Solid Omental Tumors

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 US or CT unless it is pathologically involved. Knowledge of the anatomic relations between the greater omentum and surrounding structures (see the image below) is essential for accurate diagnosis and safe resection of omental tumors.

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 two layers of mesothelial cells. The stroma of the omentum contains fibroblast, pericytes, lipocytes, and lymphoreticular bodies.

The pathologic 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,[2] and fibrosarcomas; liposarcomas[3] have been reported as well. The most common benign tumors include gastrointestinal (GI) stromal tumors (GISTs; these have malignant potential dependent on tumor size, mitotic activity, cellularity, and invasive growth), leiomyomas, lipomas, and fibromas. A few cases of perivascular epithelioid cell tumor (PEComa) of the greater omentum have been described.[4]  A primary omental neuroendocrine tumor (NET) was described in a 2021 case report.[5]

A GIST is the most common mesenchymal tumor of the GI tract, with reports of GISTs originating in the omentum.[6, 7, 8]

Leiomyosarcoma is the most common type of primary omental tumor that originates in smooth muscle, usually in the GI tract, retroperitoneum, and genitourinary tract.[9, 10, 11]

Hemangiopericytoma is a vascular tumor arising from the pericytes that surround capillaries.[12, 13] 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 that 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.[14] 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 a few cases of omental lipoblastoma have been reported in the literature.[15] 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 a few cases have been reported in the literature. Immunohistochemically, these tumors stain positive for alpha-smooth muscle actin, muscle-specific actin (HHF35), and vimentin.[16]

Teratomas are derived from all three germ layers: ectoderm, endoderm, and mesoderm. They 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 as a reflection of their benign or malignant nature, respectively.[17, 18, 19]

United States and international statistics

Because of the rarity of omental tumors and the paucity of available information, the incidence of these tumors is unknown, both in the United States and worldwide. Most of the information in the medical literature has come from case reports.[20, 9, 21, 22, 17, 23, 24, 25, 16, 26, 27, 28, 7, 29, 30, 31, 32] From a group of 131 cases identified in the literature, the authors determined the distribution shown in Table 1 below.

Table 1. Distribution of Primary Omental Tumors (Open Table in a new window)

Age- and sex-related demographics

Collected studies indicate that these tumors are found in all ages but are diagnosed most frequently in the fifth to sixth decades of life.[10, 33, 34, 18]

Because of the limited number of reported cases in some pathologic 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.[10, 18]

Patients with primary malignant tumors of the omentum have a median survival time of only 6 months. Only 10-20% of patients are alive 2 years after surgical excision. The reason for this grave prognosis is not clear, given that only a minority of these patients have distant metastasis at initial diagnosis.

Benign omental tumors are associated with long-term survival after surgical resection. In one series, patients with benign tumors demonstrated a 5-year survival rate of 75%. Patients with liposarcomas showed a 5-year survival rate of 59-70%. Survival depends on histology types. Round cell liposarcomas are poorly differentiated tumors with frequent metastasis and are associated with shorter 5-year survival rates.

The most common presentations of solid omental tumors included the following:

• Abdominal discomfort (30 of 66 cases [45.5%])
• Abdominal mass (23 of 66 cases [34.9%])
• 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.

Standard preoperative laboratory studies include the following:

• Coagulation evaluation
• Complete blood count (CBC)
• Electrolytes

Ultrasonography

Most patients undergo abdominal ultrasonography (US) as a first-line screening imaging study. This modality can indicate the presence of a mass in the midabdomen and can differentiate cystic tumors from solid tumors; however, it usually cannot identify the primary site of the tumor and its characteristics. Consequently, computed tomography (CT) of the abdomen is the study of choice in helping to diagnose omental tumors.

Computed tomography

CT provides anatomic details and usually identify the primary tumor site. It may also demonstrate displacement or compression on adjacent organs. In a review of primary omental leiomyosarcomas, all three cases revealed CT findings of a flat, pancakelike mass with multiple cystic spaces with enhancement of the solid areas of the masses.[35] The masses were located in the anterior compartment of the abdomen, usually anterior to the small bowel loops and transverse colon. This differs from the appearance of omental metastatic disease on CT, which has been described as an "omental cake"[36] owing to the thickened tumor-implanted omentum floating in ascites.

Magnetic resonance imaging

Magnetic resonance imaging (MRI) is another available modality that can aid in differentiating cystic tumors from solid tumors. MRI is not operator-dependent and requires no preparation contrast medium; however, it is more time-consuming and expensive than other imaging modalities.

Positron emission tomography

When a gastrointestinal (GI) stromal tumor (GIST) is suspected, imaging with 18F-fluorodeoxyglucose (18F-FDG) positron emission tomography (FDG-PET) can complement contrast CT in helping to differentiate benign tissue from malignant tissue and necrotic scar from active tumor. Baseline PET is recommended before initiation of treatment with imatinib because 80% of patients will exhibit response based on PET images.

