Материал: Advanced Imaging of the Abdomen - Jovitas Skucas

Resection of liver metastases is precluded in the face of extrahepatic metastases (liver metastases are discussed in more detail in Chapter 7).

Both 18F-FDG-PET and radioimmunoscintigraphy detect earlier local recurrence than is possible with CT or MRI and aid in identifying tumor involving normal size lymph nodes. Their current use, however, is still rather limited and their role in the latter clinical setting is not yet adequately established.

Theoretically, colon cancer detection is improved by combining a monoclonal antibody, which has selective tumor affinity, with PET scanning, which has increased sensitivity and resolution over conventional imaging. To test this hypothesis, an anticolorectal cancer monoclonal antibody (MaB 1A3) was labeled with copper 64, which is a positron emitting radionucleotide (178); such monoclonal antibody-PET scanning achieved a sensitivity of 71% in detecting confirmed tumor sites.

In patients with suspected recurrent colorectal or ovarian carcinoma and normal or equivocal CT or MR studies, indium 111 satumomab pendetide (OncoScint) imaging and FDG-PET imaging are similar in their tumor detection abilities; the radioimmunoconjugate OncoScint is better at detecting carcinomatosis, but PET better detects liver metastases. Indium 111 satumomab liver imaging is suboptimal due to high background levels; nevertheless, it does provide relevant information about extent and location of recurrent colorectal cancer throughout most of the abdomen. It appears especially useful in patients with normal other imaging studies but a rising or high CEA level. An occasional indium 111 satumomab study is false positive, with activity detected in another tumor, such as a nonfunctioning adrenal adenoma.

Single photon emission computed tomography (SPECT) immunoscintigraphy using Tc- 99m–anti-CEA monoclonal antibodies shows promise in follow-up after surgery. There are, however, problems in interpreting images because of variations in antibody distribution. It detects local or abdominal recurrence and appears to be more accurate than CT in the abdomen.

Scant literature exists on the usefulness of PET imaging in detecting recurrent colorectal cancer, although PET is more sensitive in detecting early recurrence than CT or MRI. In fact,

ADVANCED IMAGING OF THE ABDOMEN

PET appears to be more sensitive than CEA in detecting tumor recurrence. In previously treated colorectal cancer patients with suspected recurrence, FDG-PET detects >90% of liver and extrahepatic metastases, considerably more than CT. In fact, currently FDG-PET is the most accurate noninvasive modality for staging patients with recurrent metastatic colorectal cancer.

Anaplastic Carcinoma

Colorectal small cell anaplastic carcinomas are rare. Some contain exocrine differentiation. These tumors tend to be aggressive and metastasize early, both to lymph nodes and hematogenously. Staging should include CT of the chest and abdomen and bone scintigraphy.

Adenosquamous/Squamous Cell Carcinoma

These are rare but aggressive colorectal tumors having a predilection for the rectum; some are associated with ulcerative colitis or with other carcinomas.

About 70% of anal carcinomas are squamous and 30% are cloacogenic (179). Imaging has no role in detecting these cancers but aids in staging. They spread mostly locally to the perirectal, inguinal, and iliac nodes; distal spread is to the lungs and liver.

Traditional therapy of anal carcinoma was an abdominoperineal resection, which is rarely performed now, having been supplanted by radiation and chemotherapy.

Lymphoma

Lymphomas range from diffuse gastrointestinal tract involvement to, less often, being limited to the colon. Cecum and rectum are the most common large bowel sites. Primary colon lymphomas tend to present as large intramural infiltrating tumors. Over half of primary colonic non-Hodgkin lymphomas are diffuse large-cell lymphomas.

Clinically, some colonic lymphomas present with signs and symptoms similar to those of an adenocarcinomas; others mimic inflammatory bowel disease. Less common presentations are acute abdomen, intussusception, or simply with an abdominal tumor. A possibility of lymphoma being misdiagnosed as inflammatory

COLON AND RECTUM

bowel disease from superficial mucosal biopsies has already been mentioned (see Ulcerative Colitis). Some of these patients undergo steroids therapy until a correct diagnosis of lymphoma is made. In fact, lymphoma should be considered in the differential diagnosis of a dense lymphocytic infiltrate obtained from a segment of bowel simulating either ulcerative colitis or Crohn’s disease.

Although not common, patients with leukemia have developed colon lymphoma.

