The most common presenting feature in patients with rectal and low sigmoid colon cancer is melena (dark stools or blood in the stools). Abdominal pain is the most common feature in patients who have colon cancer above these areas. Other features include changes in bowel habits, nausea, vomiting, weakness and an abdominal mass. Sometimes colon cancer is found in patients who have anemia due to a gradual blood loss over a prolonged period of time.
Diagnostically, the most common study today is the colonoscopy. Very seldom are barium enemas utilized any longer in the diagnostic workup of colon cancer. The colonoscopy allows the endoscopist (the physician doing the procedure) to measure the distance from the anus to the lesion and to describe the lesion in full, as well as to biopsy the lesion.
Once the diagnosis of colon malignancy has been made, CT scans are performed of the lungs, abdomen and pelvis to rule out metastatic disease or lymphatic disease somewhat distant from the colon lesion. Also, preoperatively, a level of carcinoembryonic antigen (CEA) is determined from the blood. CEA is an independent prognostic factor in large bowel cancer. If it is elevated before an operative procedure and decreases after the removal of the colon cancer, it is a reliable factor for use in the follow up of patients with cancer. It if stays low, the cancer is probably not recurring; if it is high, the cancer may be coming back.
Unfortunately, about 75 percent of patients who do have recurrences of their colon and rectal cancers after operative resection will have other symptoms before their CEA rises. The 25 percent of patients who have a rising CEA before any symptoms are more likely to be cured of their recurrence.