Solid Omental Tumors 

Metastatic tumors of the omentum are common. In contrast, primary tumors of the omentum are very rare.^[1] Stout and Cassel described the first reported case of a primary omental tumor in 1942. The patient had a 60-year history of a palpable abdominal mass prior to dying of heart failure at age 92 years. An omental hemangiopericytoma was identified at autopsy.

Frequency

Because of its rarity and the paucity of information, the incidence of omental tumors is unknown, both in the United States and worldwide. Most of the information in the medical literature comes from case reports.^{[2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13]} The authors identified 131 cases in the literature (see Table below).

Table. Distribution of Primary Omental Tumors (Open Table in a new window)

The etiologic agents causing primary omental tumors are unknown.

Sex:  Because of the limited number of reported cases in some pathological categories, epidemiologic information is sparse. A slight male predominance has been suggested. In the authors' series, 35 of 66 patients (53%) with primary omental tumor were males.^{[14, 15]}

Age:  Based on collected studies, these tumors are found in all ages but are diagnosed most frequently in the fifth to sixth decades of life.^{[14, 16, 17, 15]}

The omentum is composed of a trabecular connective-tissue structure carrying arteries, veins, lymphatics, and fat pads. The lining of the omentum is composed of 2 layers of mesothelial cells. The stroma of the omentum contains fibroblast, pericytes, lipocytes, and lymphoreticular bodies. The pathological spectrum of primary omental tumors is diverse. Although the greater omentum is mainly composed of adipose, vascular, and lymphatic tissue, omental tumors have predominantly consisted of smooth muscle tissue tumors. The most common malignant lesions are leiomyosarcomas, hemangiopericytomas, and fibrosarcomas.^[18] The most common benign tumors include gastrointestinal stromal tumors (which have malignant potential dependent on tumor size, mitotic activity, cellularity, and invasive growth), leiomyomas, lipomas, and fibromas.

A gastrointestinal stromal tumor (GIST) is the most common mesenchymal tumor of the gastrointestinal tract, with reports of GISTs originating in the omentum.^[19]

Leiomyosarcoma is the most common type of primary omental tumor that originates in smooth muscle, usually in the gastrointestinal tract, retroperitoneum, and genitourinary tract.^{[3, 14, 20]}

Hemangiopericytoma is a vascular tumor arising from the pericytes that surround capillaries.^{[21, 22]} Typically, large staghorn vessels are seen within the tumor. Reticulin stain shows tumor cells external to the basement membrane of the vessels, distinguishing hemangiopericytoma from angiosarcoma. The tumor can be classified as benign or malignant. Although differentiating benign tumors from malignant tumors can be difficult, some authors have suggested the degree of anaplasia and foci of necrosis are indicative of malignant potential.

Fibrosarcoma is a tumor that most commonly arises from the soft tissues of the extremities and trunk. Rare cases of intra-abdominal fibrosarcomas have been reported arising from the viscera, retroperitoneum, mesocolon, and greater omentum.^[23] Fibrosarcomas exhibit varying degrees of differentiation, ranging from moderately differentiated regions comprised of fusiform cells arranged in a fasciculated pattern and associated with bands of collagen and reticulin to predominantly poorly differentiated areas comprised of solid areas devoid of reticulin. Histologic differentiation has been shown to be a useful prognostic indicator with soft tissue fibrosarcomas.

Lipoblastoma is a benign, soft tissue, solid tumor consisting of immature embryonal fat tissue. Only 8 cases of omental lipoblastoma have been reported in the literature.^[24] Differentiating lipoblastoma from liposarcoma may be difficult; however, new cytogenetic analysis and immunohistochemical markers have recently been reported as useful adjuncts in distinguishing between them. Reciprocal translocations involving band 8q11-8q13 and chromosome 2 have been reported as new markers in deciphering lipoblastoma from myxoid liposarcoma.

Glomus tumors resemble the modified smooth muscle cells of the glomus body, which consists of a specialized form of arteriovenous anastomosis whose function is thermal regulation. Only 2 cases have been reported in the literature. Immunohistochemically, these tumors stain positive for alpha-smooth muscle actin, muscle-specific actin (HHF35), and vimentin.^[10]

Teratomas are derived from all 3 germ layers: ectoderm, endoderm, and mesoderm. Teratomas range from benign, well-differentiated (mature) cystic lesions to those that are solid and malignant (immature).They are classified as either mature (well-differentiated) or immature referring to their benign or malignant nature, respectively.^{[6, 15]}

The most common presentations of solid omental tumors included abdominal discomfort (30 of 66 cases [45.5%]), abdominal mass (23 of 66 cases [34.9%]), and abdominal distention (10 of 66 [15.2%]). In most patients, abdominal pain was exacerbated in the supine position and eased by standing upright. Nausea and weight loss sometimes occurred. Local symptoms were the same for benign and malignant lesions.

Two main indications for surgery exist: diagnosis and treatment. One of the hallmarks of omental tumors is the inability of preoperative studies to identify specific pathologic entities. Preoperative fine-needle aspiration (FNA) and core needle biopsies are controversial. Although some surgeons find these procedures to be helpful in confirming the diagnosis of such abdominal masses, others argue that the risk of potentially contaminating the abdominal cavity with tumor cells is increased. The most effective treatment of primary omental tumors is complete resection (see Surgical Therapy). Solid omental tumors can also manifest rapidly because of bleeding or intestinal infarction, requiring emergent surgery.

The greater omentum, where most omental tumors are located, is composed of a double layer of peritoneum extending from the greater curvature of the stomach toward the pelvis anterior to the small intestine before folding over itself and the transverse colon. The left margin is continuous with the gastrosplenic ligament; the right margin extends to the proximal duodenum. Consequently, tumors arising from the greater omentum generally displace the stomach upward and the transverse colon downward. The normal omentum is thin and mainly composed of fat; therefore, it is usually not visualized on ultrasonography or CT scan unless it is pathologically involved. Knowledge of the anatomical relationship between the greater omentum and surrounding structures is essential to accurately diagnosing and safely resecting omental tumors. Gastrointestinal anatomy, as it relates to the greater omentum, is illustrated below.

Absolute contraindications for surgical resection include inability to safely resect the tumor because of local invasion.

Differential diagnoses include the following:

• Omental metastasis (from primary sites, including the colon, stomach, pancreas, or ovaries)
• Peritoneal tumors
• Mesenteric tumors
• Abdominal tumors
• Gastric submucosal tumors
• Pancreatic tumors