Книги по МРТ КТ на английском языке / Advanced Imaging of the Abdomen - Jovitas Skucas

Inflammatory Pseudotumor (Fibrosarcoma)

An unusual entity is the so-called inflammatory pseudotumor or fibrosarcoma, a condition believed to be nonneoplastic by some.A number of terms are used in the literature to describe this condition (Table 14.3) and they reflect divergent views held among pathologists about its pathogenesis. Whether the tumors described represent one entity or are a collection of several conditions, are histogenetically related or not, inflammatory or neoplastic, benign or malignant, is conjecture. These tumors were first described in the lungs but they occur throughout the body, including bowel, mesentery, liver, and extraperitoneum. Histologically they consist of fibroblasts, myofibroblasts, variable amounts of fibrosis, and occasional calcifications, and include an inflammatory infiltrate consisting of lymphocytes, plasma cells, and eosinophils. Myofibroblasts appear to play a central role, hence the inclusion of this term in some of the nomenclature. These tumors differ from granular cell myoblastomas, which are considered to be of neural origin. They also differ from xanthogranulomatosis (discussed earlier; see Diffuse Infiltration).

Pathologically, these tumors exhibit considerable heterogeneity, and a diagnosis from a needle biopsy specimen is difficult; immunohistologic studies are often not helpful. They tend to have benign-appearing cytology and often have relative hypocellularity. Clonal chromosomal aberrations have been detected in some, suggesting a malignant tendency. Polymerase chain reaction studies for Epstein-Barr virus and cytomegalovirus suggest that these viruses do not play a role in this entity.

Table 14.3. Alternate terms used for inflammatory pseudotumor (these do not necessarily represent a single pathologic entity)

Fibrosarcoma

Plasma cell granuloma

Inflammatory myofibroblastic tumor

Inflammatory myofibrohistiocytic proliferation

Xanthoma

Fibroxanthoma

Histiocytoma

Plasmacytoma

Solitary mast cell tumor

ADVANCED IMAGING OF THE ABDOMEN

Most of these tumors occur in children, but they range from neonates to elderly. Clinically, patients present with pain, anemia, a palpable tumor, or bowel obstruction. Many initially manifest an indolent clinical course and an aberrant response to tissue injury is postulated as a likely pathogenesis. Others, however, act in a malignant fashion. Recurrence is common unless widely excised. Metastases have been reported.

Primary Carcinoma

An unusual entity called normal-sized ovary carcinoma syndrome, primary papillary serous peritoneal carcinoma and other similar names, consists of diffuse malignant abdominal cavity involvement, normal size ovaries, and no obvious source for a primary malignancy found by either preoperative or operative evaluation. In general, distinguishing primary peritoneal tumors such as malignant mesotheliomas and serous surface papillary adenocarcinomas from metastatic peritoneal tumors is a challenge and involves the use of a hyaluronidase digestion test, electron microscopy, and immunohistochemical antibody studies. Comparing women with primary peritoneal carcinoma with those with primary epithelial ovarian cancer reveals very similar findings.

Ascites is common. Peritoneal involvement is either nodular or diffuse (omental cake). The omentum is affected in most, but adenopathy develops only in a minority. An occasional peritoneal primary papillary serous carcinoma contains calcifications.

These rare primary peritoneal adenocarcinomas are a diagnosis of exclusion. In most patients an adenocarcinoma of unknown primary poses a diagnostic and therapeutic dilemma. In general, the yield from multiple diagnostic procedures aimed at detecting the primary site is low and, even if detected, the therapeutic options are limited. One exception to such a nihilistic approach is in those women presenting with malignant ascites and no pelvic tumor, but who are found to have a peritoneal serous papillary adenocarcinoma. Some of these tumors consist of numerous small nodules throughout the peritoneal cavity, at times beyond the resolution of imaging. A number of these tumors respond to chemotherapy and

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.)

PERITONEUM, MESENTERY, AND EXTRAPERITONEAL SOFT TISSUES

ance on T2-weighted images. The amount of necrosis increases inversely with the degree of differentiation.

Teratomas often contain fat, but other tissue components are also present, including calcifications. Some teratomas exhibit a fluid–fluid level within their cystic component.

The most common site for extraperitoneal angiomyolipomas is in the kidneys (discussed in Chapter 10). Computed tomography and MRI of a rare extraperitoneal extrarenal angiomyolipoma reveal a fatty tumor. Renal involvement is often present, and differentiation of a renal primary with extrarenal extension is often in the differential diagnosis. These tumors tend to be multiple in patients with tuberous sclerosis and involve numerous organs. Some contain sufficient cell atypia to suggest a slow growing low grade malignancy.

An occasional angiomyolipoma contains fat necrosis and mimics a liposarcoma both with imaging and histology.

Most extraadrenal myelolipomas occur in the extraperitoneum. Their imaging findings are similar to those of adrenal myelolipomas.

Hemangiopericytoma

A group of presumed mesenchymal tumors shows a perivascular myoid differentiation and has a varying histologic appearance and classification. These tumors range from myofibromatosis, to spindle cell, to heman-

giopericytoma, with some of the latter also showing glomus tumor-like features. Whether these tumors are related neoplasms that simply manifest a spectrum of histologic and clinical findings or whether they are different entities is conjecture. Of these tumors, the hemangiopericytoma type is the most common. It occurs throughout the soft tissues, including the mesentery and omentum. These solid hemangiopericytomas are composed mostly of pericytes and capillaries and range from benign to malignant with gradations in between.

