Материал: Advanced Imaging of the Abdomen - Jovitas Skucas

COLON AND RECTUM

Figure 5.17. Transverse colon hemangioma (arrows). This pliable tumor mimicked a spastic colonic segment but it would never distend. No obstruction was present, a typical finding even with a large hemangioma.

Lymphoid Nodules

Lymphonodular hyperplasia is quite common in children and is occasionally encountered in adults. The condition occurs both in the small bowel and colon. An association between lymphonodular hyperplasia and several disorders has been described, although currently most investigators believe that in some patients, especially children, the presence of lymphonodular hyperplasia is a normal variant. It is found adjacent to a colonic segment involved by ulcerative colitis, and the presence of these nodules was proposed as a sign of early ulcerative colitis (93). Such lymphoid hyperplasia also develops in posttransplant patients who are under chronic immunosuppression.

These small polyps, up to several millimeters in diameter, are readily detected by both a double-contrast barium enema and colonoscopy. Rarely, enough lymphoid tissue accumulates at one site to result in a larger lymphoid polyp.

Lymphangioma

Cystic lymphangiomas are more common in the mesentery but do occur in the bowel wall. Ultrasonography reveals these lymphangioma to be compressible, anechoic, and possessing posterior acoustic enhancement; as expected, Doppler US identifies no flow. Computed tomography attenuation is near that of water. Endoscopic US reveals septa in the multilocular ones. A double-contrast barium enema identifies a lymphangioma as a submucosal tumor, soft, round, and smooth.

Endometriosis

Colonic endometriosis is discussed here in the tumor section because its appearance often mimics a neoplasm.

Endometriosis signifies the presence of functioning endometrial tissue in ectopic sites. The most common site of intestinal involvement is the anterior rectal region, followed by the appendix and ileocecal region. Endometriosis involving the rectovaginal septum posteriorly and then extending laterally into the uterosacral ligaments and anterior rectal wall results in fibrosis and scarring, eventually leading to a colorectal stricture and obstructive symptoms. Perforation and an acute abdomen are rare complications.

This tissue undergoes change under hormonal stimulation. In symptomatic women pain and altered bowel habits are common presentations. Clinically, endometriosis often mimics inflammatory bowel disease.

Colonoscopy reveals intact mucosa at the site of involvement; mucosal ulcerations occur in a minority and are a cause of rectal bleeding. At times only laparoscopy establishes the diagnosis. With typical anterior rectal wall involvement, barium enema findings are similar to those seen with rectal invasion by a gynecologic tumor (Fig. 5.18) or metastasis to the pouch of Douglas.

Endorectal US achieved a sensitivity and specificity of over 95% in detecting rectovaginal septal infiltration by endometriosis (94). Although these results are impressive, the role of US in providing a differential diagnosis and aiding management is not clear.

Magnetic resonance imaging reveals endometriosis >1cm in size to be homoge-

neously hyperintense on T1-weighted images and hypointense on T2-weighted images; smaller lesions tend toward a variable T2weighted signal (95). Some foci contain a cystic component (Fig. 5.19).

Surgery for rectosigmoid endometriosis is technically difficult. At times posterior vaginal

wall excision and partial rectal resection are necessary.

Xanthelasma

Xanthelasma is diagnosed histologically as a collection of lipid-laden histiocytes. It develops

COLON AND RECTUM

in the colonic wall and other structures. Imaging plays no role in this condition.

Benign Neoplasms

Adenoma

The primary importance of colon adenomas is in their potential for carcinomatous transformation. Less often they manifest by bleeding or other complications. More adenomas are detected in the left colon than the right, although over the last several decades a shift toward right-sided polyps is evident, especially in African Americans; in this patient population the malignancy potential for right-sided polyps is similar to that found on the left.

Most small adenomas are tubular or tubulovillous, and a small minority are villous. A larger villous component within an adenoma is associated with a higher risk for high-grade dysplasia and cancer. Villous adenomas are generally excised because even multiple biopsies may miss a focus of carcinoma. Some villous adenomas, however, are treated with endoscopic neodymium (Nd):YAG laser therapy, and one must ensure adequate therapy; incompletely treated ones recur. Complications of laser therapy include stricture, self-limited bleeding, and fistula.

Some villous adenomas regress after nonsteroidal antiinflammatory drug therapy.

