| Discussion: |
Colorectal carcinoma is a malignant tumor arising from the colorectal epithelium and accounting for two-thirds of all gastrointestinal malignant tumors. The large majority of colorectal carcinomas are adenocarcinomas. Its increasing frequency in the Western world and its rare occurrence in underdeveloped countries suggest etiological dietary factors such as fiber deficiency, increased fat intake and increased consumption of beef. The premalignant nature of colon polyps is now well accepted. The most common gross appearance of colon carcinoma is either polypoid or annular. More rarely a flat ulcerated lesion is seen. Polypoid carcinomas are preferentially located in the cecum, the ascending colon and the rectum whereas annular and ulcerating lesions are more frequently seen in the transverse, descending and sigmoid colon. Most colon carcinomas are slow growing. In patients found to have colorectal carcinoma, adenoma is found in about 30% and synchronous carcinoma in about 5%.
On plain films of the abdomen, one may notice very rarely fine stippled calcifications either in the tumour itself or in liver and lymph node metastasis. On barium contrast studies polypoid carcinomas are visible as a lobulated filling defect of variable size with a contour deformity along one margin of the bowel). On double contrast study, however, even large tumours may not be associated with a contour deformity because of marked adjacent lumen distension. Annular tumours show deformation of both margins with lumen narrowing. There is an abrupt transition between the tumour, where the mucosa is no longer visible and the normal aspect of the adjacent mucosa. This leads to a typical appearance of an "apple core" or an overhanging edge, called the tumor shoulder. In case of obstruction due to intussusception, a not uncommon complication, one may observe the typical coiled-spring feature. In other patients with obstruction radiographic features of colitis, characterized by narrowing and mucosal changes, is seen in the colon segment proximal to the obstructing lesion.
Abscess formation secondary to perforation will be best displayed by CT. Double contrast examination of the colon has a high sensitivity for detecting colorectal carcinoma if appropriate preparation of the colon is achieved. The main causes for a filling defect to be differentiated from colon carcinoma are: endometrosis, internal hemorrhoid, lipoma, metastatic or lymphoma localization.
Transabdominal ultrasound, which is applied frequently as the initial study for examining patients with abdominal complaints, is able to visualize colorectal carcinoma in about 25% of cases as hypoechoic masses or wall thickening. It should, however, not be used as a primary modality to detect these tumours.
Similarly CT is able to display colon wall thickening as suggestive for colorectal carcinoma on routine abdominal scans but this method is mainly useful for staging of colorectal carcinoma. Recently three-dimensional (3-D) visualization of the colon, also called virtual coloscopy, based on spiral CT or on MRI data, has become available. With this technique it is possible to identify changes of the colon mucosa and tumors originating from the mucosa. Further studies and experience are required to define the real diagnostic potential and the role of this method for the management of colon carcinoma.
The staging of colorectal carcinoma can be performed by endoluminal ultrasound (EUS), CT and MRI.
EUS is able to depict the degree of disruption of the muscular layers of the colon wall. Accuracy of EUS for T staging has been reported as around 80% and for N staging as ranging between 62% and 83%.
CT staging is based on an analysis of t resection CT and MRI are the only modalities available for evaluating recurrent tumour. As neither CT nor MRI can reliably differentiate between recurrence and fibrosis or inflammation, it is advised to perform a baseline CT or MRI study 3–4 months after surgery with repeat examinations at regular intervals thereafter.