Angiography

Angiography can be helpful in patients with a suspected omental tumor. The major arterial blood supply of the greater omentum is largely from the right and left gastroepiploic arteries, which are derived from the gastroduodenal and splenic arteries. Knowledge of these vascular structures helps in diagnosing an omental tumor. Malignant tumors are favored when angiography demonstrates a hypervascular mass with neovascularity. Angiography should be performed to determine the feeding artery of the tumor as well as its vascularity.

Preoperative fine-needle aspiration (FNA) biopsy (FNAB) and core needle biopsy (CNB) have been controversial in this setting. Although some surgeons find these procedures to be helpful in confirming the diagnosis of these abdominal masses, others argue that the risk of potentially contaminating the abdominal cavity with tumor cells is increased.

A study by Perez et al found US-guided percutaneous biopsy to be safe and effective for diagnosing omental disease.[37]

Primary omental tumors with the following histologic types have been reported:

• Leiomyosarcoma
• Fibrosarcoma
• Hemangiopericytoma
• Schwannoma
• Spindle cell sarcoma
• Liposarcoma ^{[38, 39]}
• Leiomyoma
• Lipoma
• Fibroma
• Mesothelioma
• Cysts
• Lipoblastoma
• Leiomyoblastoma ^[20]

Liposarcomas are further classified into the following four different subtypes[38, 39] :

• Myxoid
• Round cell
• Well differentiated
• Pleomorphic

The difference between benign and malignant omental tumors depends on the evaluation of many parameters (eg, size, pleomorphism, mitotic activity, necrosis, metastasis). Benign and malignant lesions are almost equally distributed.

Omental GISTs have positive staining for CD 117 (c-kit proto-oncogene protein product), which are present in the interstitial cells of Cajal (ICCs). ICCs are the pacemaker cells that are important for GI tract motility. Omental GISTs also are positive for vimentin during immunohistochemical staining. Vimentin is a protein found in cells of mesenchymal origin.

The rarity of primary omental tumors hinders adequate assessment of adjuvant therapy. Some reports have suggested that chemotherapy may be effective.

In the treatment of malignant hemangiopericytoma, several groups have observed that doxorubicin, either alone or in conjunction with other agents, can achieve response rates of up to 80%.[40]  A 12-month course of adjuvant chemotherapy with doxorubicin, cytotoxin, and dimethyltriazenoimidazolecarboxamide (DTIC) has been suggested for the treatment of fibrosarcoma. In treating leiomyosarcoma, combination chemotherapy with hydroxyurea, etoposide, and dacarbazine has been used. Other agents used include intraperitoneal cisplatin with intravenous administration of ifosfamide and pirarubicin hydrochloride.

Radiotherapy has been reported to be effective for partially excised tumors or inoperable tumors. In one study, 50 Gy was used to treat a partially excised liposarcoma. The tumor recurred after several months, necessitating a further debulking operation. The benefits of radiotherapy must be weighed against the risk of injury to abdominal viscera, particularly the bowel.

For treatment of gastrointestinal (GI) stromal tumors (GISTs), which are refractory to standard chemotherapy, imatinib in conjunction with surgical resection has been advocated.[41, 42, 43] Imatinib acts by inhibiting tyrosine kinase enzymes. Several trials have been initiated to explore the role of imatinib as an adjuvant treatment for prolonging disease-free survival, as well as overall survival.

Recurrence is a concern after treatment with adjuvant imatinib. Risk factors for GIST recurrence include high tumor mitotic count, nongastric location, large size, rupture, and adjuvant imatinib for 12 months.[44]

Indications and contraindications

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. As noted (see Workup), preoperative fine-needle aspiration (FNA) biopsy (FNAB) and core needle biopsy (CNB) are controversial for diagnostic purposes in this setting.

With respect to therapy, complete surgical excision (total omentectomy) is the recommended treatment of primary omental tumors. Even when peritoneal implants are present, omentectomy appears to improve survival significantly. Solid omental tumors can also manifest rapidly because of bleeding or intestinal infarction, requiring emergency surgery.

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

Complications

Most of the surgical procedures do not involve intestinal resections or resection of major organs. As a result, expected postoperative complications mirror those of other clean abdominal procedures. If intestinal resection is performed, the rate of infectious complications increases similar to that of clean-contaminated procedures. Other possible complications include bleeding, pancreatitis, bowel obstruction, and intestinal ischemia.

Recommendations for the follow-up care of patients with solid omental tumors have not been established. Because recurrences and metastases can occur more than 20 years after primary treatments for sarcomas, the authors recommend long-term follow-up care for these patients.