Overall, colonic lymphomas appear similar to those seen in the small bowel. A common appearance is that of an intramural infiltrating, sharply marginated tumor. Involved colon tends to be thickened and distorted, an aid in differentiating lymphomas from adenocarcinomas. A lymphomatous large, ulcerated mass is less common. A rare appearance is aneurysmal dilation of the affected colonic segment.

Unlike cecal adenocarcinomas, extension across the ileocecal valve is common with lymphomas, and the site of origin is often difficult to determine; some authors use the term ileocecal lymphoma to describe these tumors. Their differential diagnosis includes a mesenchymal tumor and localized Crohn’s disease.

Double-contrast barium enema findings in patients with peripheral T-cell lymphoma range from diffuse colonic involvement to focal, and from aphthae, gross ulcers, polyps, and circumferential narrowing to simply ileocecal deformity (180). An occasional lymphoma manifests as multiple polyps in the proximal gastrointestinal tract and numerous aphthae in the colon.

An occasional colonic lymphoma presents as diffuse polyposis. These polyps tend to vary in size, and the barium enema appearance mimics familial polyposis, although the two entities can usually be differentiated on clinical grounds. Most often such lymphomatous polyposis represents B-cell lymphoma.

Sarcoma

Leiomyosarcomas are the most common primary colorectal sarcomas. These sarcomas tend to be larger than carcinomas at first presentation. After resection the 5-year survival depends on the tumor grade.

Endoscopic US identifies rectal leiomyosarcomas as hypoechoic tumors.

Liposarcoma

Magnetic resonance imaging of liposarcomas reveal several patterns. Well-differentiated liposarcomas have MRI characteristics similar to those of a lipoma, consisting of a wellmarginated tumor hyperintense on T1weighted images, hypointense on T2-weighted images,and showing little if any contrast enhancement; less well-differentiated liposarcomas tend toward a heterogeneous appearance, with many containing varying amounts of necrosis. In general, tumor necrosis varies inversely with the degree of tumor differentiation.

Angiosarcoma

Colonic angiosarcomas are rare. A cecal angiosarcoma occasionally intussuscepts.

Histiocytoma

Primary colonic malignant fibrous histiocytomas are rare. Initially these sarcomas are confined to the colon wall, but with growth ulcerate through the mucosa and bleed, or they invade the adjacent soft tissues. Peritoneal implants and lymph node metastases are evident with some. Also, an adjacent extraperitoneal malignant fibrous histiocytoma readily invades the colon and appears as an infiltrating serosal tumor.

Imaging reveals a solid, generally large tumor suggesting a sarcoma or lymphoma.

Pathologic identification of a histiocytoma is not always straightforward. At times an inflammatory fibrosarcoma and a leiomyosarcoma are in the differential.

Carcinosarcoma

Only a few colonic carcinosarcomas have been reported. A question is occasionally raised about whether these represent two separate collision neoplasms, although most carcinosarcomas appear to represent differentiation of a single progeny into two cell types. The presence of retroviral particles in the sarcomatous cells of some of these tumors supports the theory of tumor differentiation from a carcinomatous into a sarcomatous component.

An elevated, fungated, ulcerated tumor is a typical appearance. Most carcinosarcomas carry a poor prognosis.

Melanoma

Most of the rare primary melanomas are found in the rectum, and these patients present with rectal bleeding. They tend not to obstruct.

Primary rectal malignant melanomas tend to be polypoid or fungating, but often already extending to the pelvic side wall at initial presentation (181). A cancer is often suspected. Adenopathy is common. At times a biopsy contains few melanocytes but considerable inflammation, and only after tumor excision is a correct diagnosis made.

A dual rectal melanoma and adenocarcinoma are occasionally reported; presumably such dual collision neoplasms develop by chance.

Both primary and metastatic melanomas have a high tumor-to-background PET activity and FDG-PET scanning is useful to detect unsuspected metastases; PET is also commonly employed for follow-up after therapy.

Metastasis or Direct Invasion to Colon

Metastasis to the colon is not common; more common is direct invasion from an adjacent structure. Thus gynecologic malignancies invade the rectum, or a hepatocellular carcinoma invading the adjacent splenic flexure results in massive bleeding, even to the point of exsanguination.

In a comparison of CT and MRI in predicting bladder or rectal invasion in women with uterine carcinoma, MRI was slightly, but not statistically, superior to CT (182); both provided similar results as rectoscopy.

A long segment of circumferential rectal wall thickening, having a rectal linitis plastica appearance, is most often due to metastatic gastric cancer, but it can be found with other causes of peritoneal carcinomatosis and rectal metastasis (183). Obstructions and fistulas are rare manifestations of lung and breast metastases. A renal cell carcinoma is one cause of a hypervascular metastasis.

