Hibernoma of the small bowel mesentery
Asymptomatic incidental finding of an abdominal soft tissue mass on an external CT. Patient proceeded to an MRI.
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Deep within the pelvis, within the fat, there is a 10 cm sized mass.
The mass has an insinuating contour around surrounding structures, but itself appears well circumscribed with a thin capsule or pseudocapsule. It is mixed soft tissue and fat density, with extensive nodular enhancement internally within the soft tissue components.
- Posteriorly, the mass abuts the mesorectal fascia, without infiltration into the perirectal space.
- Right laterally, the mass abuts the right internal iliac vessels against the right lateral pelvic side wall.
- Inferiorly, the mass abuts the seminal vesicle, which is displaced inferiorly, without infiltration of the structure.
- Superiorly, a loop of distal ileum is draped over the anterior/superior margin of the mass (Key image).
- Posterior/inferiorly, the sigmoid colon is draped over the posterior left lateral margin of the mass, but is separated from the mass by a thin fat plane.
- Superior right laterally, the mass abuts the right external iliac vessels, and a second loop of small bowel abuts the superior right lateral wall of the mass.
The superior rectal branch of the SMA courses through (within) the posterior margin of the mass.
No lymphadenopathy, suspicious bony lesion, or regional metastases in the field of view.
In summary, there is a mixed fat and soft tissue density, intraperitoneal mass centred within the pelvis, which should be considered to be malignant. Most likely representing a liposarcoma and closely related to surrounding structures without evidence of infiltration.
After appropriate work-up the patient proceeded to surgical resection. Intra-operatively the mass was noted to arise from small bowel mesentery and not involving surrounding structures. The mass was successfully excised, along with resection of the adjacent small bowel. Histology revealed a hibernoma (see excerpt from histology report below)
The differential for an intra-abdominal mixed density lesion is vast and includes:
- fat necrosis
- benign lesions, e.g. hibernoma
- malignant lesions, e.g. liposarcoma
MRI is the modality of choice when attempting to narrow this differential and there are a number of features which can assist with this differentiation. On the T1-weighted sequence hibernomas will demonstrate slightly decreased signal intensity compared to subcutaneous fat, and on the fat-suppressed T2-weighted images areas of incomplete signal suppression may appear.
Treatment with surgical excision is usually curative. Histological subtypes include the more common 'typical' variant and less commonly 'myxoid', 'lipoma-like' and 'spindle cell' variants.
Microscopic histology: (formal report)
"Small bowel resection: The sections show a hibernoma. There are some areas
which are very lipid rich but elsewhere the tumour is predominantly composed of
brown fat cells. There is no cytological atypia. The whole lesion is well
circumscribed. Tumour does extend through the muscularis propria into the
submucosa but is predominantly located in the mesentery. Tumour appears clear
of the margins. There are no lymph nodes identified. The overlying small
bowel mucosa is normal."
- Liu W, Bui MM, Cheong D, Caracciolo JT. Hibernoma: comparing imaging appearance with more commonly encountered benign or low-grade lipomatous neoplasms. Skeletal radiology. 42 (8): 1073-8. doi:10.1007/s00256-013-1583-x - Pubmed
- Furlong MA, Fanburg-Smith JC, Miettinen M. The morphologic spectrum of hibernoma: a clinicopathologic study of 170 cases. The American journal of surgical pathology. 25 (6): 809-14. Pubmed