IMAGING FINDINGS Figure 1
4 varieties – typical, myxoid (high water content), lipoma-like (containing adult fat, common in the thigh), and spindle cell 1,2 .
Well-circumscribed heterogenous T1 hyperintense right adductor intra/extramuscular mass with fat signal characteristics. Figure 2 Well-circumscribed heterogenous T2 hyperintense right adductor intra/extramuscular mass with fat signal characteristics. Figure 3 Well-circumscribed heterogenous STIR hyperintense right adductor intra/extramuscular mass. Figure 4 Well-circumscribed heterogeneously enhancing right adductor intra/extramuscular mass. Figure 5 Well-circumscribed heterogeneously enhancing right adductor intra/extramuscular mass.
Radiographs, Ultrasound with doppler interrogation, CT, and MRI are viable modalities to help differentiate hibernomas. On radiographs, hibernomas often appear as radiolucent masses without mineralization or osseous abnormalities. On ultrasound, hibernomas present as well-circumscribed, hyperechoic masses, with Doppler demonstrating hypervascularity. Arteriovenous (AV) shunting within hibernomas has been reported. AV shunting as well intrinsic hypervascularity, contraindicates core needle biopsy, especially involving deep lesions. The characteristics of hibernomas on imaging likely parallel the histological variants described earlier. Typical hibernomas demonstrate multiple prominent branching serpentine vascular structures on CT and MRI with low signal intensity on all MR pulse sequences. Lipoma- like (or spindle cell if located within the neck) hibernomas appear identical to fat on CT and MRI but with branching vascular structures. Myxoid type hibernomas, meanwhile, present with high water content 1,2 . Definitive treatment is resection. However, in terms of surgical resection in our case, the mass was also displacing muscle. Along the proximal deep lateral margin of the mass, one of the branches of the obturator nerve appeared to course directly into the tumor and was not salvageable. Additionally, there was an unusual area of fat collection at this same site. This collection was sent as a separate specimen to ensure that all of the tumor was resected and that no residual tumor tracked proximal along the obturator nerve. Pathology confirmed that there were fragments of hibernoma within the separate specimen. These lesions, however, have not been shown to recur, metastasize or develop into malignancies in the literature 1-3 . REFERENCES 1. Murphey MD, Carroll JF, Flemming DJ, Pope TL, Gannon FH, Kransdorf MJ. From the archives of the AFIP: benign
Figure 6 Well-circumscribed hypodense right adductor intra/ extramuscular mass.
Figure 7 Axial FDG-PET/CT fused images demonstrating significant FDG uptake within the right adductor musculature. Figure 8 3D Coronal PETCT MIP with significant abnormal uptake within the region of interest (Right Adductor Musculature) with SUV 18.7. Physiologic radiotracer uptake within the left ventricle myocardium, renal collecting system and the urinary bladder. DIFFERENTIAL DIAGNOSIS
musculoskeletal lipomatous lesions. Radiographics. 2004;24(5):1433-1466. doi:10.1148/rg.245045120
1. Hibernoma 2. Liposarcoma 3. Nonspecific Lipomatous Mass 4. Rhabdomyoma 5. Resolving hematoma FINAL DIAGNOSIS: HIBERNOMA DISCUSSION
2. Furlong MA, Fanburg-Smith JC, Miettinen M. The morphologic spectrum of hibernoma: a clinicopathologic study of 170 cases. Am J Surg Pathol. 2001;25(6):809-814. doi:10.1097/00000478-200106000-00014 3. Fnini S, et al. Hibernoma of the thigh. Ann Chir Plast Esthet 2011 Apr; 56(2):160-2. ACKNOWLEDGMENTS Mitchell Ta is a 4th year Medical Student at Tulane University School of Medicine in New Orleans, La. Ahmed T. Rashad MD is a PGYIII Resident in the Department of Radiology at Tulane University School of Medicine in New Orleans, La. Neel Dewan Gupta MD is a clinical and academic musculoskeletal radiologist in New Orleans and serves as a clinical assistant professor within the Department of Radiology at the Tulane University Medical Center. Jeremy Nguyen MD, FACR is clinical radiology professor within the Department of Radiology at the Tulane University Medical Center. Donald Olivares, Digital Imaging Specialist and Graphic Designer. ■
Hibernomas, named for resembling the brown fat of hibernating animals in 1914, are rare, benign fatty lesions that arise from vestigial fetal brown fat 1 . They are often painless and slow- growing. On physical exam, hibernomas are typically mobile, pliable masses that are warm to the touch, secondary to their hypervascularity. Macroscopically, they resemble lipomas in that they are largely well-circumscribed, encapsulated fatty lobulated masses that typically measure 5 to 10 cm in diameter. In terms of their incidence, hibernomas are most often seen in the 3rd or 4th decades of life with a slight female predominance. Hibernomas appear where normal brown fat occur, most commonly in the thigh 3 , occasionally in the shoulder, back, neck, chest, arm, and rarely within the retroperitoneum. Histopathologically, hibernomas have been categorized into
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