Материал: Advanced Imaging of the Abdomen - Jovitas Skucas

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.

A CT finding of large, solid peritoneal tumors without an evident primary site in a pediatric patient or a young adult should suggest a small cell tumor.

Neuroendocrine Tumors

One classification of neuroectodermal origin tumors is based on their origin (Table 14.5). Although these tumors can be differentiated pathologically, from an imaging perspective they often have a similar appearance, and many publications lump them together (Fig. 14.25). They are differentiated in this work as an aid to clinical management rather than to imply that each type has a specific imaging appearance, although quite often a specific diagnosis can be suggested from clinical data and tumor location.

Neuroendocrine tumors originate in the extraperitoneum, mesentery, and bowel wall. Except for neuroblastomas and ganglioneuroblastomas these tumors are found mostly in adults. Tumors adjacent to bowel wall are discussed in Chapters 4 and 5.

Ganglioneuroma

Ganglioneuromas are rare neuroendocrine tumors originating from sympathetic ganglia and are found along the paravertebral sympathetic plexus. Most are extraperitoneal in location. These occur mostly in children and young adults. A rare one is hormonally active. Imaging reveals solid tumors that are nonspecific in appearance. Some contain punctate

Table 14.5. Classification of neuroectodermal tumors

Nerve sheath (Schwann cell) origin

Neurilemoma

Neurofibroma

Amputation neuroma

Neurosarcoma

Granular cell tumor

Ganglion cell origin

Ganglioneuroma

Neuroblastoma

Paraganglioma tumors*

Pheochromocytoma

Paraganglioma

* The following are similar tumors. The term pheochromocytoma is used for paraganglioma tumors located in the adrenal medulla.

Figure 14.25. Extensive retroperitoneal poorly differentiated neuroendocrine tumor. A spine metastasis is present (arrow). Primary site of tumor origin could not be determined; both adrenal glands were involved by tumor. (Courtesy of Algidas Basevicius, M.D., Kaunas Medical University, Kaunas, Lithuania.)

calcifications. They vary in contrast enhancement. They are hypointense on T1but of varying intensity on T2-weighted MR images. Hypointense curvilinear bands have been described within these tumors on T2-weighted MR images. Some of these tumors encase blood vessels without narrowing the lumen.

Ganglioneuroblastomas contain both ganglioneuroma and primitive neuroblastoma elements and are considered to be intermediate between benign ganglioneuromas and malignant neuroblastomas. A majority occur in young children. Their imaging appearance varies from a mostly solid to a mostly cystic tumor depending on the degree of cell differentiation.

Neuroblastomas are discussed in Chapter 16.

Paraganglioma (Pheochromocytoma)

Extraadrenal paragangliomas account for about 15% of these tumors. Presumably they arise from embryonic chromaffin cells in parasympathetic tissue and in accessory adrenal glands. A minority are multicentric in origin. Most are detected during the second and third decades of life. An occasional one manifests during pregnancy.

Carney’s syndrome (or triad) consists of a gastrointestinal stromal tumor (often a gastric leiomyosarcoma), an extraadrenal paraganglioma, and a pulmonary chondroma. Whether

PERITONEUM, MESENTERY, AND EXTRAPERITONEAL SOFT TISSUES

Carney’s syndrome is an autonomic nervous system abnormality or a multiple endocrine neoplasia syndrome or even a multiple hamartoma syndrome makes for an interesting discussion.

Some paragangliomas are nonfunctional. A paraganglioma can compress and displace an adjacent kidney and induce hypertension (Page kidney). As an example, CT of a 25-year-old woman with suspected appendicitis detected a broad ligament tumor, shown to be a nonfunctioning paraganglioma (67).

Schwannoma (Neurilemoma)

Schwannomas originate from Schwann sheath cells and are most common in the peripheral neural system. Schwann cells are the peripheral neural system’s glial cells. Found mostly in young adults, more often in women, extraperitoneal schwannomas tend to be solid tumors, and their imaging findings are nonspecific. The ones involving the sacral nerves tend to widen the sacral foramina (Fig. 14.26). Preoperative biopsy is often not helpful. A pelvic schwannoma tends to mimic an ovarian neoplasm.

Both MRI and CT are useful in characterizing these tumors (Fig. 14.27). Computed tomography of extraperitoneal schwannomas reveals well-marginated, encapsulated, round or oval tumors. An occasional one undergoes central necrosis, hemorrhage, and central or peripheric calcifications. They show heterogeneous contrast enhancement, with lack of enhancement in cystic regions. Overall, they tend to be somewhat hypovascular. Computed tomography of one pelvic schwannoma revealed only the cyst wall enhancing postcontrast; in some others focal central enhancement is evident.

Neurofibromatous Tumors

Neurofibromatosis type 1 (NF1, von Recklinghausen’s disease) is a relatively common autosomal-dominant hereditary neurocutaneous syndrome consisting of neurofibromas, café-au-lait spots, and other abnormalities involving various body organs. A mutation in the NF1 gene occurs roughly in one of 3500 births; NF1 encodes a tumor suppressor, loss of which leads to neurofibroma formation. Most affected individuals are detected in childhood, but a minority contain a forme fruste type first

Figure 14.26. Schwannoma exiting sacral foramen. Noncontrast CT identifies a soft tissue tumor widening a sacral foramen on the right (arrow). (Courtesy of Patrick Fultz, M.D., University of Rochester.)

manifesting in adulthood. Although a single neurofibroma most often is an isolated event, the presence of multiple neurofibromas or a single plexiform neurofibroma is strong presumptive evidence for von Recklinghausen’s disease. About 50% of affected individuals have spontaneous mutations.

