The omentum is made up of cells that are mesothelial in origin. It is said to be a type of visceral adipose tissue with a total surface area of 1500 cm2. It hangs in front of the abdominal organs like an apron, and it is also connected to the spleen, stomach, pancreas, and colon. The omentum exhibits angiogenic, fibrotic, stem cells, and immune activities responsible for promoting vascularization, accelerated healing, and limiting infection in the abdominal cavity. These activities can also lead to pathological changes like omental tumor formation and metastasis. Primary solid tumors of the greater omentum are rare, with only 42 reported cases; however, peritoneal metastasis is fairly common in some tumors like gastrointestinal stromal tumors and ovarian cancer.
These tumors are sporadic, and etiological factors responsible for such tumors are not known.
These are rare lesions, each associated with an insidious onset.
Liposarcoma of the greater omentum is a rare tumor;19 cases are reported in the literature until July 2019. Nine cases from 1936 to 2003 and 10 cases were reported from 2003 to 2018 and one in 2019. The average age of the patients in these case reports was 51.1 years.
Leiomyosarcoma of the greater omentum are rare tumors, and 23 cases have been reported in literature till October 2016. Clinically these tumors present in the 5th decade of life. While in another study, 27 cases have been reported. The median age in these cases was 51 years, and it was slightly more common in males.
According to the reported cases of SFTs (solitary fibrous tumors) of the greater omentum, this tumor occurs in the fifth to seventh life decade with a 1:1 male to female ratio.
Fifty-four cases of omental EGISTs (extra gastrointestinal stromal tumors) have been reported, and according to these cases, the median age at diagnosis is sixty-five years, and the male to female ratio is 1 to 1.
The omentum is called a visceral fat depot. Its role as ''policeman of the abdomen'' was recognized by a British surgeon in the early 1900s, owing to its properties of moving around the peritoneal cavity and limiting infections and inflammations. The omentum contains many immune aggregates called ''milky spots'' whose function is largely unknown. It is said that both free immune cells and tumors cells bind to these aggregates.
All primary tumors of the omentum are sporadic and usually malignant. But metastatic lesions are common. The reported primary omentum tumors include fibrosarcoma, spindle cell sarcoma, liposarcoma, leiomyoma, hemangiopericytoma, lipoma, and desmoid tumor, fibroma, mesothelioma, myosarcoma, rhabdomyosarcoma, leiomyoblastoma, and endothelioma. These tumors are derived from the different elements that omentum consists of like fat tissue, vessels, and lymphatic tissue.
Primary leiomyosarcoma of omentum is a very rare tumor; more commonly, leiomyosarcomas originate in smooth muscle cells of the gastrointestinal tract, retroperitoneum, and genitourinary tract. Because of the low frequency of leiomyosarcomas of omentum, little is known about these tumors' origin and behavior.
A solitary fibrous tumor (SFT), previously called hemangiopericytoma, is a rare mesenchymal tumor often originating from pleura; the omentum's involvement is very uncommon. 78% to 88% of these tumors are benign, and 12%-22% are malignant. Only 6 reported cases of SFTs arising from greater or lesser omentum had been described in the literature up till 2015.
GIST (gastrointestinal stromal tumors) are mesenchymal tumors arising from the gastrointestinal tract wall, most of it arising from the stomach. However, rarely they can arise from the omentum, which is designated as extra gastrointestinal stromal tumors (EGISTs). GISTs and EGISTs are histopathologically and immunohistochemically similar; both express gain-of-function KIT/PDGFRA mutations. As these tumors are sporadic, their malignant potential cannot be ascertained. Sakurai et al. reported Cajal cells like CD117+ and CD34+ cells in normal omentum from which EGISTs can theoretically arise.
Liposarcomas commonly arise from extremities' deep soft tissue, and it is the most common soft tissue sarcoma. Intraabdominal locations are rare, and they are rarely arising from the greater omentum.
