proposed separating pseudomyxoma peritonei cases into two diagnostic categories: adenoma (disseminated peritoneal adenomucinosis, DPAM) or carcinoma (peritoneal mucinous carcinomatosis, PMCA) with a third category reserved for cases with intermediate features. In this classification system, cases of DPAM were characterized by peritoneal lesions composed of abundant extracellular mucin containing scant simple to focally proliferative mucinous epithelium with little cytologic atypia or mitotic activity (in other words, most cells looked fairly normal and there was no evidence of mitosis which would indicate that cells were rapidly dividing), with or without an associated appendiceal mucinous adenoma. Cases of PMCA were characterized by peritoneal lesions composed of more abundant mucinous epithelium with the architectural and cytologic features of carcinoma (irregular cells, evidence that cells were rapidly dividing, and other criteria), with or without an associated primary mucinous adenocarcinoma. Bradley et al. (2007) argued that continued use of non-malignant terms, i.e., adenoma, for those frequent cases with low-grade features (such as DPAM), is misleading because pseudomyxoma peritonei is a disease state that results from invasion of the abdominal cavity by cells with uncontrolled growth. Bradley states that an adenoma, by definition, is a tumor confined to the appendiceal mucosa with absolutely no evidence of invasion beyond the muscularis mucosae.

The term mucinous adenocarcinoma is used in different contexts depending on the reference material used by the pathologist for disease classification. For example, neoplasms characterized by high-grade features, invasive glands and or signet ring cells, are termed adenocarcinoma in pathology literature. However, some pathologists (e.g., Odze and Goldblum, "Surgical Pathology of the GI Tract, Liver, Biliary Tract and Pancreas", 2nd ed.) also use the term mucinous adenocarcinoma when referring to low-grade, well-differentiated tumors lacking high-grade features. Low-grade mucinous adenocarcinoma is used by the American Joint Committee on Cancer and World Health Organization and is nearly or completely synonymous with the DPAM designation. For low-grade mucinous adenocarcinoma, disease may be designated as "benign" because tumors do not invade deeply into tissue and rarely metastasize to parenchyma of organs; this designation may be misleading and confusing to the layperson because pseudomyxoma peritonei is not a harmless condition, fatal if untreated. High-grade or poorly differentiated mucinous adenocarcinoma has a generally poorer prognosis, though surgical treatment with heated intra-peritoneal chemotherapy (HIPEC) is yielding promising outcomes (see surgical treatment).

A Krukenberg tumor refers to a malignancy in the ovary that metastasized from a primary site, classically the gastrointestinal tract, although it can arise in other tissues such as the breast. Gastric adenocarcinoma, especially at the pylorus, is the most common source. Krukenberg tumors are often (over 80%) found in both ovaries, consistent with its metastatic nature.

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.

Diseases that affect the large intestine may affect it in whole or in part. Appendicitis is one such disease, caused by inflammation of the appendix. Generalised inflammation of the large intestine is referred to as colitis, which when caused be the bacteria "Clostridium difficile" is referred to as pseudomembranous colitis. Diverticulitis is a common cause of abdominal pain resulting from outpouchings that particularly affects the colon. Functional colonic diseases refer to disorders without a known cause, and include irritable bowel syndrome and intestinal pseudoobstruction. Constipation may result from lifestyle factors, impaction of a rigid stool in the rectum, or in neonates, Hirschprung's disease.

Diseases affecting the large intestine may cause blood to be passed with stool, may cause constipation, or may result in abdominal pain or a fever. Tests that specifically examine the function of the large intestine include barium swallows, abdominal x-rays, and colonoscopy.

From early adolescence, patients with this condition gradually (and much of the time 'silently') develop hundreds to thousands of colorectal polyps (and sometimes polyps elsewhere)—small abnormalities at the surface of the intestinal tract, especially in the large intestine including the colon or rectum. These may bleed, leading to blood in the stool. If the blood is not visible, it is still possible for the patient to develop anemia due to gradually developing iron deficiency. If malignancy develops, this may present with weight loss, altered bowel habit, or even metastasis to the liver or elsewhere. FAP can also develop 'silently' in some individuals, giving few or no signs until it has developed into advanced colorectal cancer.

Because familial polyposis develops very gradually over years, and can also manifest in an 'attenuated' form even more gradually, polyps resulting from FAP can lead to cancer developing at any point from adolescence to old age.

Depending on the nature of the defect in the APC gene, and whether it is the full or attenuated form, familial polyposis may manifest as polyps in colon or in the duodenal tract, or in any combination of these. Therefore, an absence of polyps in, for example, the rectum, may not of itself be sufficient to confirm absence of polyps. It may be necessary to consider and visually examine other possible parts of the intestinal tract. Colonoscopy is preferred over sigmoidoscopy for this, as it provides better observation of the common right-side location of polyps.

The genetic determinant in familial polyposis may also predispose carriers to other malignancies, e.g., of the duodenum and stomach (particularly ampullary adenocarcinoma). Other signs that may point to FAP are pigmented lesions of the retina ("CHRPE—congenital hypertrophy of the retinal pigment epithelium"), jaw cysts, sebaceous cysts, and osteomata (benign bone tumors). The combination of polyposis, osteomas, fibromas and sebaceous cysts is termed "Gardner's syndrome" (with or without abnormal scarring).

Gastrinoma causes the following symptoms:

- Hypergastrinemia

- Ulcers of the duodenum, stomach, and small intestine.

- Severe diarrhea.

- Generalized cancer symptoms.