Материал: Advanced Imaging of the Abdomen - Jovitas Skucas

PERITONEUM, MESENTERY, AND EXTRAPERITONEAL SOFT TISSUES

have a good prognosis, thus the importance of differentiating this entity from metastatic ovarian carcinoma.

A primary mucinous cystadenocarcinoma, in a setting of normal ovaries, can develop in the extraperitoneum. A number of these tumors originate from ovarian tissue, although some are associated with normal ovaries. A possible origin is in heterotopic ovarian tissue, a teratoma, urogenital rest, intestinal duplication, or even some other metaplasia. One mucinous cystadenoma was in contact with the isthmus of a horseshoe kidney (59); the patient also had congenital absence of the left ovary.

Metastases/Carcinomatosis

Clinical

The most common primary site of peritoneal carcinomatosis is gynecologic; less often seen are gastric and colonic primaries, and rarely breast carcinoma. On rare occasions pancreatic papillary cystic neoplasms (discussed in Chapter 9) lead to peritoneal carcinomatosis, at times after abdominal trauma. Most frustrating, in some patients a primary site simply cannot be found (Fig. 14.16).

Wilms’ tumors spread to the peritoneal cavity; they develop mesenteric, greater omental, and pelvic metastases. Ascites is an inconsistent finding. These tumors range from broad infiltration to focal masses, findings detected by imaging.

Testicular cancers initially metastasize to the extraperitoneum. After successful chemotherapy residual enlarged nodes pose a dilemma: Do these represent necrotic, nonviable tumor or do they contain cancer? Complicating this issue is the rare metastatic testicular germ cell tumor evolving into a mature teratoma after being treated with chemotherapy and resection.

Metastasis to the umbilicus, in the United States often called Sister Mary Joseph nodes, is more common than a primary malignancy at this location. Most of these tumors have a gynecologic origin, with an occasional one being a renal cell carcinoma or some other sites. These metastases appear before, during, or after the primary tumor is detected. Associated peritoneal implants are common.

Tissue is generally needed for specific diagnosis, although even this may provide limited information. Image-guided 18-gauge needle biopsies of “omental cake” (which refers to extensive omental infiltration), peritoneal, or adnexal tumors in women with undiagnosed peritoneal carcinomatosis suggested a primary tumor site in only 77% (60); a poorly differentiated adenocarcinoma with an immunohistochemical profile suggesting ovarian cancer was found in another 20%. A peritoneal core biopsy, together with immunohistochemical analysis, establishes a site-specific diagnosis in most of these women.

Cytologic material is obtained from peritoneal washings as a follow-up in some women with ovarian cancer. A peritoneal reservoir can be implanted after tumor debulking and peritoneal washing cytology obtained at severalweek intervals; a recurrence is detected in some women by peritoneal cytology prior to other positive findings.

Figure 14.16. Primary peritoneal carcinomatosis in a man results in multiple sites of bowel obstruction.

Imaging

Imaging identifies peritoneal tumors, detects any cystic component, and at times suggests the site of origin. Imaging usually does not differentiate between benign and malignant, nor does it differentiate between primary and secondary

neoplasms. Also, the rare diffuse peritoneal leiomyomatosis is in the differential diagnosis. Although most imaging modalities detect widespread peritoneal carcinomatosis, early or small tumors are difficult to identify. Both CT and MR detect enlarged nodes.

Currently CT is the most often used imaging modality to study peritoneal carcinomatosis, although fat-saturated, spin echo contrastenhanced MR sequences appear superior, being aided by peritoneal contrast enhancement (61). Computed tomography findings in patients with peritoneal carcinomatosis most often consist of peritoneal implants followed by ascites, mesenteric implants, and omental implants (Fig. 14.17). Tumor appearance varies with the site of involvement and the imaging technique used. Peritoneal implants range from small nodules, diffuse infiltration, to an irregular, thickened “cake.” Ascites aids CT identification of small nodules. Mesenteric and omental implants likewise have a broad imaging spectrum, including omental cake. Contrastenhanced CT of carcinomatosis and some infections often shows enhancement of a thickened peritoneum and does not differentiate between these entities. Visible peritoneal calcifications suggest metastatic ovarian carcinoma; most other metastases do not calcify heavily. Prior peritonitis can, of course, result in calcifications and extensive sheet-like deposits generally are due to benign disease rather than a malignancy.

