When imaging tells the whole story: A pediatric omental mass from scan to surgery - A case report

INTRODUCTION

Lipoblastoma is a rare benign tumour of embryonal fat cells seen in young children. It usually presents as a painless, progressive swelling with symptoms due to mass effect. On imaging, it appears as a well-defined, fat-containing solid mass, most often located in the extremities. Intra-abdominal cases are uncommon and typically involve the retroperitoneum, while omental origin is extremely rare. Here we present a case, where the lesion’s location and characteristic mass effect on cross-sectional imaging was clue to its greater omental origin.

CASE REPORT

A 1-year-old boy presented to our tertiary care center with a history of progressive abdominal distention for 4 months, with recent worsening of abdominal pain and fullness. He also complained of a loss of appetite associated with a loss of weight. On clinical examination, he had a grossly distended abdomen with a vague mass palpable per abdomen. The rest of the examination was unremarkable, with no palpable lymph nodes or hepatosplenomegaly. He was evaluated elsewhere for these complaints and underwent an ultrasound that documented a large, solid abdominal mass.

The patient’s basic laboratory parameters were within normal limits. Imaging evaluation at our institution began with an initial sonographic assessment of the abdomen, which revealed a large, homogeneous, hyperechoic mass occupying the abdominal cavity and exerting mass effect on adjacent structures [Figure 1].

A subsequent contrast-enhanced computed tomography (CT) scan of the abdomen demonstrated a large, lobulated mass with predominantly fat attenuation, interspersed with thin, enhancing septae and small isoattenuating solid areas [Figure 2]. The mass extended superiorly to the transverse colon and the left lobe of the liver, while inferiorly, it reached into the pelvis, abutting the dome of the urinary bladder. Anteriorly, it contacted the peritoneal cavity. The lesion itself was positioned anterior to the small bowel loops, which were displaced posteriorly.

There was no evidence of extension into the retroperitoneal space or the lesser omentum. No infiltration into surrounding structures was observed. In addition, the lesion lacked calcifications, and significant lymphadenopathy was absent. Based on its location and the observed pattern of mass effect, the imaging findings strongly suggested that the tumor had originated from the greater omentum.

For a lipomatous mass originating from the greater omentum in a pediatric patient, the primary differential considered was a simple lipoma. However, the progressive growth of the mass and the presence of solid components contradicted this diagnosis. The clinicoradiological picture of a lipomatous mass with a history of progressive enlargement suggested either a lipoblastoma or a myxoid liposarcoma. However, the lesion’s location was atypical for lipoblastomas, which are predominantly found in the extremities. In addition, liposarcoma is extremely rare in pediatric patients. Finally, a dermoid cyst was considered; however, this was deemed less likely due to its unusual location and the absence of internal calcifications. The radiological differentials are summarised in Table 1.

The patient underwent laparotomy, during which intraoperative findings revealed a large fatty lipomatous mass originating from the greater omentum. The mass shared a common blood supply with an adjacent ileal loop. Surgical intervention involved excision of the omental mass, along with resection and anastomosis of the affected segment of the ileum.

Histopathological examination confirmed the diagnosis of omental lipoblastoma, consistent with the imaging and surgical findings [Figure 3].

After reassuring the family regarding the benign nature of the disease, the child was followed up after 1 month and was noted to be doing well with a healthy scar and no palpable mass in the abdomen.

DISCUSSION

Lipoblastoma is a rare, benign neoplasm of immature adipose cells, seen almost exclusively in the pediatric age group,^[1] typically within the first 3 years of life, with a male preponderance.^[2] Although it lacks malignant potential, lipoblastoma can exhibit local aggressiveness and has a propensity for post-surgical recurrence. A diffuse, non-encapsulated, infiltrative variant – termed lipoblastomatosis shares pathological similarities with lipoblastoma.^[3]

Lipoblastoma most commonly occurs in anatomical sites with large amounts of immature fat, including the subcutaneous plane of the extremities, the head-and-neck region, and the trunk. Intra-abdominal involvement is rare, predominantly affecting the retroperitoneum. The omental location of this tumor is exceedingly uncommon.^[3] The hallmark of lipoblastoma is its painless, progressive enlargement, with symptoms arising secondary to mass effect. While typically asymptomatic, rare complications such as torsion^[4] and volvulus^[5] have been documented.