Malignant mesothelioma commonly arises from the pleura and peritoneal cavity but rarely can arise from the greater omentum. Its relationship to asbestos is less obvious as compared to pleural mesothelioma.
Histopathology depends upon the type of tumor. These tumors' embryological origin varies as omentum is made up of different tissues like vessels, lymphatic, and fats.
Primary leiomyosarcoma of omentum on histopathology may show long fusiform cells with high mitotic index along with areas of coagulative tumors necrosis and myxoid change; immunohistochemistry may be positive for desmin and smooth muscle actin and strongly negative for S100 and CD34 as in leiomyosarcoma.
SFT is derived from mesenchymal cells; it is mostly composed of spindle-shaped cells and collagen bundles. Confusion exists in the literature about the origin and classification of SFT, as hemangiopericytomas (HPC) and SFTs share many pathologic findings; they are now considered part of the same entity. On immunohistochemical staining, these tumors are found to be negative for the following markers CD117, cytokeratin, calretinin, CD99, CD68, desmin, F8, S-100, epithelial membrane antigen. These tumors are positive for CD34, α-smooth muscle actin (α-SMA), BCL-2, and vimentin (VIM). CD34 is a useful positive biomarker for SFT; few studies have shown a 100% positivity rate.
According to WHO, liposarcoma is classified into five types, well-differentiated, pleomorphic, round cell, myxoid, and dedifferentiated. Liposarcoma may often have benign and malignant areas. These tumors' prognosis depends on the histologic type; the dedifferentiated subtype has a worse prognosis and high recurrence rate. A well-differentiated liposarcoma has thick septa, a lesion size of >10cm, containing globular non-adipose areas, and a lesion component of <75% fat.
The origin of extra gastrointestinal stromal tumors (EGISTs) is not fully known, but their histology and immunophenotype are similar to GISTs. It is thought they are GISTs that have been separated from the gastrointestinal tract, or they can arise from the mesenchymal cells of the omentum. EGISTs like GISTs are typically positive for KIT, alpha-smooth muscle actin, and negative for desmin and s-100 protein. These tumors similar to GISTs are shown to be made of elongated spindle cells, epitheliod cells, or mixed cells with high cellularity and low mitotic activity.
Histological examination of mesothelioma of omentum may show an epithelial, sarcomatoid, or biphasic pattern. Epithelial type tumors comprise 75% of cases and vary from well-differentiated with tubulopapillary pattern to solid sheets of round or polygonal cells. Such tumors may mimic carcinomas, and it can be difficult to distinguish epithelioid mesothelioma from a metastatic carcinoma, particularly adenocarcinoma. The sarcomatoid type tumors may be indistinguishable from fibrosarcomas on histology alone. Immunohistochemistry can help differentiate sarcoma and adenocarcinoma. These tumor cells are positive for calretinin, cytokeratin, and epithelial membrane antigen and negative for S-100 protein, Leu-M1, CEA, thrombomodulin, and placental alkaline phosphatase, positive immunoreactivity for calretinin greatly increase the accuracy of the diagnosis.
Primary omental tumors are mostly asymptomatic, but most patients present with abdominal discomfort, palpable abdominal mass, nausea, early satiety, and weight loss. Other symptoms based on different case reports present in literature are abdominal pain, abdominal distention, constipation, anorexia, vomiting, and fever. Physical examination may reveal palpation of a painless abdominal mass laterally ballotable, based on case reports in the literature available; the mass may be palpable in the epigastrium, hypogastrium, upper or lower right or left quadrant or involving the whole of the abdomen.
Patients can also have tenderness of the mass, and guarding may be present on per abdomen examination. In some case reports, patients also had ascites. Patients having ascites, weight loss, and peritoneal implants usually signify malignancy. The most common clinical presentation of the primary omental tumor is abdominal discomfort (56%) and mass (35%). In one case report patient presented with diffuse abdominal pain without palpable mass on physical examination while in another case report patient with primary leiomyosarcoma presented with unexplained hemorrhagic pseudoascities. While symptoms are largely nonspecific, they may also present as incidental findings on imaging. EGISTs of omentum can remain clinically silent despite huge tumor size.
