INTRODUCTION
The small bowel mesentery is a broad fan shaped fold of peritoneum that suspends the small bowel loops from the posterior abdominal wall. The two layers of peritoneum forming the mesentery contain a variable amount of fat through which run the major arteries, veins and lymphatics of the small bowel. Its root extends diagonally from its origin at the ligament of Treitz inferiorly and to the right towards the ileocecal valve.
The mesentery is
a frequent route for the spread of malignant neoplasms through the abdominal
cavity. Primary tumors arising in the mesentery are relatively less common[1].
Patients with mesenteric neoplasms usually present with non specific symptoms
of abdominal pain, weight loss, a palpable abdominal mass or diarrhea. CT plays
a critical role in achieving an accurate diagnosis to guide patient management.
The objective of this exhibit is to illustrate the appearances of mesenteric
neoplasms as depicted at CT. Images generated from three-dimensional volume
rendering will be used to emphasize the major pattern of spread of tumors in
the mesentery and small bowel. The differential diagnosis and various pitfalls
will also be illustrated.
Tumors originating
in the abdomen or elsewhere in the body can disseminate to the mesentery in
four major ways:
1. Direct spread along the mesenteric vessels and surrounding fat
2. Extension via the mesenteric lymphatics
3. Embolic hematogeneous spread
4. Intraperitoneal seeding
Although convenient, this classification is somewhat arbitrary, since many neoplasms
can spread by more then one route.
DIRECT SPREAD TO THE MESENTERY
Gastrointestinal
carcinoid tumor
Gastrointestinal carcinoid tumors arise from neuroendocrine cells in the bowel
mucosa or submucosa and are the most common malignant neoplasms of the small
intestine. Approximately 40 to 80% spread to the mesentery, either by direct
extension or via the local lymphatics [2]. The ileum is the most frequent location
of the primary lesion. The mesenteric involvement is usually discovered first,
when patients present with non specific abdominal pain. Alternatively, patients
with hepatic metastases may present with the carcinoid syndrome caused by the
release of vasoactive substances into the systemic circulation.
At CT, the most common manifestation of mesenteric carcinoid is that of an enhancing
soft tissue mass with linear bands radiating in the mesenteric fat. Radiologic-pathologic
correlation has shown that these radiating strands of soft tissue result from
the intense fibrotic proliferation and desmoplastic reaction in the mesenteric
fat and the adjacent mesenteric vessels caused by the release of serotonin and
other hormones from the primary tumor. Calcifications are visible in up to 70%
lesions at CT [2]. Thickening of adjacent small bowel loops caused by tumor
infiltration or ischemia as well as angulation and or obstruction secondary
to fibrosis are common associated findings [3]. The primary tumor is often small
and not always diagnosed at CT.
Desmoid tumor
Desmoid tumors are rare locally aggressive non encapsulated masses resulting
from a benign proliferation of fibrous tissue. Although they can occur sporadically
and develop anywhere in the abdomen including in the musculature of the abdominal
wall, the retroperitoneum and the pelvis, abdominal desmoids developing in the
mesentery are especially common in patients with Gardner syndrome, particularly
if the patient has undergone abdominal surgery [4] [5] At CT, they present as
soft tissue masses, often with poorly defined borders and strands radiating
into the adjacent mesenteric fat [6]. Large size (over 10cm), multiple desmoids
as well as extensive infiltration of the small bowel and entrapment of the ureters
are poor prognostic signs [7].
Other neoplasms
Several intra-abdominal malignancies, including gastric, pancreatic and colon
cancer may extend directly into the leaves of the mesentery or spread along
the mesenteric vessels. About 40% of patients with newly diagnosed adenocarcinoma
of the pancreas have unresectable, locally advanced disease with tumor extension
along the root of the mesentery and encasement of the major mesenteric vessels
[8].
Differential diagnosis
Sclerosing
mesenteritis
Sclerosing mesenteritis is a rare inflammatory condition of unknown etiology
affecting the root of the mesentery. The mesenteric fat is involved with variable
amount of inflammation, fatty necrosis and fibrosis. When the inflammation predominates,
the so- called mesenteric panniculitis, patients generally present with acute
pain. On CT, this entity presents as a focal area of increased attenuation within
the mesenteric fat surrounded by a pseudocapsule. Areas of fibrosis within the
inflamed fat appear as linear bands of soft tissue attenuation [9]. In retractile
mesenteritis, the fibrosis predominates and the disease manifest itself as large
masses of soft tissue attenuation which may contain calcifications. Some masses
are poorly defined with whiskers of soft tissue thickening extending into the
adjacent fat [9]. The infiltrative nature of the fibrosis may lead to result
in serious complications including thrombosis the mesenteric vessels with secondary
variceal bleeding. Scarring with retraction of the mesentery and encasement
of small bowel loops can lead to ischemia or obstruction.
Lymphoma
Lymphoma is the most common malignant neoplasm affecting the mesentery[10].
Approximately 30 to 50% of patients with Non Hodgkin Lymphoma harbor disease
in the mesenteric lymph nodes. Patterns of mesenteric lymphoma at CT include
multiple rounded homogeneous masses, often encasing the mesenteric vessels and
producing the "sandwich sign"[11], a large lobulated "cake like"heterogeneous
mass displacing small bowel loops or ill defined infiltration of the mesenteric
fat, particularly after chemotherapy[10] [12]. Bulky retroperitoneal adenopathy
commonly accompanies the mesenteric disease and should be a clue to the diagnosis
[1].
