Screening Techniques for Colorectal Cancer
Colorectal cancer, the second leading cause of cancer death in the U.S., is particularly amenable to screening since the disease evolves from small, premalignant polypoid masses detected via a variety of examinations:
- The hemoccult test to check for blood in stool
- Barium enema
- Colonoscopy
- Helical CT ("virtual" colonoscopy)
The hemoccult test is the most widely available and easily implementable test. It is also the least accurate and specific.
Air contrast barium enema is minimally invasive and requires no sedation. The technique has only modest sensitivity (50%-80%) for polyps <1 cm or for Dukes Stage A and B cancers (55%-85%). The low sensitivity is due to inadequate visualization of bowel segments and errors of interpretation. The increasing utilization of endoscopic techniques is causing a decline in the numbers of patients referred for double contrast barium enemas.
The current gold standard-colonoscopy-misses 15%-20% of polyps <1 cm and up to 6% of larger lesions. It should also be noted that colonoscopy is an invasive technique associated with a varying level of morbidity depending upon the experience level of the endoscopist, and the patient's clinical condition. An advantage of colonoscopy over CT colongraphy is that both visual diagnosis and biopsy of suspicious polyps can be performed in one procedure.
Thus, there are hopes for the noninvasive helical CT technique which uses "virtual reality" computer visualization. The patient must undergo pre-imaging bowel preparation similar to the protocol prior to colonoscopy. The colon is then inflated with air or carbon dioxide before imaging begins.
Ongoing multicenter trials will eventually determine if helical CT is cost effective and accurate enough to replace invasive colonoscopy as the gold standard in this regard. It should also be noted that MRI colonoscopy is also undergoing extensive evaluation with some impressive preliminary results.
Staging Primary Colon Cancer
The initial immediate therapy for colon cancer is wide surgical resection of the involved segment and removal of regional lymph nodes. Risk of recurrence increases with initial spread (e.g., 5% in lesions confined to the mucosa or submucosa but 25% or higher for tumors extending in to the pericolonic fat). Adenopathy is a particularly ominous associated finding, greatly elevating the risks of recurrence. Recurrent disease occurs in up to 40% of patients who undergo a theoretically curative initial resection.
Cross-sectional imaging is the most useful staging protocol. Endoscopic ultrasound is a useful tumor staging tool which can also help stage surrounding nodes adjacent to the colonic wall. MRI or CT require colonic cleansing and adequate distension in order to well delineate an intraluminal tumor. Both CT and MRI are, however, powerful tools for evaluating extension beyond the bowel wall, as well as distant metastatic disease. Contrast-enhanced MRI facilitates multiplanar imaging which is particularly useful in patients with involvement of adjacent organs.
Hepatic metastases can be removed by liver resection, assuming the tumor burden in the liver is not too high. Therefore, it is critical to fully delineate the number of lesions and their precise locations. CT and MRI both offer excellent noninvasive alternatives, as opposed to the gold standard arterial portography and intraoperative sonography. MRI with liver-specific enhancement agents like FERIDEX I.V.® (ferumoxides injectable solution) has impressive utility. MRI with conventional contrast-enhanced techniques with gadolinium agents like MAGNEVIST ®(gadopentetate dimeglumine) are continually being refined, as well.
While extrahepatic disease in the abdomen and peritoneal spread can both be assessed equally well by MRI or CT, lung lesion detection is most reliably performed by CT.
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