Colorectal Carcinoma

Overview
  • Colorectal Carcinomas represent the major malignancy of the large intestine and rectum. Colorectal Carcinomas are thought to arise from precursor lesions of colorectal adenoma due to progressive genetic and epigenetic lesions which derange cell cycle control.
Epidemiology
  • Colorectal Carcinomas are the second most common cause of cancer death and typically arise in the elderly after the fifth or sixth decade. A variety of risk factors have now been identified, the most important of which appears to be diet.
  • Diets high in animal fats and low in fiber appear to be represent the greatest risk, although it is unclear if foods common to these diets directly possess carcinogenic compounds or instead enhance growth of bacteria which convert bile acids into carcinogens. Alternatively, risk-prone diets may indirectly promote colorectal carcinomas by inducing the true proximate causes such as insulin resistance and deranged host metabolism.
  • Genetics is also suspected of playing a key role as quarter of patients possess some family history of the disease. Heredity plays a much clearer role in those with Multiple Polyposis Syndrome who can develop colorectal carcinoma before their fourth decade.
Morphology
  • Colorectal carcinomas are all adenocarcinomas and are staged according to the depth of their invasion, local spread to lymph nodes, and presence of distant metastasis. It should again be pointed out that these adenocarcinomas are thought to arise from colorectal adenomas with greater probability from the villous subtype. Initially, tumors are limited within the large intestine mucosa, but over time burrow into the submucosa, and ultimately into and through the muscularis propria. Metastasis typically occurs to local lymph nodes initially but can spread distantly, usually to the liver, indicating far worse prognosis.
Clinical Consequences
  • Overview
    • Sadly, colorectal carcinomas remain asymptomatic for years before they present. When symptomology manifests it often depends on the anatomic location of the tumor as described below.
  • Right Colon
    • Because feces is fairly liquid in the cecum and ascending colon, symptomology of tumors at these locations is not related to the quality of stool itself. Rather, ulceration of these tumors leads to occult lower GI bleeding which over the long-term can manifest as a hypochromic microcytic anemia, representing chronic iron deficiency anemia with associated fatigue.
  • Transverse and Descending Colon
    • Tumors of the transverse and descending colon tend to grow in a napkin ring-like pattern around the bowel, thus narrowing the lumen. Because feces is more solid at this stage of the alimentary tract, the narrowed bowel lumen can result in mild abdominal pain associated with bowel obstruction.
  • Rectosigmoid Colon
    • Tumors of the sigmoid colon and rectum also tend to reduce the alimentary lumen at these segments. Consequently, pateints may present with narrowed stool caliber or some anal bleeding.
Laboratory
  • Carcinoembryonic Antigen (CEA) Levels are used to predict tumor recurrence following surgical excision. CEA is a protein normally only produced during fetal development but is also generated by a number of tumor types including colorectal carcinomas. Consequently, is used only for following treatment progress rather than diagnosis as it is not a specific marker for colorectal carcinoma.