Almost all colon cancers are primary adenocarcinomas.
Tumors of the colon arise as intramucosal epithelial lesions, usually
in adenomatous polyps or glands. As cancers grow, they invade the
muscularis mucosa and lymphatic and vascular structures to involve
regional lymph nodes, adjacent structures, and distant sites, especially
The most common site for a bowel tumor in the adult is around the
caecum. It is useful to target this area in patients with altered
bowel habit in whom bowel carcinoma is suspected, although detection
with ultrasound is usually incidental
The normal bowel has a typical appearance on sonography. In general
it has a bull's-eye or target-like appearance with a central hyperechoic
component and an outer hypoechoic ring. There are three hyperechoic
layers and two hypoechoic layers: The inner hyperechoic layer arises
from the interface reflection between the lumen and the surface
of the mucosa. The second layer is hypoechoic and arises from the-combined
deep mucosa and the muscularis mucosa. The third layer is hyperechoic
and arises from the submucosa. The fourth layer is hypoechoic and
arises from the muscularis propria. The final outer layer is hyperechoic
and arises from the interface reflection between muscularis propria
and the serosa or adventitia (peri-intestinal fat). These five layers
are routinely se on endoscopic ultrasound and are intermittently
seen transcutaneous scans.
On US, Colonic neoplasms give rise to hypoechoic or heterogeneous
bowel wall thickening, the target or atypical target sign. A colonic
tumor appears typically as an echo-poor or mixed mass with a hyperechoic
and eccentric gas-filled lumen. This cannot be differentiated, however,
from an inflammatory mass on ultrasound. Vigorous Doppler flow can
usually be visualized in both inflammatory and malignant masses.
Other findings include localized irregular colonic wall thickening,
an irregular contour, lack of normal peristalsis, and an absence
of the normal layered appearance of the colonic wall.
With high resolution scans disruption of the layered bowel pattern
may be seen. Malignant lesions are more common in patients with
greater wall thickness, asymmetric involvement, loss of stratification,
absence of perigut findings and involvement of short segments.
When a colonic lesion is seen, a search for possible lymph node
and live metastases should be made. Bowel obstruction is not an
unusual mode of colonic carcinoma presentation. With a fluid-filled
proximal bowel the obstructive bowel lesion may be directly visualized.
The finding of a colonic mass would normally prompt a barium enema,
to delineate the nature, extent and position of the mass, with subsequent
staging by CT if malignancy is confirmed. The advantage of ultrasound
over barium enema is that of displaying the tumor itself, rather
than just the narrowed lumen. The role of ultrasound in patients
with known bowel carcinoma is to identify and document the presence
of distant metastases, particularly in the liver, as metastases
from colorectal carcinoma are particularly amenable to curative
Hepatic metastases from a colonic primary tumor are usually hyperechoic
(increased echogenicity in relation to normal liver; but also may
be hypoechoic (decreased echogenicity).
Large bowel lymphoma represents 1.5 % of all abdominal tumors. It
is usually indistinguishable from a carcinoma causing an atypical
or typical target sign. However the bowel thickness tends to be
more hypoechoic in lymphoma than bowel carcinoma.
*Inflammatory bowel disease.
*Diverticular mass or abscess.