colorectal carcinoma also known as colon cancer is when malignant or cancerous cells arise in the large intestines which includes the colon and rectum it's the most common type of cancer of the gastrointestinal tract and a major cause of death and disease around the world the large intestine is found in the abdominal cavity which can be thought of as having two spaces the intraperitoneal space in the retroperitoneal space the intraperitoneal space contains the first part of the duodenum all of the small intestines the transverse colon sigmoid colon and the rectum the retroperitoneal space contains the distal duodenum ascending colon descending colon and anal canal so the large intestines essentially weave back and forth between the intraperitoneal and retroperitoneal spaces now the walls of the gastrointestinal tract are composed of four layers the outermost layer is called the serosa for the intraperitoneal parts and the adventitia for the retroperitoneal parts next is the muscular layer which contracts to move food through the bowel after that is a submucosa which consists of a dense layer of tissue that contains blood vessels lymphatics and nerves and finally there's the inner layer of the intestine called the mucosa which surrounds the lumen of the gastrointestinal tract and comes in direct contact with digested food the mucosa has invaginations called intestinal glands or colonic crypts and it's lined with large cells that are specialized in absorption most colorectal carcinomas are adenocarcinomas meaning that they arise from the cells lining the intestinal glands most cases of colorectal tumors happen because of sporadic mutations but a small number are caused by known genetic mutations that run in a person's family an example of this is the adenomas polyposis coli gene or apc gene which is a tumor suppressor gene normally the apc protein identifies when a cell is accumulating a lot of mutations and forces it to undergo apoptosis or program cell death but when the apc gene is mutated the mutated bowel cells don't die and instead some start dividing uncontrollably giving rise to polyps over time these polyps might accumulate more mutations in other tumor suppressor genes like the kras gene or the p53 gene and ultimately it might become a malignant tumor meaning that the cells might be able to invade neighboring tissues another well-known example are genetic mutations in dna repair genes which help fix up mutations in cellular dna when they're out of action though cells accumulate mutations and over time can develop into polyps and eventually adenocarcinomas so broadly speaking adenocarcinomas are the malignant evolution of polyps and polyps arise when cells start dividing faster than usual there are many different types of polyps and some are more prone to becoming malignant those are called premalignant or neoplastic polyps and these pre-malignant polyps can be classified into adenomas and serrated according to how they look under the microscope typically adenomatous polyps have an apc mutation and the cells look like normal colonic mucosa cells whereas serrated polyps have defects in dna repair genes and have a sawtooth appearance when tumors become cancerous they can be categorized into stages stage 0 is carcinoma in situ meaning that the tumor has not grown beyond the mucosa stage 1 is when the tumor has grown beyond the mucosa but has not spread to lymph nodes or distant organs stage 2 is when the tumor has invaded the whole colonic or rectal wall and may have reached nearby organs or tissues but still has not spread to lymph nodes or distant organs stage three is when the tumor has spread to lymph nodes but still hasn't spread to distant organs finally stage four is metastatic meaning that the tumor has spread to distant organs the most common site of metastasis for colon cancer as the liver and for rectal cancer it's the lungs there are some non-modifiable risk factors for colorectal cancer like being an elderly male and having inflammatory bowel disease but there are also some modifiable risk factors as well like smoking cigarettes eating a lot of red meat not eating a lot of fiber and being obese colorectal cancers are also associated with specific disorders like familial adenovidus polyposis as well as hereditary non-polyposis colorectal cancer initially colorectal carcinoma is often asymptomatic but as the disease progresses symptoms can develop depending on the tumor's location tumors in the ascending or right colon generally grow outward beyond the surface of the mucosa and that can cause vague abdominal pain and weight loss typically these tumors don't cause bowel obstruction which means that they can grow quite large before causing symptoms and that also means that there's often a late diagnosis these tumors can ulcerate and bleed and over time the bleeding can lead to iron deficiency anemia tumors located on the descending or left colon are generally infiltrating masses meaning that they tend to be ring-shaped masses that involve the whole circumference of the colonic wall this causes lumen narrowing referred to as napkin ring constriction so symptoms of bowel obstruction typically happen early on bowel obstruction can cause colicy abdominal pain and blood streaks stools called hematokesia of colorectal cancer typically requires colonoscopy which is when a camera has inserted retrograde into the colon and rectum to take pictures of abnormal looking polyps and get a biopsy in addition fecal occult blood testing is often done to look for evidence of gastrointestinal bleeding typically in colorectal cancer there's an elevation in the tumor marker cea which is a glycoprotein involved in cell adhesion this however is not a very specific finding a barium enema can be useful as well and that's where a liquid is injected into the rectum through a small tube and an x-ray is taken to look for abnormalities in the large intestines on a barium enema a classic sign of colorectal cancer is the apple core sign which shows the constriction of the lumen most often in the descending colon to identify colorectal cancer early routine colonoscopy and fecal occult blood tests can be done to look for neoplastic polyps or early carcinomas so that they can be removed at an early stage this is particularly important for individuals with relatives that have colorectal cancer and for those with disorders like familial adenomas polyposis and hereditary non-polyposis colorectal cancer treatment for colorectal cancer depends on the stage of the cancer early cancers that are confined to the wall of the colon are often surgically resected and those that have spread to nearby lymph nodes are often treated with chemotherapy unfortunately metastatic cancers are typically incurable but sometimes chemotherapy or surgery can be used to ease symptoms all right as a quick recap colorectal carcinoma is a very common and deadly cancer that affects the large intestines the majority of cases are sporadic and do not have a clear cause but modifiable risk factors include smoking cigarettes eating a lot of red meat not eating a lot of fiber and being obese symptoms can vary based on the tumor's size and location tumors of the right colon generally don't cause bowel obstruction and instead can cause occult bleeding which can lead to iron deficiency anemia tumors of the left colon on the other hand are generally ring-shaped masses which can cause bowel obstruction with constipation as well as abdominal pain helping current and future clinicians focus learn retain and thrive learn more you