The role of Helicobacter pylori infection in colon adenoma and carcinoma formation is not clear. Prevalence of H. pylori antibodies in patients with colorectal adenomas and carcinomas is greater than in controls.

Adenomas range from pedunculated to sessile. Some have a long stalk, in others a stalk is barely visible. Sessile adenomas range from those with a discrete intraluminal component to relatively rare flat ones, with the latter consisting of an intramural tumor with a nodular or reticular surface and thus being especially difficult to detect either by imaging or endoscopy. Villous adenomas are classically described as frond-like and containing numerous interstices, but such an appearance in found only in a minority. Imaging cannot differentiate most villous adenomas from their tubular counterpart.

Computed tomography suggested several anecdotal villous adenomas by identifying a

convoluted surface or residual oral contrast in polyp interstices (96).

Detection of one adenoma implies an increased future risk for another adenoma. Studies provide somewhat conflicting results, but, in general, among patients followed for at least 10 years, those with a large initial adenoma have a significantly higher risk of developing metachronous adenomas.

Leiomyoma

Colorectal leiomyomas are uncommon. Most are asymptomatic, although the larger ones tend to be associated with pain, change in bowel habits, or rectal bleeding. Some are detected incidentally during a rectal examination. Leiomyosarcomas tend to be larger and more inhomogeneous than leiomyomas.

Endorectal US reveals rectal leiomyomas as hypoechoic tumors, similar to other stromal tumors. It is helpful in evaluating tumor extent, but not in differentiating benign from malignant.

Lipoma

Lipomas are intramural in origin and most are single, although diffuse colonic lipomatosis does occur. They vary in size considerably. More often than other stromal tumors, lipomas tend to become pedunculated and act as a lead point for an intussusception. Some lipomas undergo spontaneous autoamputation.

Uncomplicated lipomas have a CT fat density. Intussuscepting ones tend to contain more of a soft tissue density, probably because of associated infarction and necrosis. The lack of fat detected by CT in the lead point of an intussusception thus does not exclude a lipoma.

Endoscopic US identifies colon lipomas as hyperechoic tumors.

Magnetic resonance imaging reveals lipomas as typical fat-density tumors—hyperintense on T1and hypointense on fat-suppressed T1weighted images.

Angiomyolipoma

Extrarenal angiomyolipomas are rare. If large enough, an ileocecal valve angiomyolipoma or one in the left colon can obstruct.

Polyposis Syndromes

Not all patients with familial polyposis can be placed in a discrete syndrome. Thus one large family had a tendency to develop mixed colonic adenomatous and hyperplastic polyps, with the characteristic lesion being an atypical juvenile polyp, although some polyps contained mixed histology (97); some of these polyps became malignant. Inheritance in this family appeared to be autosomal dominant.

Familial Adenomatous Polyposis

Familial adenomatous polyposis is a genetic disorder with an autosomal-dominant inheritance and penetrance approaching 100%. Typically these patients develop hundreds to thousands of adenomatous polyps throughout the colon and often also in the rest of the gastrointestinal tract. Gardner’s syndrome and familial polyposis coli have the same genetic defect but exhibit different phenotypic expression. Many investigators describe both entities with the term familial adenomatous polyposis.

The Swedish Polyposis Registry noted an incidence rate during 1977 to 1996 of 0.86 per million and a prevalence 32 per million at the end of 1996 (98); the median age at polyposis diagnosis of probands was 39 years, while in the call-up group it was 22 years. Importance of earlier detection is evident: 67% of probands and 3% of call-up patients already had colorectal cancer at initial diagnosis, with corresponding mortality rated being 44% and 2%, respectively.

The discovery of a defective gene changed the diagnosis of this condition and now allows direct carrier identification by mutation screening in some but not all patients. The mutation abnormality is complex, and numerous mutations in the adenomatous polyposis coli gene have been identified. The defective gene is a tumor suppressor gene having a role in carcinogenesis. Although gene mutations are often evident in sporadic colorectal adenomas and carcinomas, familial adenomatous polyposis patients already have mutations in this gene.

Polyp count and age are directly related to the risk of cancer. Patients with >1000 polyps had a 2.3 times greater risk of having a cancer than

ADVANCED IMAGING OF THE ABDOMEN

those with <1000 polyps (99); each 10-year age group led to a 2.4-fold difference in cancer risk. Nevertheless, occasional cancers are detected in patients younger than 30 years of age and those having less than 1000 polyps.

