Laboratory Studies
Standard preoperative laboratory studies include the following:
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Coagulation evaluation
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Complete blood count (CBC)
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Electrolytes
Imaging Studies
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. [38] 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" [39] 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.
Biopsy
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. [40]
Histologic Findings
Primary omental tumors with the following histologic types have been reported:
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Hemangiopericytoma
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Spindle cell sarcoma
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Fibroma
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Cysts
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Lipoblastoma
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Leiomyoblastoma [23]
Liposarcomas are further classified into the following four different subtypes [41, 42] :
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Myxoid
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Round cell
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Well differentiated
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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.
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Diagram of subdivisions of the omentum. The greater omentum is attached to the caudal border of the greater curvature of the stomach and consists of the hepatoduodenal, gastrocolic, and gastrosplenic ligaments. The lesser omentum is divisible into 2 parts: the hepatogastric ligament and the hepatoduodenal ligament.
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The anterior double-layered fold of the greater omentum descends from the stomach and the first part of the duodenum in front of the small intestine and ascends behind itself as far as the transverse colon.
Tables
Tumor Histology |
Number of Cases |
% of Total |
Leiomyosarcoma |
22 |
17 |
Hemangiopericytoma |
8 |
6 |
Sarcoma |
3 |
2 |
Myosarcoma |
2 |
1.5 |
Fibrosarcoma |
3 |
2 |
Reticulosarcoma |
1 |
1 |
Spindle cell sarcoma |
1 |
1 |
Liposarcoma |
1 |
1 |
Rhabdomyosarcoma |
1 |
1 |
Leiomyoma/leiomyoblastoma |
14 |
11 |
Lipoma |
5 |
4 |
Fibroma |
3 |
2 |
Fibromatosis |
2 |
1.5 |
Mesothelioma |
2 |
1.5 |
Endothelioma |
1 |
1 |
Myxoma |
1 |
1 |
Neurofibroma |
1 |
1 |
Malignant fibrous histiocytoma |
1 |
1 |
Gastrointestinal stromal tumor (GIST) |
21 |
16 |
Glomus tumor |
2 |
1.5 |
Teratoma |
28 |
21 |
Lipoblastoma |
8 |
6 |
Total |
131 |
100 |
