Diagnostic Value of CT Colonography in Colorectal Carcinoma

DISCUSSION

Colorectal carcinomas are common malignant tumors of the lower gastric intestinal tract. The incidence rate and case-fatality rate are very high in the West, and have been gradually elevating in China with development of the economy and changes in eating habits^[1]. In the past, the diagnosis of colorectal lesions depended mainly on endoscopy and administration of a barium enema. Although these two examinations are easy to apply, they can only observe the interior of the enteric cavity, but not the bowel wall and its environment. So these methods are inadequate for diagnosis and staging. Because CTC is based on a multi-spiral CT scanning technique that produces images reconstructed by advanced imaging software, it can not only examine the occupying lesions in the enteric cavity, but also identify the abnormalities in the bowel wall and environment. CTC has high spatial resolution, that can locate polyps with diameters of 5 mm or less^[2]. In our cases, the diameter of the minimal lesion was approximately 10 mm. These features make CTC much better than either endoscopy or barium enema examinations, thus avoiding missed diagnosis caused by barium masking or difficulty in passing of the endoscope. CTC examinations are more consummate and accurate by scanning the abdomen overall.

CTC involves many reconstruction techniques. CTVE simulating colonoscopy can observe masses at a short distance from the intracavitary to describe the surface details of the masses. SSD and Raysum simulating barium enemas, can display compression and stenosis and interruption of the abnormal colon or rectum. MPR can view the spread of a mass inside and outside of the enteric cavity, and display the details within the mass^[3]. These reconstruction techniques combined, provide a complete review of the masses, display clearly the size, shape and surface of the mass, enabling an accurate diagnosis. The coincidence of our case in typing was 90% in total. It was highest for the mass type reaching to 100%, but was less specific because of some infiltrative lesions whose surface was uneven which was might be mistaken for the ulcerative type.

Even more, CTC can specifically identify colorectal carcinoma from other occupying lesions adjacent to the colon or rectum. The appendicular pseudocyst and interstitialoma were diagnosed accurately by CTC because they were clearly disclosed staying outside the intestine. The nonspecific inflammation was not rare in the colonic lesions. In our cases, the 5 cases were all in the right hemicolon. With CTC, a slight inflammation would be mucosal fold thickening, when serious the colonic wall would be thickening even sometimes forming a mass similar to carcinoma. For inflammation, the lesion region gradually continued with the normal colonic wall, and the infiltration was more extensive and contracted rapidly after anti-inflammatory treatment which is helpful in differentiating it from carcinoma.

The nature of an adenoma is related to its size, with the canceration probability of an adenoma smaller than 10 mm, 10~20 mm, and larger than 20 mm in diameter being 1%, 10%, and 30%, respectively^[4]. When the diagnoses were made only based on the size of the adenomas, it would lead to false positive or false negative results. In our study, an adenoma approximately 28 mm in diameter was mistaken for cancer. So the other morphological features should be noticed. A broad pedicle, cauliflower-like shape and notch mostly indicate the canceration of an adenoma ^[5].

Lymphoma is another malignant tumor in the colon, usually found in the ileocecal junction, which is similar to a carcinoma, and hard to identify. But lymphomas were usually accompanied with multiple lymphadenectasis in the retroperitoneum or other regions of the body, and obviously shrink after chemotherapy in a short term. So CTC must be combined intimately with clinical observations.

Tuberculoses of the colon, whether a proliferative or ulcerative type, are similar to colon carcinomas. The differential diagnosis must be combined with clinical history and laboratory examinations. One TB case in our study was mistaken for carcinoma for lack of a typical clinical appearance. The residual stool masses in the enteric cavity might be similar to occupying diseases. MPR can differentiate them from solid tumors by finding air in the stool masses. On the whole, the diagnostic accuracy of CTC for coloretum carcinoma is fairly high. The sensitivity and specificity was 100% and 84.6% respectively, in the cases we studied.

Preoperative staging is one part of CTC diagnosis. The accuracy of CTC staging diagnosis is relatively high, because CTC possesses high spatial resolution, can observe the relation between the lesions and surroundings from various angles, and can verify metastasis in the regional lymph nodes and abdominal viscera. According to Dukes’ staging classification: Dukes’ A cancers have not spread beyond the mucosa into the submucosa or muscularis propria; Dukes’ B cancers have invaded through the muscularis propria and into neighboring tissues (fat or other organs); Dukes’ C cancers have spread to local lymph nodes; Dukes’ D cancers have metastasized to distant organs^[1]. On CTC, tumors of Duke’s A appear as a polypoid mass in the enteric cavity, without bowel wall thickening; tumors of Duke’s B appear in the bowel wall as lesions exceeding 5 mm, the fat layer is clear or slightly turbid, the boundary with surrounding organs is clear; tumors of Duke’s C appear with peripheral or regional lymph node swelling; tumors of Duke’s D appear with metastasis in distant organs or lymph nodes, a fat layer surrounding the lesion is obviously turbid, with extensive infiltration in the surrounding organs or abdominal wall.

Although CTC staging diagnosis of colorectum carcinoma is more accurate than with other techniques, its capability to disclose the extent of infiltration and lymph node metastasis is still limited. In our cases, the accuracy of staging carcinoma with Dukes’ tumors was 75%, close to other similar studies^[6]. According to the liturature and our results, many factors can influence the accuracy of CTC staging diagnosis such as the following: (l)On CTC, the bowel wall only presents one layer, even when the enteric cavity is not fully aerated, so the infiltrative depth in each layer is difficult to identify. Only when the bowel wall thickening exceeds 5 mm, is serous membrane infiltration considered. Therefore the lesions of Stage B are underestimated. In addition, if there is edema in the serous membrane, the lesions of Stage A may be overestimated to Stage B^[7]. (2)On CTC, the malignancy of a lymph node is mainly identified by its size, so a small lymph node metastasis below a threshold may be missed^[8], resulting in a lesion of Stage C being mistaken for Stage B. (3)By scanning the whole abdomen, CTC can locate liver metastasis, distant lymph node metastasis, and peritoneal seeding promptly and accurately, but it is difficult to identify the surrounding infiltrated organ, especially when the patient was very thin and the fat layer between organs was absent, which mainly affected the accurate diagnosis of Stage D. Even though, CTC staging diagnosis was useful in evaluating a radical operation, choosing a treatment plan and elevating operative success rate^[9].

The multiple reconstruction techniques of combined CTC can clearly observe colorectal carcinomas as well as the overall metastasis in the surrounding and distant areas, and thus provide more useful diagnostic information than a coloscopy or barium enema. Although CTC can not provide a pathologic diagnosis, identify the color of the lesion and is insensitive to a flat lesion^[10], its distinct dominance will make it of increasing importance in diagnosing colorectal carcinoma.