On imaging, lipoblastoma, being a predominantly fat-containing solid mass, appears hyperechoic lesion on ultrasound, demonstrates low attenuation on CT, and appears hyperintense on T1-weighted and T2-weighted magnetic resonance imaging (MRI) sequences, with signal suppression on fat-suppressed sequences. Ultrasound serves as the primary imaging modality, and prior awareness of this entity can help prevent misdiagnosis. Cross-sectional imaging, including CT and MRI, is often required to evaluate the tumor’s internal characteristics and determine its extent.^[6]

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Figure 1:

A 1 year old boy with omental lipoblastoma. Ultrasound abdomen with (a) low frequency and (b) high frequency examination shows a large, predominantly hyperechoic solid lesion (orange arrows) occupying the entire abdominal cavity. (a) The bowel loops are seen displaced posterior to the mass.

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Figure 2:

A 1-year-old boy with omental lipoblastoma, Contrast enhanced CT abdomen (a) axial and (b) coronal sections shows a large, lobulated mass that fills the entire abd omen. It is of predominantly fat attenuation (yellow ROI) with few thin enhancing septae (curved orange arrow) and solid areas of iso-attenuation (blue ROI) within. Superiorly abuts the left lobe of the liver and inferiorly abuts the dome of the urinary bladder. The lesion itself lies anterior to the small bowel loops (orange arrow), which are displaced posteriorly.

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Figure 3:

A 1 year old boy with omental lipoblastoma. (a-b) Gross specimen shows a globular, well-encapsulated, soft tissue mass (orange arrow) with a yellow fibrofatty cut surface and no areas of necrosis or hemorrhage (orange circle). (c-d) Histopathology in x200 (c) and x400 (d) magnification shows a multilobulated tumor composed of sheets of predominantly mature adipocytes (orange circle) of varying size with interspersed aggregates of signet ring-like lipoblasts (purple circle).

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However, due to the variable presence of internal septations and enhancing non-fatty solid components, imaging alone cannot always definitively differentiate lipoblastoma from other fat-containing lesions.

The most common radiological mimic of lipoblastoma is myxoid liposarcoma, an aggressive malignant entity. However, liposarcoma is extremely rare in children under 10 years of age, making lipoblastoma the most likely diagnosis for a lipomatous mass in this age group, regardless of solid component presence.^[7] Other differentials include lipoma, which is rarely seen in childhood, and teratoma, which can occur in similar locations. However, teratomas typically exhibit calcifications, a feature not observed in lipoblastoma.^[8]

Histologically, lipoblastoma consists of a spectrum of lipoblasts, mesenchymal cells, myxoid stroma, and mature adipocytes, organized into lobules separated by fibrous septa. In contrast, myxoid liposarcoma exhibits nuclear atypia and pleomorphism, which are absent in lipoblastoma.

In cases where histological differentiation is challenging, cytogenetic analysis can be employed. Lipoblastoma is characterized by a rearrangement of the long arm of chromosome 8 (8q11–13), involving the PLAG1 oncogene.^[9] In contrast, liposarcomas demonstrate a chromosomal translocation t(12;16)(q13;p11).^[10]

Complete surgical resection with tumor-free margins is the preferred treatment, given the high potential for local recurrence. MRI is recommended for post-operative surveillance, particularly in cases of incomplete resection. Current guidelines suggest a follow-up period of at least 5 years.^[2]

DIFFERENTIAL DIAGNOSIS

Differential diagnosis considered for a lipomatous omental mass [Table 1].

CONCLUSION

Omental lipoblastoma is an exceptionally rare pediatric abdominal neoplasm. Cross-sectional imaging, is critical for accurate characterization, localization, and assessment of mass effect. Recognition of its imaging features facilitates differentiation from other fat-containing lesions and informs surgical planning. Complete surgical excision is curative, and imaging guides both preoperative planning and postoperative follow-up.

TEACHING POINTS

1. Proper knowledge of the peritoneal anatomy is essential. As in our case, the location of the lesion and mass effect exerted gave us clues to its organ of origin.

2. Lipoblastomas appear as lipomatous solid masses with interval growth and are indistinguishable from liposarcoma; however, liposarcoma is extremely rare in an age group of <10 years.

3. Although lipoblastomas are commonly seen in the extremities and the head and neck, it is important to be aware that they do present in unusual locations, such as the omentum, as in our case.