Adenocarcinomas of the colon and rectum make up 95 percent of all colorectal cancer cases. In the gastrointestinal tract, rectal and colon adenocarcinomas develop in the cells of the lining inside the large intestine. Adenocarcinomas of the colon and rectum typically start as a growth of tissue called a polyp. A particular type of polyp called an adenoma may develop into cancer. Polyps are often removed during a routine colonoscopy before they may develop into cancer.
Less common types of colorectal cancer include primary colorectal lymphomas, gastrointestinal stromal tumors, leiomyosarcomas, carcinoid tumors and melanomas. Types of colorectal cancer include:
Colorectal adenocarcinoma: "Adeno" is a prefix meaning glands. "Carcinoma" is a type of cancer that grows in epithelial cells that line the surfaces inside and outside the body. Adenocarcinomas of the colon or rectum develop in the lining of the large intestine. They often start in the inner lining and then spread to other layers. There are two less common subtypes of adenocarcinoma:
Gastrointestinal carcinoid tumors: Carcinoid tumors develop in nerve cells called neuroendocrine cells, which help regulate hormone production. These tumors are among a group of cancers called neuroendocrine tumors (NETs). Carcinoid tumors cells are slow-growing and may develop in the lungs and/or gastrointestinal tract. They account for 1 percent of all colorectal cancers and half of all cancers found in the small intestine.
Other types of rare colorectal cancers combined account for less than 5 percent of all cases and include:
Primary colorectal lymphomas: A type of non-Hodgkin lymphoma, this cancer type develops in the lymphatic system, specifically in cells called lymphocytes. Lymphocytes are a type of white blood cell that helps the body fight infections. Lymphoma may develop in many parts of the body, including the lymph nodes, bone marrow, spleen, thymus and the digestive tract. Primary colorectal lymphomas account for just 0.5 percent of all colorectal cancers and about 5 percent of all lymphomas. This colorectal cancer type usually occurs later in life, and is more common in men than women.
Gastrointestinal stromal tumors: GISTs are a rare type of colorectal cancer that forms in a special cell found in the lining of the gastrointestinal (GI) tract called interstitial cells of Cajal (ICCs). More than 50 percent of GISTs develop in the stomach. While most other GISTs form in the small intestine, the rectum is the third most common location. GISTs are classified as sarcomas, or cancers that begin in the connective tissues, which include fat, muscle, blood vessels, deep skin tissues, nerves, bones and cartilage.
Leiomyosarcomas: Another form of sarcoma, leiomyosarcoma essentially means “cancer of smooth muscle.” The colon and rectum have three layers of the type of muscle affected by leiomyosarcoma, and all three work together to guide waste through the digestive tract. This rare type of colorectal cancer accounts for about 0.1 percent of all colorectal cases.
Melanomas: Most commonly associated with skin cancer, these may occur anywhere, including the colon or rectum.
Next topic: What is metastatic colorectal cancer?