CASE REPORT

Fibroma of the Omentum Resembling an Ovarian Tumorin the Pelvis

Masanori Ono,1 Kyoko Tanaka,1 Nana Ikezawa,1 Yusuke Kobayashi,1 Yoko Inoue,1 Megumi Yokota,1 Satoko Matsumura,1Ichiro Uehara,1Yoshihisa Hattori,1 Takashi Kurahashi,1 Tetsuya Shimada2 and Hiroyuki Nakagawa1
1Department of Obstetrics and Gynecology,
National Hospital Organization Saitama National Hospital, Saitama, Japan
2Department of Pathology, National Hospital Organization Saitama National Hospital, Saitama, Japan(Received for publication on May 9, 2009)(Revised for publication on June 7, 2009)(Accepted for publication on June 25, 2009)

Abstract
We would like to report the case of a patient with fibroma of the omentum that resembled anovarian tumor in the pelvis. Since primary tumours of the omentum are rare, there is a paucity
of information about the biology of such tumors in the basic texts and literature. An ultrasoundexamination of the patient revealed a mass, likely of ovarian origin, which consisted of liquidand solid components. It was suspected to be a malignant ovarian tumor. However, laparotomy
demonstrated it was an omental tumor. This case shows that it can be difficult to pre-operatively diagnose omental fibromas because of their close resemblance to ovarian tumors. (KeioJ Med 58 (4) : 234－236, December 2009)

Keywords: omental fibroma, omentum, surgery, ovarian tumor, pelvic mass

Introduction
Fibromas are rare tumors that consist of dense collagenbundles and a variable number of mature fibroblasts. Tumors of the omentum also occur infrequently, and only a
few cases of omental fibroma have been reported previously.1 We recently experienced a case in which MRIand ultrasonography (US) analyses suggested the presence of a malignant ovarian tumor. However, laparotomy
revealed it was a fibroma of the omentum. Thus, since
primary tumours of the omentum are rare and there is a
paucity of information in the basic texts and literature regarding their biology,1, 2 fibromas of the omentum may
be readily misdiagnosed as more common ovarian tumors.
Case Report
A 32-year-old woman was admitted to the general
medicine service with complaints of leg edema and anemia. Physical examination revealed a large and tender

pelvic mass. The mass was smooth and mobile. It was
soft and measured about 14 cm at its widest diameter. Achest X-ray showed no evidence of metastatic involvement. Gastrofiberscopy and colonoscopy did not detect
any tumors within the gastrointestinal tract. Based onthese findings, an ovarian tumor was suspected. All laboratory findings were normal except for low hemoglobin
and mildly elevated CA125 levels (hemoglobin 10.1g/dl, CA125 54.2 U/ml). To characterize the mass, US
imaging of the pelvis was performed. This revealed amass with cystic and solid region (Fig. 1). MRI revealed
a well-circumscribed mass with cystic and solid part(Fig. 2). Upon laparotomy, a firm, mobile mass with increased vascularity was found in the greater omentum.
There was no adjacent organ or metastatic involvement.
Invasion around the mass was not observed. Total surgical resection was performed (Fig. 3). We obtained an intraoperative frozen section and the result was a benign
omental fibroma. The bilateral adnexae were intact. Thetumor was excised with adequate margins and an omentectomy was not performed. The gross specimen mea-Reprint requests to: Masanori Ono, MD, Department of Obstetrics and Gynecology, National Hospital Organization Saitama National Hospital, Tel:
+81-48-462-1101, Fax: +81-48-464-1138, E-mail: masanori@sc.itc.keio.ac.jp234

Keio J Med 2009; 58 (4): 234－ 236

Fig. 1 US showing the pelvic cystic mass with a papillary region(bar, 4 cm).

Fig. 2 MRI showing the well-circumscribed mass with apapillary region.

sured 14 x 10 cm. It consisted of a single cystic structure
containing reddish-brown serous fluid and lacked central
necrosis on its cut surface. Microscopically, the mainpart of this tumor consisted of fibroblasts arranged incollagen bundles (Fig. 4). The pathologist classified thetumor as an omental fibroma with endometriotic cyst.
The patient had an uneventful recovery. She remains
well with no evidence of tumor recurrence three years
after the resection. This study was approved by the Ethics Committee of the hospital, and the written informed
consent was obtained from the patient.

235

Fig. 3 Picture of the total surgical resection.

Fig. 4 The mass was composed of moderately cellular andfibrotic tissue with spindle tumor cells interspersed among
bundles of dense collagen (H.E. x 40).

