Image of the Month

Figure 1. Contrast-enhanced computed tomographic scan of the abdomen and pelvis demonstrating a large, well-circumscribed, homogeneous mass anterior to the right iliopsoas muscle with medial displacement of the right colon and the small bowel. Scale is in centimeters.

Figure 2. Intraoperative photograph demonstrating the intra-abdominal component of the mass with lateral displacement of the iliofemoral neurovascular bundle.

The preoperative diagnosis of a retroperitoneal tumor of adipose tissue may represent a particularly challenging endeavor. Primary retroperitoneal lipomas are rare benign tumors that usually present as an abdominal mass or with patient complaints of pressure symptoms on adjacent organs. Histologically, they originate from mature adipose tissue of the retroperitoneum, mesentery, or the Gerota fascia.¹ Although they are relatively more common in adults, they can occur in infants and small children.² Retroperitoneal lipomas may also undergo malignant transformation to sarcomas.¹ Classic lipomas have CT and magnetic resonance imaging (MRI) signal characteristics similar to subcutaneous fat (between −65 and −120 Hounsfield units).³ Resection is the treatment of choice.

Liposarcoma is the most common soft tissue sarcoma of the retroperitoneum that accounts for most retroperitoneal fatty tumors. The usual presentation is that of a nontender palpable mass. A low grade liposarcoma is difficult to differentiate from a benign lipoma based solely on CT scan or MRI findings, but heterogeneity, areas of enhancement or necrosis, and irregular margins are often seen on the CT scan of a liposarcoma.³ Pathologic examination for mitotic activity, cellular atypia, necrosis, and invasion allows for a definitive diagnosis. Aggressive excision with clear margins remains the only effective treatment.⁴

A retroperitoneal lipoblastoma is a rare benign neoplasm of infancy and early childhood that usually manifests as a slowly growing asymptomatic mass. The characteristic microscopic appearance includes variable amounts of mature lipocytes, lipoblasts, and poorly differentiated mesenchymal cells in a myxoid stroma. Complete excision is the treatment of choice, although there is a reported 25% recurrence rate.⁵

Retroperitoneal hibernomas are extremely rare, slowly growing benign tumors of brown fat. Microscopically, hibernomas consist of multivacuolated cells, small eosinophilic cells, and univacuolated adipocytes.⁶ Although the CT scan and MRI findings cannot always differentiate a hibernoma from other adipose tissue tumors, increased vascularity (enhancement) and higher attenuation numbers similar to muscle (brown fat) may be suggestive of the diagnosis.³ Total excision is the treatment of choice.⁷

Corresponding author and reprints: Ioannis Raftopoulos, MD, PhD, 856 W Nelson St, Apt 1002, Chicago, IL 60657 (e-mail: iraftopoulos@hotmail.com).