Neurofibromas are complex, noncapsulated tumors originating from Schwann cell lineage in peripheral nerves. Histologically, they differ from neurilemomas. Mast cells appear to play a role in their growth (68). Both somatic and autonomic neural tissue is involved. Because these nerves are ubiquitous and neurofibroma formation sporadic, a wide manifestation of this entity is encountered. Tumors are more common in extraperitoneal tissues, including paraspinal regions, than within the bowel wall. Focal involvement of one organ is unusual but has been reported. Some tumors in the mesentery compress adjacent blood vessels and result in focal ischemia. Extensive extraperitoneal neurofibromas mimic adenopathy.

Neurofibromas tend to be focal, solid, homogeneous, and somewhat hypodense with CT, presumably due to their rich lipid content (Fig. 14.28). Often their appearance mimics that of adenopathy. Cystic degeneration is more common in neurilemomas, but does occur in a minority of neurofibromas. Many are symmetrical bilaterally, with a presacral location also being common. Asymmetry should suggest malignant degeneration. Postcontrast CT reveals a hypovascular soft tissue tumor. Occasional tumors with either peripheral or central contrast-enhancement are described, with the

less vascular portion representing lipid-rich Schwann cells and cystic degeneration, while denser segments tend to be more vascular.

Plexiform neurofibromas infiltrate and are poorly marginated. In fact, soft tissue involvement tends to mimic a diffuse infection. Magnetic resonance imaging of some mesenteric plexiform neurofibromas reveals ring-like structures within the tumor on T2-weighted and contrast-enhanced images.

One should keep in mind, however, that mesenteric involvement in von Recklinghausen’s disease needs to be distinguished from hereditary intestinal neurofibromatosis, which is a separate genetic disease.

Carcinoid

A mesenteric origin for a carcinoid is not uncommon. In fact, a distinction between bowel

Figure 14.28. Neurofibroma. Contrast-enhanced CT outlines a tumor with target-like enhancement (arrow). The hyperdense center represents nerve tissue, surrounded by more peripheral myxoid degeneration. (Source: Rha SE, Byun JY, Jung SE, Chun HJ, Lee HG, Lee JM. Neurogenic tumors in the abdomen: tumor types and imaging characteristics. Radiographics 2003;23:29– 43, with permission from the Radiological Society of North America.)

PERITONEUM, MESENTERY, AND EXTRAPERITONEAL SOFT TISSUES

and mesenteric origin is often not possible and some of these tumors are simply identified adjacent to a loop of bowel. Because of the oftenassociated dense desmoplastic reaction, even CT often cannot identify the exact site of origin for these tumors.

A typical CT appearance is that of a mesenteric tumor surrounded by radiating stellate folds, and such a finding is presumptive evidence for a carcinoid. Adjacent bowel loops are either infiltrated or simply displaced. Atypical appearances, however, are common. A rare carcinoid presents as a large necrotic mesenteric tumor mimicking a necrotic stromal tumor. Associated enlarged lymph nodes are an inconsistent finding.

A percutaneous biopsy is often nondiagnostic; tumor cells are difficult to obtain because of the exuberant fibrosis.

Other Neuroendocrine Tumors

An extrapancreatic gastrinoma is occasionally located in a lymph node. Primary gastrinomas are rare at this location and a small primary pancreatic tumor should be excluded.

Presacral Tumors

Certain tumors are more common in the presacral soft tissues than in other locations and these are discussed here. Overall, however, presacral tumors are uncommon. They originate from any of the soft tissue elements located in this space and range from solid to cystic. Primary presacral neoplasms can invade the sacrum or rectum. Larger presacral tumors tend to displace the rectum laterally. These tumors tend to be insidious, often are quite large when first detected, and thus are diagnosed late.

A triad of anorectal, sacral, and presacral anomalies was described by Currarino et al. (69) in 1981 and is known as Currarino’s triad. Both a complete triad and an incomplete form exist. A deformed sacrum is common; anorectal anomalies range from stenosis to an imperforate anus and presacral tumors consist of teratomas, meningoceles and various cysts or cystic neoplasms. Conventional radiographs identify an abnormal sacrum, called a scimitar sacrum, but the full extent of abnormalities is better detected with MRI.

Teratoma

The most common presacral tumor in neonates is a teratoma. Most teratomas are benign, although the risk of malignancy increases considerably in older children. Most are large at first presentation. Their imaging appearance is similar to teratomas at other locations and consists of a heterogenous, well-marginated tumor containing both cystic and solid components. The presence of varying amounts of fluid, fat, and calcifications, including teeth, is diagnostic. Malignant ones tend to contain more soft tissue and less of a cystic component than benign ones.

Meningocele

A presacral meningocele is a rare congenital abnormality consisting of herniated meninges and other spinal canal content. An occasional one is familial.Associated abnormalities include sacral abnormalities, spina bifida, and genitourinary defects. Imaging reveals a presacral cystic mass filled with cerebrospinal fluid and containing varying amounts of soft tissue. An associated sacral defect, at times detected even with conventional radiography, should suggest the diagnosis.

Other Presacral Tumors

A chordoma is a low-grade malignant neoplasm resulting in sacral destruction. Extension into the presacral soft tissues is common. A complication after resection of a sacral chordoma is posterior rectal herniation, presumably due to weakness of the posterior pelvic floor.

Widening of a sacral neural foramina should suggest neurofibromatosis.

The rare presacral neuroblastoma has an imaging appearance similar to its counterpart in the adrenal glands.

Ectopic Pregnancy

In general, 1% of all pregnancies are ectopic, and almost all occur in the fallopian tubes; among ectopic pregnancies, about 1% or fewer are intraabdominal. They are thus rare, but their importance lies with the associated high maternal death rate, if undetected. They are believed