The diagnosis of liposarcoma is challenging, and to differentiate it from other tumors is difficult. The appearance of well-differentiated liposarcoma on CT scan and MRI is similar to fat and other abdominal tumors.
The pre-operative diagnosis of primary leiomyosarcoma of omentum is difficult, and the diagnosis is usually made postoperatively. Accurate diagnosis is only obtained by histopathological examination of the tumor.
Intravenous CECT of the abdomen and pelvis and MRI are the best imaging methods for diagnosing leiomyosarcoma of omentum. It can give accurate information about the origin of the tumor. These lesions have similar imaging features in most cases, presenting as a solid-cystic and multilobulated tumor. Ultrasound can detect the tumor and show the lesion's internal structure but can fail in determining the origin. Sometimes these tumors have associated ascites and elevated CA-125, which can suggest ovarian cancer.
The radiological features of omental EGISTs which don't have myogenic and neurogenic differentiation is not established. They may be similar to omental leiomyoma and leiomyosarcoma. Most GISTs with the myogenic feature appears to be hypervascular and have a clear margin on CT scan and angiography.
According to Ishida (1998), a CT scan is the imaging of choice for greater omental tumors. CT appearance of such tumors is usually multilobulated, flat, pancake-like, with enhancing solid and multicystic densities. Angiography is a useful investigation in the diagnosis of omental tumors. A diagnosis of an omental tumor is made when a feeding artery to the tumor arises from the omental blood supply. A highly vascular lesion having neovascularity more likely means a malignant tumor.
A definitive diagnosis can be achieved by laparoscopy or open surgery with biopsy. Histology and immunohistochemistry are used to diagnose and differentiate mesothelioma from adenocarcinomas and sarcomas.
Ultrasound and CT-scan can be used to follow up for recurrences.
CT-scan and MRI are the best diagnostic modalities for the diagnosis of solitary fibrous tumors of omentum. They help show the proliferation of fibrous tissue in the abdominal cavity, assess the relationship between the tumor and surrounding structures, and help to decide the excision of the tumor. However, CT-scan and MRI cannot differentiate between SFT and mesenchymal tumors. Therefore it can be misdiagnosed as stromal tumors. Needle aspiration does not help in the diagnosis because the tumor is made up of acellular and hypercellular portions, and it doesn't provide enough tissue for cytological analysis. Apple et al. reported accurate diagnosis could be achieved through fine needle aspiration, but it needs investigation in many patients. Tumor samples can be collected through the Ryle tube for small tumors and exploratory laparotomy in large tumors.
Diagnosis of SFT of omentum is difficult as it resembles other tumors like leiomyoma and mesothelioma. Moreover, if a pedunculated tumor is arising from omentum, it can be mistaken as other pelvic tumors. Immunohistochemistry helps to establish the diagnosis. These tumors are positive for CD34, CD99, and BCL-2 and occasionally SMA, but they are usually negative for S-100, desmin, and cytokeratins. SFT appears as highly vascularized solid tumors with well-defined margins on ultrasound imaging, and CT-scan may show the same findings. However, imaging is not a specific investigation, and pre-op diagnosis becomes impossible.
Radical resection of the tumor is recommended for liposarcomas; adjuvant therapy has not been established as an effective treatment.
Complete resection of the tumor results in a good prognosis in leiomyosarcoma of omentum. Some authors have used non-standardized chemotherapy schemes in non-resectable disease with the variable results. In patients with disseminated disease, multimodal therapy has also been used. Due to such tumors' aggressive nature, relapse is a common problem, and long-term survival is unknown.