Patients with leukemia, particularly of the chronic lymphocytic type often harbor
extensive abdominal adenopathy.
Other malignancies
Metastases from colon cancer, ovarian carcinoma, breast and lung cancer as well
as melanoma can affect mesenteric lymph nodes. Compared with lymphomatous nodes,
the degree of nodal enlargement is less and the distribution more localized
[10]. Metastases from leiomyosarcoma often undergo degeneration and cystic changes.
Differential diagnosis
Several infections and inflammatory conditions produce mesenteric nodal enlargement
mimicking lymphoma or metastatic disease. However, in the majority of cases,
inflammatory adenopathy remains discrete, while lymphomatous nodes tend to coalesce,
a helpful distinguishing feature.
Atypical
mycobacterial infection and tuberculosis
The rising incidence of abdominal atypical mycobacterial infection and the re-emergence
of tuberculosis can be attributed to the increasing number of immunocompromised
hosts, particularly patients with HIV infection, chronic steroid therapy and
intravenous drug use. Abdominal tuberculosis is transmitted via three major
routes: ingestion on infected milk or sputum carries the infection through the
intestine to local lymph nodes; hematogenous spread from the lungs to abdominal
and paraaortic lymph nodes; and direct spread from the serosal surface of infected
organs such as the fallopian tubes. Intra abdominal lymphadenopathy is the most
common manifestation of abdominal tuberculosis and infection with Mycobacterium
Avium Complex (MAC). Affected nodes often demonstrate rim enhancement in the
peripheral inflammatory reaction and low attenuation center in the central caseous
necrosis or a multilocular appearance [13] [14]
Other inflammatory conditions
Enlarged mesenteric nodes can also be seen in some non infectious inflammatory
conditions such as Celiac sprue, Crohn disease, Whipple disease, systemic mastocytosis
and sarcoidosis [15, 16]. Rare cases of mesenteric Castleman's disease presenting
as intensely enhancing mesenteric adenopathy have been reported [17].
Mesenteric hematoma
Organizing mesenteric hematoma, either post traumatic or related to overzealous
anticoagulation therapy can occasionally mimic a neoplasm at CT.
Embolic metastases
from melanoma, breast and lung can reach the antimesenteric border of the small
bowel via small mesenteric arterial branches and grow into enhancing mural nodules
in the bowel wall. These tumor deposits can act as a lead point for intussuception.
The small bowel and its mesentery are the most common site of gastrointestinal
metastases from melanoma [18]. In a series of 230 patients with melanoma reviewed
by Kawashima and al, 7.4% had CT evidence of small bowel involvement [19]. Metastases
are even more commonly described in autopsy series, found in up to 35 to 58%
of cases.
Because of the
natural flow of fluid in the peritoneal cavity, the mesentery close to the terminal
ileum in the right lower quadrant is a common site of intraperitoneal tumor
seeding. Tumor deposits within the leaves of the mesentery can appear as focal
masses or produce a diffuse infiltration of the mesenteric fat, the so called
"stellate appearance of the mesentery".
The stellate appearance of the mesentery is caused by thickening and rigidity
produced by microscopic infiltration of tumor within the fat along the mesenteric
blood vessels.
Carcinomatosis
The stellate appearance to the mesentery is more commonly seen in association
with peritoneal carcinomatosis particularly if caused by breast cancer, gastric,
pancreatic or ovarian cancer [16]. Compared to infiltrating ductal carcinoma,
lobular breast carcinoma more frequently metastasize to the mesentery and gastrointestinal
tract [20].
Peritoneal lymphomatosis (figure V King) results from peritoneal seeding of
primary gastrointestinal lymphomas and cannot be distinguished from carcinomatosis
based on CT appearance [21].
Malignant peritoneal
mesothelioma
Malignant peritoneal mesothelioma is a rare and usually lethal neoplasm arising
from the mesothelial cells lining the serosal surface of the peritoneal cavity.
The majority of patients have a history of asbestos exposure [22]. CT manifestations
include ascites in variable amount, enhancement of the peritoneum after administration
of intravenous contrast, focal peritoneal soft tissue masses and infiltration
of the omentum. Spread to the mesentery is common and appears as increased attenuation
in the mesenteric fat, perivascular soft tissue thickening and rigidity of the
vascular bundles [22]. This so called "stellate appearance" is caused
by microscopic infiltration of tumor within the fat along the mesenteric blood
vessels [23]. Associated pleural calcifications, thickening or pleural effusions
are common.
Differential diagnosis
Tuberculous
peritonitis
Involvement of the peritoneum and mesentery with tuberculosis generally occurs
secondarily to infection in the gastrointestinal tract. Differentiating this
condition from carcinomatosis at CT can be quite challenging. In addition to
diffuse thickening and fine nodularity of the mesentery and infiltration of
the mesenteric fat, CT features that suggest the diagnosis include enhancement
and smooth thickening of the peritoneum, high density ascites, thickening of
the bowel wall, particularly the terminal ileum and the cecum and low attenuation
mesenteric nodes [12] [13] [24].
Superior
mesenteric vein thrombosis
Thrombosis of the superior mesenteric vein often produce focal mesenteric edema,
with a focal increase in the attenuation of the mesenteric fat surrounding the
thrombosed vessel and poor definition of the vessel wall.
CT remains the dominant imaging modality for the diagnosis of mesenteric neoplasms. Table 1 presents a systematic approach to the differential diagnosis of mesenteric lesions detected at CT.
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