Carcinogens may be ingested in the diet but are more likely to be produced from dietary substances or from biliary or intestinal secretions, probably by bacterial action. The exact mechanism is unknown. Colorectal cancer spreads by direct extension through the bowel wall, hematogenous metastasis, regional lymph node metastasis, perineural spread, and intraluminal metastasis. Symptoms, Signs, and Diagnosis Adenocarcinoma of the colon and rectum grows slowly, and a long interval elapses before it is large enough to produce symptoms. Early diagnosis depends on routine examination.
Symptoms depend on the lesion's location, type, extent, and complications.
The right colon has a large caliber and a thin wall. Because the colon contents are liquid, obstruction is a late event. Tumors, which are usually fungating, may grow large and be palpable through the abdominal wall. Bleeding is usually occult. Fatigue and weakness caused by severe anemia may be the only complaints.