Colorectal cancer (CRC) is the second leading cause of cancer death in the United States, second only to lung cancer. Rectal cancer accounts for approximately one-third of colorectal tumors. Rectal cancers differ from colonic carcinomas in terms of their embryologic origin, treatment, and sites of metastasis.

The majority of rectal cancers are adenocarcinomas, which arise from a background of adenomatous dysplasia. The risk factors for developing CRC are numerous. There are adenomatous polyposis syndromes such as familial adenomatous polyposis, hereditary nonpolyposis colorectal cancer, and MUTYH-associated polyposis (MAP) that significantly increase a person's risk of developing CRC. Smoking is also a risk factor. A personal or family history of sporadic CRCs or adenomatous polyps is also concerning. Other inflammatory conditions such as inflammatory bowel disease (especially ulcerative colitis) are risk factors for the development of CRC.

CRC rates are highest among African Americans compared with other populations in the United States. Patients with a low socioeconomic status have also been shown to have an increased risk of developing CRC. The incidence of rectal cancer is higher among men than women. Overall, the incidence of colorectal cancer in older patients, particularly those aged 65 years and older, is declining, likely due to a combination of screening and changes in risk factors (eg, decreased smoking). But for patients aged younger than 55 years, the incidence of colorectal cancer has been rising.

Patients with cancers of the rectum or rectosigmoid region often present with hematochezia, tenesmus, and narrowing of the stool. Anemia is uncommon at presentation.

The most common sites of metastasis of colorectal adenocarcinoma are the liver and lungs; rarely, metastatic oral lesions occur.

The US Preventive Services Task Force (USPSTF) recommends screening for CRC using noninvasive blood tests, sigmoidoscopy, or colonoscopy in adults, beginning at age 50 years and continuing until age 75 years. Earlier screening is appropriate for patients with a family history of CRC. Some experts recommend screening everyone beginning at age 45 years.