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Description
According to the U.S. Centers for Disease Control, cancer ranked second (23%) as the cause of death in 2000. (1) The American Cancer Society (ACS) 2003 cancer statistics report ranked cancer of the colon and rectum third in both incidence and death rates for all cancers occurring in the U.S. population. (2) Early detection has the greatest impact on patient survival, both at the onset of the disease and in the case of recurrence after treatment. Diagnostic imaging has a role in both circumstances, although no imaging application has been clearly identified as the primary, technique for screening the average-risk population.
Diagnostic imaging identifies CRC by demonstrating characteristic alterations in colorectal anatomy. Determining the usefulness of each imaging technique depends on understanding its ability to show the relevant anatomical alterations. To facilitate this understanding, this article begins with a review of anatomy.
Colorectal Anatomy
The large intestine comprises the colon and rectum. The proximal colon begins with the large, pouch-like cecum with its worm-like appendix. The terminal ileum enters the cecum at its medial and posterior aspect. The ascending colon begins al the level of the terminal ileum and becomes the hepatic flexure at the level of the inferior margin of the liver. The transverse colon extends across the upper abdomen, between the hepatic and splenic flexures. The descending colon extends from the splenic flexure to the sigmoid colon, which has a redundant course ending at the rectum. (See Fig. 1.) The colon's largest diameter (7.5 to 8.5 cm) is at the cecum, and it gradually decreases in diameter to approximately 2.5 cm where the sigmoid colon terminates at the rectal junction. The rectum is 12 to 15 cm in length, extends from the sacral promontory to the anal canal and follows the curve of the sacrum. The anal canal is approximately 4 cm long and has the external appearance of an anteroposteriorly oriented slit.
[FIGURE 1 OMITTED]
The cecum, transverse colon and sigmoid colon are intraperitoneal, meaning their outer surfaces are fully covered with serosa that is continuous with a mesentery of varying length. The sigmoid colon has the longest mesentery, so it is most prone to volvulus because of the lack of fixation. The ascending colon, descending colon, posterior surfaces of the hepatic and splenic flexures, and the rectum are retroperitoneal because some portion of their outer surface is not covered with a serosal layer.
The tissue layers of the colon wall are mucosa, submucosa, circular muscle, longitudinal muscle and serosa. The mucosa is the innermost layer, consisting of columnar epithelium in an arrangement similar to the small intestine, but with a smoother surface because of the lack of villi. The tubular crypts of the mucosa extend from the colon lumen to the muscularis mucosa that separates the mucosal layer from the submucosa. The submucosa contains numerous veins, arteries and lymphatic vessels. External to the submucosa is the muscularis propria that consists of an inner circular muscle layer and an outer longitudinal muscle layer. The inner, circular smooth muscle fibers form a continuous, circumferential layer, whereas the outer longitudinal smooth muscle fibers are arranged in 3 narrow bands, called the teniae coli, that are spaced equidistant around the circumference of the colon. The haustra are sacculations (out pouches) between the teniae coli, and the plicae semiluminares (crescent-shaped mucosal folds) that separate adjacent haustra. The serosa is the outermost, mesothelial-derived cell layer that covers the peritoneal surface of the colon wall. This layer secretes serous fluid to lubricate the contact surface of the colon with adjacent structures, facilitating easy movement during peristalsis.
Origin of Colorectal Cancer
CRC begins, in most cases, as dysplasia of the colorectal mucosa and progresses to malignancy. (3-6) This phenomenon is referred to as the adenoma-carcinoma sequence or, more recently, as the dysplasia-carcinoma sequence. (3) However, progression of an adenoma to CRC is not inevitable, with only about 5% becoming malignant. (7) Dysplasia leading to an adenoma is characterized by disorganized cell proliferation, with dividing cells distributed throughout the full thickness of the mucosa rather than confined to the nor-real proliferative zone in the lower three quarters of the crypts (recesses). (6) This dysplasia expands to involve clusters of crypts, resulting in aberrant crypt foci (ACF). Although there is evidence that ACF can develop into adenoma, the mechanism has not been clearly established. (8)
The development of ACF to a small adenoma is an early morphologic sign of the dysplastic process. An adenomatous polyp is characterized by a well-defined lump of dysplastic epithelium projecting above the mucosa surface. (6) The term "polyp" applies to these adenomas because a polyp refers to any circumscribed mass of cells that projects above the normal mucosal surface. It has been discovered in recent years that not all adenomas are polypoid. Flat adenomas have the dysplastic features of an adenoma, but as the name implies, do not project above the mucosal surface, making them more difficult to see with imaging techniques that rely on visual changes in the topology of the colorectal mucosa. It is important to identify these flat adenomas because there is evidence that the malignant potential of these lesions is greater than for polypoid adenomas of similar size. (9)
The histology of the adenoma is classified as tubular, tubulovillous or villous. The more common tubular adenoma has a cellular structure similar to normal colorectal mucosa, tends to be smaller and is more likely to contain a stalk or pedicle. (See Fig. 2.) The villous subtype consists of finger-like projections (villi) covered with dysplastic epithelium, is frequently larger and is more likely sessile (stalkless), growing on a broad base directly from the mucosal surface.
[FIGURE 2 OMITTED]
The tubulovillous adenoma, as the name implies, contains a cellular morphology with both tubular and villous components. Muto et al (10) correlated the size of an adenomatous polyp and its histology. They determined that more than three fourths of the smaller adenomas (<1 cm) they studied were of the tubular subtype, 46.8% of intermediate-sized adenomas (1 to 2 cm) were tubulovillous and 60% of adenomas larger than 2 cm were villous. This relationship of size and adenomatous subtype is important because there is growing evidence that size, grade of dysplasia and degree of villous structure are significant predictive factors for malignancy development within a polyp. (11,6) Highly dysplastic, villous adenomas are more likely to become malignant, (12) with carcinoma in situ found in 30% to 50% of this adenomatous subtype. (13) (See Fig. 3.)
[FIGURE 3 OMITTED]
The majority (95%) of... |
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