You are in: eMedicine Specialties > General Surgery > Abdomen Solid Omental TumorsArticle Last Updated: Apr 29, 2008AUTHOR AND EDITOR INFORMATIONSection 1 of 12
  Author: Quintessa Miller, MD, Staff Physician, Department of General Surgery, Keesler Air Force Base Medical Center Quintessa Miller is a member of the following medical societies: American College of Surgeons and National Medical Association Coauthor(s): A Letch Kline, MD, Program Director, Department of Surgery, Keesler USAF Medical Center; Clinical Assistant Professor, Department of Surgery, Uniformed University of the Health Sciences Editors: Juan B Ochoa, MD, Assistant Professor, Department of Surgery, University of Pittsburgh; Francisco Talavera, PharmD, PhD, Senior Pharmacy Editor, eMedicine; David L Morris, MD, PhD, Professor, Department of Surgery, St George Hospital, University of New South Wales, Australia; Paolo Zamboni, MD, Professor of Surgery, Chief of Day Surgery Unit, Chair of Vascular Diseases Center, University of Ferrara, Italy; John Geibel, MD, DSc, MA, Vice Chairman, Professor, Department of Surgery, Section of Gastrointestinal Medicine and Department of Cellular and Molecular Physiology, Yale University School of Medicine; Director of Surgical Research, Department of Surgery, Yale-New Haven Hospital Author and Editor Disclosure Synonyms and related keywords: abdominal mass, omental metastatic disease, omental hemangiopericytoma, leiomyosarcoma, sarcoma, myosarcoma, fibrosarcoma, reticulosarcoma, spindle cell sarcoma, liposarcoma, rhabdomyosarcoma, leiomyoma, leiomyoblastoma, lipoma, fibroma, fibromatosis, mesothelioma, endothelioma, myxoma, neurofibroma, fine-needle aspiration, FNA, core needle biopsy, radiotherapy, chemotherapy, omentectomy INTRODUCTIONSection 2 of 12
  Metastatic tumors of the omentum are common. In contrast, primary tumors of the omentum are very rare. 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. FrequencyBecause 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. The authors identified 131 cases in the literature (see Table below). Distribution of Primary Omental Tumors
EtiologyThe 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. 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. PathophysiologyThe 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. 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. Leiomyosarcoma is the most common type of primary omental tumor that originates in smooth muscle, usually in the gastrointestinal tract, retroperitoneum, and genitourinary tract. Hemangiopericytoma is a vascular tumor arising from the pericytes that surround capillaries. 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. 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. ClinicalThe 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. INDICATIONSSection 3 of 12
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. RELEVANT ANATOMYSection 4 of 12
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 (see Media files 1-2). 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. CONTRAINDICATIONSSection 5 of 12
Absolute contraindications for surgical resection include inability to safely resect the tumor because of local invasion. Differential diagnoses include the following:
WORKUPSection 6 of 12
Lab Studies
Imaging Studies
Other Tests
Diagnostic Procedures
Histologic FindingsPrimary omental tumors with the following histologic types have been reported:
Liposarcomas are further classified into 4 different subtypes: myxoid, round-cell, well-differentiated, and 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 gastrointestinal tract motility. Omental GISTs also are positive for vimentin during immunohistochemical staining. Vimentin is a protein found in cells of mesenchymal origin. TREATMENTSection 7 of 12
Medical therapyThe rarity of primary omental tumors has prevented an 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%. 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 is reportedly 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, requiring a further debulking operation. The benefits of radiotherapy must be weighed against the risk of injury to abdominal viscera, particularly the bowel. In treating GISTs, imatinib in conjunction with surgical resection is currently being advocated. Imatinib acts by inhibiting tyrosine kinase enzymes. Several trials are currently being conducted exploring the role of imatinib as an adjuvant treatment to prolong disease-free as well as overall survival. GISTs are refractory to standard chemotherapy. Surgical therapyComplete surgical excision (total omentectomy) is the recommended treatment of primary omental tumors. Even when peritoneal implants are present, omentectomy appears to significantly improve survival. Follow-upRecommendations for the follow-up care of patients with solid omental tumors have not been established. Because recurrences and metastasis can occur more than 20 years after primary treatments for sarcomas, the authors recommend long-term follow-up care for these patients. COMPLICATIONSSection 8 of 12
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. OUTCOME AND PROGNOSISSection 9 of 12
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 unclear because 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. FUTURE AND CONTROVERSIESSection 10 of 12
Complete surgical excision offers the best chance of cure. The role of adjuvant therapy in the management of malignant omental tumors is uncertain. Because of the rarity of these lesions, the effectiveness of chemotherapy and radiation therapy has not been established. Advancements in these treatment modalities may improve future survival rates for patients with malignant primary omental tumors. MULTIMEDIASection 11 of 12
REFERENCESSection 12 of 12
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