ADVANCED IMAGING OF THE ABDOMEN

Ascites provides an acoustic window for US. In a setting of carcinomatosis US evaluates the extent of ascites, tumor nodules, and adenopathy, and can also image omental and mesenteric involvement.

A fat-suppression MR technique aids visualizing subtle carcinomatosis. Postcontrast MRI identifies an enhancing, thickened peritoneum, achieving sensitivities and specificities of about 90% in detecting peritoneal tumor spread. Peritoneal implants become hyperintense on postcontrast delayed MR images. An MRI finding of enhancing ascites 15 to 20 minutes after IV contrast is not uncommon in peritoneal carcinomatosis. Magnetic resonance lymphography using ultrasmall SPIO particles is a potential aid in detecting nodal disease, but these studies are currently more research than clinical.

An induced pneumoperitoneum during CT visualizes most tumor implants in the peritoneum, with the exception of the pelvis and some peritoneal recesses. A pneumoperitoneum allowed detection of intraabdominal adhesions.

Computed tomography does not detect subtle peritoneal metastases; for some of these, gallium-67 scintigraphy or FDG-PET appear to be superior, but keep in mind that occasionally even disseminated peritoneal carcinomatosis is not detected by PET; only limited data are available on this point. Indium-111–satumomab pendetide planar and SPECT imaging are useful in detecting some carcinomatosis.

In some parts of the world both tuberculous peritonitis and peritoneal carcinomatosis are encountered. The CT findings in both overlap (Table 14.4). Computed tomography, endoscopy, and even biopsy can suggest Crohn’s disease, when in reality the patient has peritoneal carcinomatosis. Perforation of an ovarian cystic teratoma with intraperitoneal spill of cyst content also mimics carcinomatosis.

Although not common, hepatocellular carcinoma can result in intraperitoneal metastases, including omental seeding. Imaging reveals discrete single or multiple hypervascular nodules, with larger ones containing necrosis; adjacent vessel engorgement,including prominent draining veins, is identified in some.

Figure 14.17. Peritoneal metastases from ovarian carcinoma. Computed tomography outlines an irregular cake-like tumor (arrows). (Courtesy of Algidas Basevicius, M.D., Kaunas Medical University, Kaunas, Lithuania.)

Mesenchymal Tumors

A full range of mesenchymal neoplasms are encountered in the peritoneum and extraperi-

toneal thickening. A thickened mesentery and ascites are evident in some. Pleural plaques and nodules are uncommon.

Malignant

A peritoneal mesothelioma is a rare tumor, being considerably less common than its pleural counterpart. Most patients have had previous asbestos exposure, generally decades earlier, but those with peritoneal mesothelioma tend to be a decade younger than those with a pleural tumor. Nevertheless, pleural thickening and even a concomitant pleural mesothelioma are not uncommon associated findings. At times these tumors grow gradually for several years. The prognosis is grim.

Asbestos comes in two major forms, chrysotile and amphiboles, with most of the asbestos used commercially consisting mainly of chrysotile. The relative carcinogenesis varies between the various types, but why a particular type is more carcinogenic is not known. Asbestos is not directly mutagenic. Asbestos fiber size and other physical and chemical properties appear to be relevant factors in carcinogenesis, but contaminants such as tremolite (a form of amphibole) may also play a role. Chrysotile fibers appear to be cleared from human lungs rapidly,while amphibole clearance half-life is measured in years or decades. A lag period of several decades is necessary after asbestos exposure before a mesothelioma becomes evident. Heavy asbestos exposure increases the risk for abdominal mesothelioma, although considerable individual variability exists. The presumed pathway for abdominal involvement is expectoration of inhaled asbestos fibers, which are swallowed and some then penetrate the bowel wall into lymphatic and splanchnic circulations. Why mesothelial cells are primarily affected after asbestos exposure is speculation. Numerous but inconstant gene changes have been reported. An occasional tumor contains a sarcomatous component.

