Age / Sex : 40 / M

Chief complaint : pain and numbness of right leg

What is your impression?
Two weeks later, you can see the final diagonosis with a brief discussion of this case (Please submit only one answer).

Courtesy of In Sook Lee, Pusan National University Hospital   

Discussion

Answer:  Hibernoma 

Findings: coronal T1-weighted MR image of pelvis (a) showed a large, well-defined fatty mass with signal intensity identical to that of the adjacent fat. The prominent vascularity (black arrows) within the mass was noted. Right sciatic nerve (white arrows) was displaced by the mass. coronal T2-weighted MR image revealed a similar signal intensity with prominent vessels (arrows). On diffusion weighted image (c), the mass showed dark signal intensity, representing no diffusion restriction. Axial dynamic contrast enhanced MR image (d) showed well-enhancing vessels within the mass. On perfusion MR image (e), the graph curvature represented arterial flow within the mass.  

Differential Diagnosis: angiolipoma, hemangioma, well-differentiated liposarcoma

Discussion:

Hibernomas are rare slow-growing benign lipomatous tumors originating from residual brown fat cells. They affect chiefly adults in the 3^rd and 4^th decades of life. Hibernoma is the only tumor known to occur within brown fat and can grow at any location where brown fat remains. It usually grows in the vestiges, where brown fat is found in fetuses and infants, such as the shoulder, neck, axilla, the periscapular and interscapular area, mediastinum, upper thorax and retroperitoneum. other uncommon locations include the abdomen, thigh, buttock, popliteal fossa and intracranial sites. The AFIP series demonstrates a slight male predominance (58% of cases) with the thigh being the most common location (30% of cases).

From a macroscopic aspect, hibernomas are well-defined, encapsulated soft, lobulated masses and the color ranges from tan to red brown. Microscopically, the tumor is characterized by multivaculoated cells with eccentric nuclei and granular eosinophilic cytoplasm, univaculoated cells with peripheral nuclei, and smaller round cells with granular cytoplasm

CT and MRI features are not specific and vary with the nature and amount of lipid component. three distinct patterns are identified on CT and MR imaging. the typical hibernoma images, similar, but not identical, to mature adult fat. These lesions also show prominent septations, with some lobular growth and serpentine/branching vessels. diffuse hyperintensity is reported throughout the lesion on STIR imaging. Somewhat less commonly, the lipomalike hibernoma presents as a lesion largely composed of fat, with prominent septations and serpentine/branching vessels, with the vascularity demonstrating areas of both high and low flow

Non contrast CT usually demonstrates a well-demarcated soft tissue mass with tissue attenuation intermediate between that of fat and skeletal muscle, depending on the lipid content of the tumor. Internal linear, curvilinear or branching septations-like densities may be contained. On post contrast scans, enhancement of the septa as well as more diffuse uptake, usually occurs.

Incomplete excision results in local recurrence of the tumor; therefore marginal but complete resection is considered as the treatment of choice for these lesions.

References

1. Gupta P, Potti TA, Wuertzer SD, Lenchik L, Pacholike DA. Spectrum of fat-containing soft-tissue masses at MR imaging: The common, the uncommon, the characteristic, and the sometimes confusing. Radiographics 2016;36:753-66


2. Papathanassiou ZG, Alberghini M, Taieb S, Errani C, Picci P, Vanel D. Imaging of hibernomas: a retrospective study on twelve cases. Clin Sarcoma Res 2011;25:3


3. Burt AM, Huang BK. Imaging review of lipomatous musculoskeletal lesions. SICOT J 2017;3:34


4. Kransdorf MJ, Murphey MD. Imaging of soft tissue tumors. 2^nd Lippincott Williams & Wilkins.