Small Bowel: Mestenteric Neoplasms: Patterns of Disease Spread at CT

SHEILA SHETH, KAREN M HORTON, MELISSA GARLAND AND ELLIOT K FISHMAN

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.

MAJOR PATHWAYS FOR THE SPREAD OF TUMOR TO THE MESENTERY

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.

EXTENSION VIA THE MESENTERIC LYMPHATICS

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 HEMATOGENOUS SPREAD

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.

INTRAPERITONEAL SEEDING

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.

CONCLUSION

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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