A predisposition to adenomatous polyps in familial adenomatous polyposis is best known, but both benign and malignant tumors also develop at other sites. More common extracolonic manifestations include desmoid tumors, often extraperitoneal in location, and gastroduodenal neoplasms. These patients tend to have gastric fundic gland polyposis. Skin, subcutaneous tissue, and bone tumors develop. Some studies suggests an increase in adrenal tumors. Papillary thyroid carcinoma are detected in about 1% to 2% of these patients.

Colon polyps develop and start growing roughly at the time of puberty, and in patients at known risk surveillance is started shortly thereafter. A double-contrast barium enema readily detects familial polyposis, although most gastroenterologists perform endoscopy.

The nonsteroidal antiinflammatory drug sulindac is currently used to treat patients with familial polyposis. Long-term use leads to a marked reduction in the number and size of polyps and, at times, even in their disappearance. Occasionally, however, polyps recur and a malignancy develops.

Because the predominant risk in many of these patients is colon cancer, colon resection is recommended. Procedures performed include the following:

1.Proctocolectomy with ileostomy

2.Total colectomy with ileorectal anastomosis and postoperative rectal surveillance

3.Colectomy, rectal mucosectomy, and ileorectal pull-through (ileal pouch)

Unsatisfactory results with proctocolectomy and ileostomy led surgeons to a total colectomy with an ileorectal anastomosis, but an intact rectal mucosa leaves these patients at risk for developing a future rectal carcinoma. In a Czech study of 55 patients undergoing colectomy and ileorectal anastomosis for familial polyposis, 16% developed a malignant tumor within 16 years (100). Many of these patients have either nodal or distant metastases at the time of rectal cancer detection, and a number of studies

COLON AND RECTUM

have concluded that a proctocolectomy and ileoanal anastomosis should be the primary operation, instead of colectomy and ileorectal anastomosis.

The Soave procedure—a colectomy, rectal mucosectomy, and ileorectal pull-through—is currently preferred by many surgeons. One unanswered question concerns therapy when small bowel polyps are present. Also, these patients are at increased risk of developing ileal pouch polyps. Among 85 ileal pouches examined, 35% contained adenomas (101); the risk of developing an adenoma at 5, 10, and 15 years was estimated to be 7%, 35%, and 75%, respectively. Those with a pouch adenoma were more likely to also have duodenal adenomas.

Complications of an ileorectal Soave pullthrough include anastomotic stricture and leak. Some of these strictures are dilated using either endoscopic or imaging guidance. Most focal perforations heal with time. Although a new ileorectal pull-through is feasible after breakdown of an initial anastomosis, a number of these patients are left with a permanent ileostomy.

As mentioned previously, these patients are at increased risk for mesenteric fibromatosis, or desmoid tumors. Some desmoids form spontaneously, although their prevalence increases after surgery. Desmoids are considered benign, but they are locally aggressive tumors associated with considerably morbidity and mortality. Desmoids are most common in the mesentery, followed by abdominal wall. Abdominal wall desmoids can be excised but recur in about half of patients; mesenteric desmoids are more difficult to excise. A not uncommon history is for a patient to undergo total colectomy, a desmoid tumor develops in the abdominal wall or close to some other surgical incision, a second surgical procedure is performed to resect the desmoid, a more extensive desmoid recurs, etc. An occasional desmoid becomes infected and forms an abscess.

At times skeletal scintigraphy with 99mTcHDP shows diffuse uptake in regions of fibromatosis.

Medical therapy is feasible for some desmoids. Some regress with tamoxifen therapy.

Data in the Familial Gastrointestinal Cancer Registry in Toronto show that most deaths of

patients with familial adenomatous polyposis are still from colorectal cancer (102); the proportion of deaths caused by extracolonic disease is rising, however, probably due to better screening and earlier prophylactic colectomies. Desmoid tumors are especially difficult to manage. Death due to desmoid tumor complications is not uncommon.

Turcot’s Syndrome

Turcot’s syndrome is a rare hereditary disorder consisting of central nervous system malignant gliomas and colon polyposis. It has manifested in the pediatric age group.

In a 30-year-old Japanese man with histopathologically confirmed glioblastoma multiforme and colonic well-differentiated adenocarcinoma, somatic mutations of K-ras and APC genes were identified in the colon carcinoma but not in the brain tumor (103); alterations in DNA repair genes presumably play a role in these tumors.