Some patients are relatively asymptomatic in spite of a large tumor, while others develop hypertension, hypoglycemia, or both. One patient with a metastatic hemangiopericytoma developed hypoglycemic coma (64); she had an abnormal insulin-like growth factor and low blood insulin and growth hormone levels that reverted to normal after resection.

Imaging reveals a solid, contrast-enhancing soft tissue tumor.

Recurrence is common after resection.

Leiomyomatous Tumors

Both leiomyomas and leiomyosarcomas occur in the peritoneal cavity and adjacent structures. Detection of a peritoneal leiomyosarcoma raises an obvious question: Did it originate in the peritoneum or is it a metastasis? A rare leiomyoblastoma develops in the omentum (65) or adjacent structures (Fig. 14.22).

One variant is disseminated leiomyomatosis, also called leiomyomatosis peritonealis disseminata. It is a rare, often benign condition consisting of smooth muscle tumors growing along the peritoneal surface, but an occasional one undergoes malignant degeneration. Whether these tumors originate from metaplasia of submesothelial mesenchymal cells is conjecture. The condition occurs mostly in premenopausal women and is associated with the use of birth control medications or pregnancy. Some of these tumors may regress spontaneously or after oophorectomy. Endometriosis is an associated finding in some these patients. The findings can be quite complex; thus one woman had disseminated leiomyomatosis, endometriosis, and a multicystic mesothelioma (66); preoperatively she was believed to have stage III ovarian cancer.

Computed tomography shows leiomyomatosis as multiple peritoneal nodules mimicking carcinomatosis, except that some of these patients have no ascites. The differential includes ovarian carcinomatosis, leiomyosarcomatosis, mesothelioma, multiple dermoids, lymphoma, and some infections such as peritoneal tuberculosis.

Leiomyosarcomas at other gastrointestinal sites do metastasize to the peritoneum and differentiation from disseminated leiomyomatosis is often not possible. Primary mesenteric or omental leiomyosarcomas have a slow growth

ADVANCED IMAGING OF THE ABDOMEN

rate and are generally quite large at presentation (Fig. 14.23). Occasionally an abscess develops in these leiomyomatous tumors.

A rhabdomyosarcoma is probably the most common sarcoma in the extraperitoneum. Its smooth muscle counterpart, a primary extraperitoneal leiomyosarcoma, is quite rare. A heterogeneous imaging appearance is common, with the larger malignant tumors due to necrosis. Adjacent vessels are often either encased or displaced.

Histiocytic Tumors

Histiocytic tumors range from benign (fibrous histiocytoma) to malignant. A malignant fibrous histiocytoma is not rare. These tumors readily invade adjacent structures; rectal bleeding develops with bowel involvement.

These tumors range from solid to mostly cystic and tend to be large on initial presentation. The site of origin is difficult to establish once extensive invasion of adjacent structures develops. Pelvic ones in women mimic an ovarian neoplasm, a differential diagnosis that can be excluded if a normal ovary is identified.

Imaging reveals malignant fibrous histiocytomas as complex heterogeneous tumors due to their cystic component and necrosis and hemorrhage. They tend to be hypoechoic. Doppler US identifies prominent feeding and draining

PERITONEUM, MESENTERY, AND EXTRAPERITONEAL SOFT TISSUES

vessels. More cystic ones contain thick septa. In an occasional one extensive hemorrhage even suggests a hematoma.

Not uncommonly, a diagnosis is confirmed only after resection. Recurrence is common with malignant ones.

Osteogenic Tumors

An extraskeletal (soft tissue) osteosarcoma is rare. Most occur in adults in the extraperitoneum or in extremities. Some are associated with prior trauma or radiation therapy. Calcifications or even ossification are common in these otherwise homogeneous tumors. Imaging can suggest the tumor extent, but a biopsy is needed for diagnosis.

Malignant Melanoma

Malignant melanoma metastasizes widely throughout abdominal tissues rather than exhibiting a peritoneal carcinomatosis pattern, and for this reason tumor detection strategies differ from those for most other metastases. Computed tomography and MR are often employed, with special attention paid to soft tissues, including abdominal fat. Abdominal wall metastases are seen as soft tissue nodules, often surrounded by cutaneous or properitoneal fat. A FDG-PET scan detects metastases if they are larger than about 1.5 to 2mm in size; overall, FDG-PET achieves greater sensitivities and

specificities than CT in detecting these metastases. Likewise, it is superior to CT in detecting recurrence during follow-up.

Germ Cell Tumors

A primary extraperitoneal germ cell tumor is rare. Most represent metastases from a testicular primary; extensive spread is common on initial presentation (Fig. 14.24). An occasional patient develops peritoneal metastasis after resection; carcinomatosis tends to be rather subtle and is difficult to detect with CT—either Ga-67 scintigraphy or FDG-PET appears more sensitive.

An extraperitoneal choriocarcinoma is associated with elevated b-human chorionic gonadotropin (b-hCG) levels but normal a- fetoprotein levels; CT and MR postchemotherapy may reveal residual tumor, but cannot differentiate between viable tumor and necrotic tissue. Some of this tissue eventually calcifies.

Small Cell Carcinoma

Most peritoneal small cell carcinomas (at times simply called small cell tumors to reflect their unknown origin) occur in infants and young males. Imaging in the few reported patients reveals large, lobulated, solid-appearing tumors; some induce a surrounding desmoplastic reaction. Ascites is an inconstant finding.