Resection of liposarcoma of the greater omentum: A case report and literature review

Highlights • Intraabdominal liposarcoma including omental liposarcoma is rare.• Liposarcoma can be difficult to distinguish from other lipomotous tumor.• We surgically managed a case of liposarcoma of omentum ovserved as lipoma 3 years ago.


Introduction
Liposarcoma is one of the most common soft tissue sarcomas. It accounts for approximately 10% of all soft tissue sarcomas, and its peak incidence occurs around the fifth to sixth decades of life [1]. Intra-abdominal liposarcoma, including omental sarcoma, is rare [2]. We herein report a surgical case of liposarcoma of the greater omentum along with a review of the literature. It is reported in line with the PROCESS criteria [3].

Presentation of case
A 60-year-old woman underwent screening blood tests, which revealed a high serum amylase level. Computed tomography (CT) showed an 8-× 8-× 4-cm abdominal mass on the cranial side of the bladder, an intra-abdominal lipoma was suspected and observed ( Fig. 1).
Three years later, she underwent a medical examination, and an elastic hard mass with slight tenderness was palpated in the abdomen. She had no abdominal pain, nausea, constipation, or other symptoms. Contrast-enhanced CT and magnetic resonance imaging (MRI) showed a large, well-defined abdominal mass with low attenuation and fat density measuring 20 × 17 × 7 cm. The mass was adjacent to another abnormal region measuring 6 × 6 × 5 cm with septae and a capsule. Contrast-enhanced CT also revealed omental artery involvement in the mass, and an omental tumor was suspected. MRI showed no evidence of invasion to other organs, including the digestive tract, bladder, or great vessels. A liposarcoma (smaller region) with a lipoma (larger mass) was suspected, and no metastatic lesions were observed (Fig. 2a, b). After the conference of the surgeons and physicians, confirming the possibility of the tumor resection being with other structures in case of invasion, the patient consented the plan for the surgery and surgical resection was performed.
Intraoperatively, 20 cm midline incision of the abdominal wall was made. A huge, yellowish soft mass with a dark reddish-gray region adjacent to the mass was found under the abdominal wall   without invasion, including colon, intestine, mesentery, abdominal wall, bladder, uterus, and retroperitoneum. After ligation of the feeder vessel originating from the omental artery, existing cranial side of the mass, the mass was resected en bloc ( Fig. 3a, b). No evidence of intra-abdominal metastasis was found. The surface of the mass was carefully treated and not raptured.
The resected specimen, which comprised a yellowish mass and reddish-gray region, weighed 3750 g and measured 27 × 20 × 10 cm (Fig. 4a, b). Histopathological examination showed that within the reddish-gray region (black arrowhead in Fig. 4b), neoplastic spindle cells with atypical nuclei containing condensed chromosomes were present in the septae (Fig. 5a). Lipoblasts and inflammatory cells were present. Necrosis of fat was also observed. Near the reddish-gray region in the yellowish mass (black arrow in Fig. 4b), malignant cells were also seen (Fig. 5b). Far from the reddish-gray region in the yellowish mass (white arrow in Fig. 4b), mature adipocytes with uniform nuclei resembling normal fat tissue were observed (Fig. 5c). Immunohistochemical analysis revealed the MDM2+/CDK4+ immunophenotype (black arrowhead and black arrow in Fig. 4b) and the MDM2−/CDK4− immunophenotype (white arrow in Fig. 4b). The diagnosis was a well-differentiated liposarcoma (black arrow and black arrowhead in Fig. 4b) and a lipoma (white arrow in Fig. 4b) based on the histopathological features. The resected margin was microscopically negative (R0).
The patient had an uneventful postoperative period. No recurrence or significant problems had occurred at 9 months postoperatively.
The prognosis of a liposarcoma in the trunk, including the intra-abdominal region, retroperitoneum, and thoracic cavity, is worse than that of a liposarcoma in the extremities. In one study, the median 5-year disease-free survival rates for liposarcomas in the trunk vs. extremities were 41.9% vs. 66.7% (P < 0.001), and the 5-year overall survival rates were 64.5% vs. 84.5% (P < 0.001), respectively [11]. The prognosis of these tumors also depends on the histological subtype. The 5-year disease-free survival rate is worse for dedifferentiated, round cell, and pleomorphic liposarcomas (high-grade group) than for well-differentiated and myxoid types (low-grade group) (16.9% vs. 65.7%, P < 0.001); the 5-year overall survival rate is also worse (47.8% vs. 83.5%, P < 0.001) [11,12].