Discussion
Considering the widespread distribution of connective
tissue throughout the body, fibromas are a surprisingly
rare tumor. Despite the proliferation of non-invasive imaging studies, the incidence of fibromas remains low.3
Fibromas are usually small tumors that are firm, encapsulated, and pearly gray on cross section.4 They are composed of typical spindled fibroblasts that are packed
closely together with collagen. On microscopic examination, our case showed the typical histopathological morphology, namely, spindled fibroblasts and scant interven-

Ono M, et al: Omental Fibroma Suspected to be an Ovarian Tumor236

ing collagen deposits.
Fibroma of this case was similar to an ovarian fibroma
but differed from a soft tissue fibroma such as of the skinor tendon. This omental fibroma has oval or short spindle-shaped nuclei with very little atypia. Omental fibroma also has spindle-shaped cells with a small amount ofcytoplasm proliferating in a disorganized manner. The
growth pattern is fascicular or storiform. In some cases,
intercellular collagen fibers form in a wave-like patternwith hyalinization. Cytoplasm could include a small
amount of lipids, but nuclear division is rare. It is possible to use α-inhibin, CD99, and calretinin for demonstration of ovarian derivation; smooth muscle actin and desmin for differentiation from smooth muscle tumors; andc-kit, CD34, S100 for differentiation from GIST.
In this case, the mass was smooth and mobile. All laboratory findings were normal except for low hemoglobin
and mildly elevated CA125 levels (hemoglobin 10.1
g/dl, CA125 54.2 U/ml). Retrospectively, the reason ofslight anemia was considerd due to hypermenorrhea, andthe elevation of CA125 was occurred due to endometriosis. To characterize the mass, MRI revealed a well-circumscribed mass with cystic and solid part.
Primary peritoneal, omental and mesenteric tumors arerare. The incidence of fibromas varies; fibroma of tendon
sheath, for example, is a comparatively common tumor,
but the incidence of omental fibroma is not known.5 Solid omental and mesenteric tumors have similar pathological features and may arise from any elements found inthese structures.6 In this case, the omental fibroma arosefrom fibrous elements within the omentum. Tumors
originating in the omentum include lipomas, fibromas,
fibromatosis, infantile myofibromatosis, angiofibromas,
myxoid hamartomas, lymphangiomas, mesotheliomas,
neurofibromas, leiomyomas, fibromyxomas, hemangiomas, hemangiopericytomas, elastofibroma, and desmoids.7-9 The incidence of fibromas varies: for example,
while fibroma of the tendon sheath is a comparatively
common tumor, cases of omental fibroma are rare. However, although omental fibromas are unusual, they should
nonetheless be considered in the differential diagnosis ofan abdominal or pelvic mass.
Some fibromatoses (desmoid) occur in the peritoneum(mesentery, greater omentum, and retroperitoneum), anddifferentiation is necessary.6 Macroscopically, they are
similar since they are fibrous tumors with well-definedmargins. Histologically, fibromatosis has infiltrative
growth pattern, and thus, it can be excluded. Anothercharacteristic of fibromatosis is abundant dense collagen.
A desmoplastic fibroblastoma (collagenous fibroma) is a
fibrous tumor which occurs subcutaneously in the limbs

and neck of adults. This type of tumor is also similar because it has well-defined margins. However, a desmoplastic fibroblastoma usually has a low density of spindle-shaped cells interspersed in a collagen-rich stroma.
To our knowledge, few US and MRI analyses of omental fibroma have been reported previously.2, 4 On US, thelesion in our case was clearly demarcated and contained
cystic areas. The imaging results suggested a fibroma,
but they were not sufficiently typical to be diagnostic.
Fibromatosis, inflammatory lesions, desmoids, organizedhematomas, and sarcomas may be confused with one another in a pre-operative evaluation. Even with a characteristic radiological feature, such as the enhanced solid
areas of a leiomyosarcoma, it is nearly impossible to secure a pre-operative diagnosis solely on the basis of a radiological examination. At present, studies evaluating
the utility of fine-needle biopsy in the diagnosis of an
omental tumour have not been performed, which meansthat surgical resection is the only way to obtain a histological diagnosis. Since fibromas rarely recur and nevermetastasize, surgical resection is usually curative.2, 4 Although omental fibromas are very rare, they should beconsidered in the differential diagnosis in pelvic and abdominal lesions. The imaging findings are suggestive ofthe diagnosis but do not seem sufficiently typical to allow a clear distinction from other neoplasms. This caseillustrates the difficulty in pre-operatively diagnosing afibroma of the greater omentum.
References
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