Not all patients with malignant mesothelioma have a history of asbestos exposure. Prior radiation therapy has been occasionally implicated. A remote history of Thorotrast exposure is found in some. The relationship of a rare peritoneal mesothelioma developing in a setting of recurrent bouts of peritonitis is conjecture.

ADVANCED IMAGING OF THE ABDOMEN

Spread tends to be intraabdominal and to the pleural cavity, probably by direct extension. Distal metastases are rare.A not uncommon end stage is extensive bowel involvement and small bowel obstructions.

Extraperitoneal malignant mesotheliomas are rare. They readily invade adjacent structures.

Chest radiographs and CT identify pleural plaques in about half the patients with malignant abdominal mesothelioma. A barium study typically shows extensive mesenteric and bowel wall infiltration, thickening and distortion of the valvulae conniventes, bowel wall thickening, sharp angulation of bowel loops, and a narrowed lumen. Some of these tumors have an almost pathognomonic radiographic appearance. Computed tomography findings range from a hypodense tumor with peripheral contrast enhancement to diffuse omental thickening, described as an omental cake; in many patients no discrete tumor masses are identified (Fig. 14.19). Thickening of the parietal peritoneum is more common than visceral peritoneal involvement. Extensive mesenteric thickening is found in some patients. Calcifications are rare. Ascites is common with asbestos-associated mesotheliomas, but it tends to be more limited than that seen with carcinomatosis. It tends to be rather viscous and difficult to remove. Fluid cytology appears worthwhile but tends to be nondiagnostic. Laparoscopic biopsy is useful to establish the diagnosis, although tract seeding by tumor is a potential complication.

The differential includes carcinomatosis and occasionally an infection such as tuberculosis. Even laparotomy has misdiagnosed a mesothelioma as carcinomatosis.

Postoperative chemotherapy is largely ineffective, although anecdotal reports suggest a response to arterial infusion chemotherapy.

Benign

A type of abdominal mesothelioma-like tumor occurs in a younger age group, has no known association with asbestos exposure, women predominate 2:1, and some of these tumors contain a cystic component. The term solitary benign fibrous tumor is used by some authors to differentiate this tumor from its malignant counterpart. Pathogenesis is unknown; some authors

B

C

These are aggressive tumors with a poor prognosis.

Fat-Containing Tumors

Some lipomas are discovered incidentally. An occasional mesenteric lipoma acted as a nidus for small bowel volvulus. These lipomas are readily detected with CT and MRI; they are well defined and of fat density, are hyperintense on T1and hypointense on T2-weighted images, and show little contrast enhancement. Color Doppler US and angiography confirms their avascular nature. Central necrosis is not seen. The rare tumor lipomatosis is readily differentiated from obesity (Fig. 14.21).

Most liposarcomas present either as a palpable tumor or as sequelae of compression and impingement on an adjacent structure.An occasional one bleeds. Some of these large, bulky tumors are associated with fever, presumably due to central tumor necrosis. Imaging shows most liposarcomas as large, heterogeneous, poorly marginated and invasive tumors. A cystic component is common. The amount of fat within a liposarcoma varies; more undifferentiated ones tend to contain little fat and consist

mostly of soft-tissue–density material, while well-differentiated ones mimic a lipoma. Their CT and MR findings are reflected accordingly. Poorly differentiated liposarcomas are mostly heterogeneous, and are mostly hypointense on T1and tend toward a hyperintense appear-

Figure 14.21. Lipomatosis in a woman with tuberous sclerosis. Computed tomography reveals numerous fat-density tumors and small bowel intussusception (arrow). (Courtesy of Algidas Basevicius, M.D., Kaunas Medical University, Kaunas, Lithuania.)