Primary omental liposarcoma is rare, and De et al. [13] reviewed nine cases from 1936 to 2003. Since that review, 10 cases were reported in the English-language literature from 2003 to 2018, including our case. These 19 cases were reviewed in the present study ( Table 1). The average age of the patients was 51.1 years (range, 11-83 years). Although our patient was asymptomatic, the patients in previous reports exhibited various symptoms including abdominal pain, swelling, fever [13], constipation [14], and abdominal distention [15]. Cases of liposarcoma of the greater omentum presenting as inguinal hernia and torsion have also been reported [16]. Our patient developed ischemic change in the reddish-gray, elastic hard region of the mass. We considered that this ischemic mass may have developed by torsion; nevertheless, the patient had no episodes of acute abdominal pain. Complete tumor resection is recommended for greater omental liposarcoma [16]. Our review showed that the tumors in 15 patients (78.9%) were resectable and that wide tumor resection was needed for 4 patients (26.7%). In terms of histologic subtypes, three tumors (15.8%) were well-differentiated, five (26.3%) were myxoid, three (15.8%) were pleomorphic, four (21.1%) were round cell, one (5.3%) was dedifferentiated, and three (15.8%) were not classified. Although our review of these 19 cases included a long-term survivor (13 years) with myxoid liposarcoma, 6 patients died during the follow-up period. A recent study suggested that postoperative radiation therapy may improve outcomes in patients with retroperitoneal liposarcoma, especially for subtypes other than well-differentiated tumors [17]. Because of the risk of radiation enteritis and the rarity of the disease, adjuvant radiation for omental liposarcoma remains controversial. Although adjuvant chemotherapy also remains controversial, chemotherapy seems promising in the treatment of liposarcoma [18][19][20]. (b) Malignant cells were also seen in the area near the reddish-gray region in the yellowish mass (black arrow in Fig. 4b). (c) The area far from the reddish-gray region in the yellowish mass (white arrow in Fig. 4b) was composed of mature adipocytes with uniform nuclei resembling normal fat tissue. Liposarcoma often has different histological components including both benign and malignant areas [21,22]. The tumor in the present case had two sections: a reddish-gray area with ischemic change and a huge, soft fatty yellowish mass. Liposarcoma was diagnosed in the reddish-gray area, and the near side of the fatty yellowish mass was formed by lipoblasts. Mature fat cells were observed in the far side of the yellowish mass, and this region was diagnosed as lipoma. A recent study suggested biologic potency of transformation of benign lipoma into welldifferentiated liposarcoma [23,24]. Nevertheless, the pathogenetic concept of liposarcoma arising from benign lipoma is generally not accepted [24,25].
The CT and MRI appearances of a well-differentiated liposarcoma are similar to those of normal fat and other abdominal tumors [21]. A well-differentiated liposarcoma is characterized by a lesion size of >10 cm, the presence of thick septa, the presence of globular and/or nodular non-adipose areas or masses, and a lesion component of <75% fat [26]. Resection should be considered for huge intra-abdominal lipomatous tumors.

Conclusion
Liposarcoma of the greater omentum is rare, and 19 cases were reviewed. Differentiation of liposarcoma from other tumors is challenging. Adjuvant therapy has not been established as an effective therapy, and radical resection of the tumor is recommended.

Funding
This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

Ethical approval
This is a case report and it did not require ethical approval from ethics committee. We have got permission from the patient to publish.

Consent
Written consent to publish this case report was obtained from the patient.

Author contribution
Shintaro Hashimoto, Junichi Arai, and Hidetoshi Fukuoka were responsible for the study concept and performed the operation. Masato Nishimuta, Hirofumi Matsumoto, Masashi Muraoka, Masahiro Nakashima, and Hiroyuki Yamaguchi collaborated in the patient's medical care. Hiroyuki Yamaguchi reviewed the manuscript. All authors approved the final article.

Registration of research studies
Not Applicable.

Provenance and peer review
Not commissioned, externally peer-reviewed.

Declaration of Competing Interest
None.