Gastrointestinal Cancers

A basic understanding of the spread of cancers of the gastrointestinal tract relies upon the depth of mural penetration. Originating in the epithelial layer, the cancer's invasion of the wall may variously extend into the lamina propria, muscularis mucosae, submucosa, muscularis propria, and subserosal connective tissue or through the serosa with or without direct invasion of adjacent structures. This determination forms the basis of the universally adopted TNM staging classification,1-3 encompassing all three prognostic factors of tumoral mural penetration (T), nodal involvement (N), and distant metastatic spread (M) in one staging system.

Staging is the process by which the anatomic extent of a tumor is defined at a certain point of time. Onco-radiology has had a major influence on the TNM staging system, which has incorporated changes to accommodate the information available by the widening spectrum of diagnostic and interventional imaging and endoson-ography.4 Depth of tumor invasion (T) is the main criterion for clinical classification, replacing factors such as tumor size, circumferential extent of involvement, and gross morphology. In carcinoma of the esophagus and stomach, depth of invasion as the major determinant of prognosis was clearly established in Japanese studies of more than 3000 cases5 and 15,000 cases,6 respectively. This was achieved because of developments in endoson-ography (EUS) and CT. Similarly, the current TNM classification for colorectal carcinomas is based on the accuracy of cross-sectional images. Developments in sectional imaging contribute significantly to the earlier detection of disease, to determine the stage of a malignancy, to substantiate criteria for resectability and non-resectability of a cancer, to monitor posttreatment tumor response, and to evaluate residual or recurrent disease.

Fundamental to the staging system is the concept of possible independence of the TNM states. Because lymph node involvement (N) is designated as a separate category, there is no assumption that the full thickness of the gut wall must be penetrated before lymph node involvement occurs, as is the assumption behind the Dukes classification of rectal carcinoma.7 The concept of the stages of a cancer does not imply a regular progression from stage I to stage IV.

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