Multiple Hamartoma Syndrome

(Cowden’s Disease)

Familial multiple hamartoma syndrome, also known as Cowden’s disease, is an autosomaldominant condition having high penetrance but variable expressivity. The gene involved, believed to be a tumor suppressor gene, has been mapped to the long arm of chromosome 10 at 10q22–23. Mutations in this gene were identified in 81% of 37 Cowden’s disease families (104); these include missense and nonsense point mutations, deletions, and insertions over the entire length of this gene.

Numerous hamartomatous polyps are the usual gastrointestinal tract manifestation of Cowden’s disease. One patient presented with numerous round gastric and duodenal polyps and elongated (filiform) small bowel polyps (105). Extraintestinal manifestations vary considerably and range from macrocephaly to various genitourinary defects and other deformities.

Some affected patients develop gastric or colonic carcinomas, although the premalignant status of these polyps is not established. These patients also form various other cancers; in the

abdomen they are at increased risk for renal cell carcinoma, neuroendocrine neoplasms, germ cell tumors, and endometrial neoplasms.

Peutz-Jeghers Syndrome

Peutz-Jeghers syndrome manifests primarily as multiple hamartomatous polyps in the stomach, duodenum, small bowel, and colon. Patients with this syndrome often develop malabsorption and profound anemia or present with recurrent bouts of small bowel obstruction. Intussusceptions and even prolapse of a colonic polyp through the anus have been reported. Skin pigmentation is common.

A hamartoma-to-adenoma transition sequence probably exists, with an increased risk of carcinomatous development, although the cancer risk in these patients is considerably less than with familial polyposis.

A 16-year-old boy with Peutz-Jeghers syndrome diagnosed at age 8 years developed a rectal carcinoid (106).

Cronkhite-Canada Syndrome

Cronkhite-Canada syndrome consists of hamartomatous polyposis in the stomach, small bowel, and colon in association with alopecia and onychodystrophy. It is nonfamilial.

Prior to polyp detection these patients develop changes in taste sensation, dystrophic nails, and alopecia areata, findings suggesting the syndrome during the initial patient contact. Malabsorption is common and leads to hypoproteinemia.

Numerous polyps containing foci of inflammation and cystic regions develop in the stomach and small bowel. Not all of these polyps are hamartomatous; these patients also develop adenomatous polyps and thus are at risk for malignancy.

Juvenile Polyposis Syndromes

The presence of multiple juvenile polyps suggests one of the familial juvenile polyposis syndromes. Three types have been described: polyps limited to the colon, polyps throughout the gastrointestinal tract, and gastric juvenile polyposis. Some patients appear to be at increased risk of developing colon cancer.

ADVANCED IMAGING OF THE ABDOMEN

The coexistence of familial juvenile polyposis and hereditary hemorrhagic telangiectasia has been described both sporadically and in families. An association of colorectal juvenile polyposis and hereditary spherocytosis has also been raised.

A severe form of juvenile polyposis is seen in the first several years of life. Some of these infants have bloody diarrhea and malabsorption to the point that life cannot be sustained.

Malignant Neoplasms

Adenocarcinoma

Clinical

In North America and Western Europe, colorectal carcinoma ranks second in incidence after lung cancer in men and breast carcinoma in women. It is considerably less common in developing countries. The incidence is increasing in Eastern Europe and Japan. Published cancer data vary not only among countries but also between university and nonuniversity centers.A number of studies show a correlation between meat consumption and colorectal cancer. Still, this topic remains controversial. A French review of available studies concluded, “One cannot state, nor exclude, that meat promotes colorectal cancer” (107).

Regular ingestion of aspirin or other nonsteroidal antiinflammatory drugs decreases the risk of colorectal cancer, although the length of time necessary before a reduction in risk occurs is debated.

Although not common, colorectal cancer does occur in young adults, with case reports describing even patients under 20 years of age. Some studies report clinical presentation and tumor grade to be similar to that in the older population, others report a lower morbidity and mortality; still others suggest dismal results because of advanced stage at diagnosis and an aggressive tumor.

An association between Streptococcus bovis bacteremia and the presence of a colon carcinoma is well known. The reason for this bacteremia is puzzling, because bowel S. bovis colonization rates in those with colorectal cancer and controls appear similar (108). A similar association exists with S. sanguis and colon cancer,and presumably other streptococci species also lead to bacteremia in a setting