Colorectal cancer is a common malignancy associated with substantial morbidity and often leads to death. It would be nice to have a single preferred imaging study for diagnosis and workup of colorectal cancer, yet even with advances of modern imaging such a single imaging modality is not universally accepted.
Currently, imaging of colorectal cancer can be divided into either the investigation of a possible colon cancer or the workup of a known colon cancer which has already been diagnosed, usually by colonoscopy.
Let’s first look into imaging of a possible colon cancer. This usually will involve a CT examination of the abdomen and pelvis for symptoms such as abdominal pain, bloating, change in bowel habits or blood in the stool. Symptoms may be nonspecific in that myriad other disorders of the internal abdomen and pelvis can be associated with similar problems. In these cases, the detection of a colon cancer depends in large part on the size of the lesion. Considering that cancer may present at the earliest stage as a polyp, at times less than 1 cm in size, the detection of an early-stage tumor may only approach 50% at best with standard barium oral contrast CT.
As disease progresses, there will be a stage of focal colon wall thickening often followed by narrowing of the colon lumen. Finally, as disease progresses further, there may be a well-defined mass. CT detection rates increase as disease becomes more severe but still may only approach 80%. A dedicated CT screen exam called CT Colonography, or virtual colonoscopy, has a much higher colon cancer detection rate as a screening exam than does a barium enema. CT Colonography can approach optical colonoscopy for lesion detection. A feature of CT Colonography is that it is considered less invasive than colonoscopy; a limitation is that if it does detect a lesion, the patient will still then need to undergo the colonoscopy for tissue biopsy for diagnosis/lesion removal.
CT imaging plays an important role as a staging tool in the workup of already diagnosed colon cancer, comparable to MRI. CT can detect the spread of disease beyond the colon wall, can evaluate for the spread of disease to regional or distant lymph nodes, and can be used to assess the spread to other body organs as metastatic disease. Many surgeons prefer CT imaging for staging prior to colon cancer surgery. Both CT and MRI have limitations in evaluation of lymph nodes in that only the size of a node is assessed. While enlarged nodes are more likely to harbor spread of disease, there can be enlarged lymph nodes that are not involved with tumor, just as there can be small lymph nodes thought to be ‘normal’ size which do contain microscopic disease. This is a major reason for increased use of a functional imaging exam known as PET/CT in the workup of colorectal cancer to evaluate increased cellular metabolism of a labeled sugar compound to evaluate extent of disease spread either regional or to distant organs such as liver.
To summarize, CT continues to detect many unsuspected carcinomas. The high-resolution multidetector CT at Gastro Health Imaging provides excellent detail for colon imaging. Your Gastro Health physician has access to the full armamentarium of imaging studies available to best workup your presenting signs and symptoms, some of which can be associated with colon and rectal cancer. Even with significant imaging advancements in recent years, there is still not a single imaging study to accomplish both colon screening and workup of a potentially detected mass. Therefore, most would agree colonoscopy remains the gold standard in diagnosis of colorectal cancer.