Gastrointestinal stromal tumors are currently believed to originate from the interstitial cells of Cajal, a pace-maker cell that controls GI track peristalsis and the only GI track cell that exhibits the CD117+
immunophenotype, which is the diagnostic hallmark of GIST.13
The tumors can occur anywhere that these cells exist in the gastrointestinal tract, including the stomach (40–60%), small intestine (30–40%), ano-rectum (7%), colon, and esophagus.1,2
Sakurai et al. reported in the normal omentum CD117+/CD34+ mescenchymal cells, like Cajal cells, from which the EGIST may theoretically arise.
The median age at diagnosis of GISTS is about 60 years and is slightly more prevalent in males than in females.1,2,5
In a Medline search, we found 54 omental EGISTs reported in the form of small series or isolated cases.5,9,14–40
According to these cases, the median age at diagnosis is 65 years, with a male female ratio 1:1. Also, we ascertained that there is no difference in incidence between lesser and greater omentum.
Miettinen et al.
reported that only 3% of Gists are diagnosed before the age of 21 years and GISTs arise only rarely in children.12
Omental EGISTs can remain clinically silent despite the large tumor size. The most frequent presenting complaint is an abdominal mass, but patients are often diagnosed incidentally during investigations for other medical conditions.
The omental GIST in our patient showed no myogenic or neural differentiation of myogenic features and neural attributes. Only 16 omental GISTs without myogenic or neurogenic features, including the present case, have ever been reported ().
Reported cases of lesser omentum gastrointestinal stromal tumor.
The tumor arose in 7 men and 8 women ranging in age from 31 to 89 years. Tumor diameter ranged from 2.5 to 36 cm (median 15.7 cm). Macroscopically, most were large, solid masses exhibiting cystic changes.
The radiological features of omental GISTs without myogenic or neurogenic features have not been established. Generally, they may be similar to those of omental leiomyomas and leiomyosarcomas. Most GISTs with myogenic features are demonstrated as hypervascular tumors with clear margins on CT and angiography.41–43
It is difficult to differentiate a GIST in the lesser omentum from a GIST in the lesser curvature of the stomach, despite the use of advanced radiological imaging techniques. About half of all omental GISTs are misdiagnosed as extra mucosal tumors of the stomach.41,42,44
Additionally, omental EGISTs seem to be morphologically and immunohistochemically identical to their gastric and intestinal counterparts. They are cellular tumors consisting of elongated spindle and epithelioid cells that are typically positive for c-kit (CD117) and, less consistently, for CD34. They may show smooth muscle actin positivity but are negative for desmin and S-100 protein.
C-kit (CD117) may be negative in GISTs (2–5%), either due to limited sampling in the tumor with focal variation or, more rarely, due to a unique subset of CD117-negarive GIST with epithelioid morphology. CD34 strongly and diffusely stains approximately 70% of GISTs. The other 30% of GISTs have patchy weak to moderate intensity staining. In GISTs with neural differentiation, fewer cells stain postive for CD34 and the intensity is less compared to GISTS with smooth muscle differentiation. Most GISTs are desmin negative. Strong desmin staining occurs in approximately 2% of GISTs, although approximately 33% stain focally and weakly. S-100 protein stain the cytoplasm and/or nuclei focally in approximately 50% of neoplasms.45–47
Due to the rarity of omental EGISTs, there are no specific treatment data from clinical trials and surgical resection is the only effective modality, and their management follows the guidelines applicable to classical GISTs.3
It should be noted here that, according to the National Institutes of Health algorithm for assessing malignancy of classical GISTs,4
most omental EGISTs would be classified as high-risk
due to their large size alone, as in at least 55% of published cases it exceeds 10 cm. However, the tumor size is not a reliable prognostic parameter in the case of omental EGISTs.