SFTs are generally benign. 15 to 20% of them are malignant, especially tumors greater than 10 cm. Histologic features like high cellularity, high mitotic activity, pleomorphism, atypia, tumor necrosis, and hemorrhage denote malignancy. Tumor size and mitotic activity play an essential role in predicting SFT originating from the omentum. Features like tumor size, mitotic activity, cellularity, and pleomorphism to predict SFT behavior can be used for risk assessment. Malignant factors are associated with high recurrence and metastasis. The treatment of choice is surgical resection, but malignant tumors can recur even several years after the surgery. The beneficial role of adjuvant therapy has not been established yet, but some reports suggest adjuvant radiotherapy and show response to chemotherapy, but their effectiveness has not been proved. Tumor size is a good prognostic factor, but high mitotic activity is associated with a bad prognosis.
Radical resection is the treatment of choice for omental GISTs; adjuvant therapy with imatinib after surgical resection in limited disease is also given. Adjuvant imatinib therapy can prevent relapse and can prolong long-term survival.
Regarding mesothelioma of omentum, an intensive loco-regional treatment strategy has been adopted in several independent phases one and two trials, including cytoreductive surgery supplemented with perioperative hyperthermic intraperitoneal chemotherapy. Early postoperative intraperitoneal chemotherapy can also be added to it. It is reported that this form of treatment improved median survival as a result. Vogelzang et al. conducted a multicenter, controlled, randomized phase III trial, and pemetrexed-cisplatin is shown to be the gold standard in case of non-operable malignant pleural mesothelioma. This treatment yielded an objective response when used for the mesothelioma of omentum.
The definitive treatment for SFT of omentum is the surgical resection of the tumor with a negative margin. Post-op long term follow up is essential as these tumors commonly recur. Malignant tumors are aggressive, locally invasive with increased growth and metastasis. Therefore post-op chemotherapy is recommended in malignant tumors. Moreover, half of the malignant SFTs were positive for c-kit, and tyrosine kinase inhibitors like imatinib and sunitinib, which is also effective for GISTs, have been used SFTs too. Tumor greater than 10 cm has increased malignant potential, the worse outcome for metastasis, and increased recurrence.
The differential diagnosis of primary tumors of omentum is
These tumors can be mistaken for pelvic tumors, ovarian tumors, and GISTs.
The prognosis of the trunk's liposarcomas, including the intra-abdominal region, is worse than liposarcoma of the extremities. In one study mean 5-year survival rate for liposarcoma of the trunk was 41.9% as compared to that of extremities, where it was 66.7%. The prognosis is also dependant on the histological subtype of the liposarcoma. The 5-year disease-free survival is worse for tumors of the high-grade group (dedifferentiated, round cell, and pleomorphic) than for low-grade tumor groups (well-differentiated and myxoid type).
Because of the rarity of omental EGISTs, their malignant potential and overall prognosis is difficult to ascertain. Compared to conventional GIST, EGIST has a less favorable prognosis because such tumors are accompanied by high mitotic indices, large size, and distant metastasis.
The prognosis of malignant mesothelioma of omentum is extremely poor because effective treatment is still not present. Most patients die in one year of diagnosis. According to some retrospective studies, the median survival after surgery aimed at palliation combined with systemic or intraperitoneal chemotherapy is about one year. In patients where aggressive surgery with hyperthermic intraperitoneal chemotherapy was done, the median survival has approached five years.
Intraabdominal rupture of the tumor resulting in the acute abdomen has been noted in some patients.
As primary tumors of the omentum are rare, knowledge about such tumors' behavior is still lacking, and it needs more cases and research.
Primary omental tumors are sporadic tumors. These tumors are either asymptomatic or signs and symptoms of presentation are nonspecific, which only appears late due to the tumor's mass effect. Imaging modalities like CT scans, largely diagnose them. To treat such tumors and improve the outcomes, more research is needed to know about the etiology, progression, and best treatment options in such tumors. Although surgeons are the main caregiver in managing these patients, radiologists play a vital part in diagnosing patients with these rare tumors. Other health professional staff involved are providers dealing with patients' comorbid conditions if present, nurses, paramedics, and rehabilitation, and social workers who help patients in the post-op period.
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