A recurrent bladder or prostatic carcinoma invading the rectum can simulate a rectal leiomyoma or result in an annular constricting lesion.

A malignant colon obstruction, either primary colonic or extrinsic, most often due to spread of a gynecologic tumor or peritoneal seeding, is a difficult management problem.

ADVANCED IMAGING OF THE ABDOMEN

Most of these tumors are unresectable, but palliation of bowel obstruction is desirable. Palliation consists, at best, of a proximal colostomy. An occasional option with a single major obstruction is metallic stent placement for decompression if access under fluoroscopic guidance is feasible. Most such treatable obstructions are in the rectum and rectosigmoid, but occasionally a stent can be inserted through a more proximal obstruction (184). Many of these patients, however, have widespread metastases, including to the small bowel, and no viable bypass is feasible.

Rhabdoid Tumor

The rare colonic malignant rhabdoid tumor is diagnosed by a pathologist detecting rhabdoid cells. This tumor is more common in the kidneys. No specific imaging features have been described.

Neuroendocrine Tumors

Colonic neuroendocrine tumors are uncommon, and some are difficult to place in proper perspective.An occasional colorectal poorly differentiated neuroendocrine carcinoma presents with widespread liver metastasis.

These tumors range from benign to malignant. Most are solid, intramural tumors, with an occasional mesenteric one appearing as an extraserosal tumor.

Carcinoid

Rectal carcinoids are more common than colonic ones; a cecal location is most common in the colon. Synchronous carcinoids occur occasionally. Similar-appearing rectal carcinoids have developed in siblings. The malignant potential of rectal carcinoids varies considerably.

Small rectal carcinoids are palpable on digital examination as firm nodules. Endoscopy visualizes small polyps covered by normal-appearing mucosa, with either erythema or a central depression occasionally being found.

Carcinoid syndrome develops mostly in a setting of metastases, with only an occasional rectal carcinoid directly responsible for this syndrome.

Table 5.5 summarizes the clinical findings from two studies. Between 1964 and 1988, the Alberta Cancer Registry compiled 36 malignant colon carcinoids (excluding ileocecal region and rectum) (186); average age at diagnosis was 68 years, and at presentation 22% of patients were already Dukes C and 86% had invaded pericolic fat. Many malignant carcinoids have already metastasized at initial presentation, with the most common metastatic sites being the liver and lung. In fact, the presence of metastases is often the unequivocal finding establishing malignancy of these tumors. Survival with colonic carcinoids is lower than with rectal or appendiceal carcinoids (or even with colon adenocarcinomas).

Carcinoids range in appearance from a simple polyp to an apple-core infiltrating tumor mimicking an adenocarcinoma. An unusual barium enema finding in the presence of a carcinoid is colon jejunization. Such jejunization presumably is secondary to colonic wall foreshortening induced by the desmoplastic reaction commonly associated with these tumors. A carcinoid located in the posterior rectal wall or adjacent tissues widens the presacral space.

Transrectal US in patients with rectal carcinoids reveals increased echogenicity and a heterogeneous internal echo pattern in some. Small rectal carcinoids tend to be hypoechoic. Pathologically, these findings are associated with increased fibrotic interstitium around

nodular tumor nests. Transrectal US can also often reveal depth of invasion and suggest lymph node metastasis.

I-123-metaiodobenzylguanidine (MIBG) scintigraphy evaluates metastatic carcinoids.

Some small carcinoids have been resected endoscopically, although most require surgical excision, similar to adenocarcinomas.

Other Tumors

Schwannomas, or primary nerve sheath tumors, originate more often from peripheral nerves and are rare in the colon. Most are benign. Their imaging appearance is similar to other stromal tumors. A cystic component is occasionally detected.

A rare colonic ganglioneuroma presents as filiform polyposis.

Neurofibromatosis type 1 (von Recklinghausen’s disease) is discussed in Chapter 14. Gastrointestinal neurofibromatosis is uncommon and is a late manifestation of von Recklinghausen’s disease. Only rarely is colonic neurofibromatosis an initial presentation.

Gastrointestinal neurofibromas range from solitary, to multiple, to plexiform in appearance. An occasional colonic plexiform neurofibroma and neuronal hyperplasia result in disordered mobility, a megacolon, and proximal bowel dilation, similar to other causes of adynamic ileus. Some manifest through gastrointestinal bleeding.