Case report 842. Benign hibernoma

A Case Report of Axillary Hibernoma: US, CT, MR, and Histopathologic Findings

Hibernoma is a rare benign tumor of brown adipose tissue. Herein, we report a case of axillary hibernoma in a 53-year-old female and discuss the various radiologic findings. The US revealed a 4.5-cm well-defined oval heterogenous hyperechoic mass in the right axilla with anterior displacement of the axillary vessels. Non-enhanced chest CT showed a 5.0-cm well defined, oval, and low-attenuated mass. MRI demonstrated a 5.5-cm mass with heterogeneous intermediate-to-high signal intensity on T1-and T2-weighted images and irregular enhancement at the peripheral portion. The patient underwent an US-guided core needle biopsy and the final diagnosis was hibernoma.

Hibernoma: MRI features in eight consecutive cases

AIM

To describe the preoperative magnetic resonance imaging findings of eight histologically-proven cases of hibernoma.

MATERIALS AND METHODS

The site, size, and signal characteristics of eight consecutive hibernomas were retrospectively assessed on T1-weighted spin-echo and short T1 inversion recovery (STIR)/fat-saturated T2-weighted fast spin echo magnetic resonance images. Four patients also had gadolinium-enhanced fat-saturated T1-weighted spin echo imaging. Patient age and sex, and duration of symptoms were recorded.

RESULTS

Three female and five male patients with an average age 36 years (range 16-53 years) were included. Seven lesions occurred in the thigh, four in the anterior compartment and three in the posterior compartment. One lesion occurred superficial to the scapula. All cases demonstrated common magnetic resonance imaging findings of a well-defined, heterogeneous mass, slightly or clearly hypo-intense to subcutaneous fat on T1-weighted spin-echo images, with prominent thin low signal bands throughout the tumour. The lesions failed to fully suppress on STIR or fat-saturated T2-weighted images. Only one of the four contrast-enhanced studies demonstrated increased vascularity in the tumour.

CONCLUSION

The MRI findings of a lesion that is diffusely slightly hypointense to surrounding subcutaneous fat, should prompt the operator to consider hibernoma in the differential diagnosis.

Hibernoma--correlation of histopathology and magnetic-resonance-imaging features in 10 cases

OBJECTIVE, DESIGN AND PATIENTS: Hibernoma is an uncommon, slow-growing, benign soft-tissue tumour resembling brown adipose tissue. The histological appearances are well-documented, but there are relatively few descriptions of the magnetic resonance (MR) imaging features. We report a retrospective comparison of the histological and MR appearances of ten hibernomas of the extremities, classified histologically into lipoma-like [<70% multivacuolated adipocytes (MVAs)] and non-lipoma-like hibernomas (>70% MVAs).

RESULTS

The lipoma-like hibernomas measured 4-27 cm in maximum size. All were well-defined on MR imaging and histology except for one subcutaneous lesion that blended in with surrounding fat histologically. All lesions were isointense with subcutaneous fat on T1- and T2-weighting apart from one lesion that was predominantly slightly hypointense on T1-weighting and predominantly slightly hyperintense on T2-weighting and STIR. Two slightly inhomogeneous lesions contained thin (<5 mm thickness) tortuous vessels. One patient received intravenous contrast, but the lesion did not enhance. The six non-lipoma-like hibernomas measured 2.5-15.5 cm in maximum size and all were unencapsulated. Three were well-defined and three partly ill-defined on MR imaging. There were no significant differences in the MR characteristics of the non-lipoma-like variants. On T1-weighting, the non-lipoma-like hibernomas that contained >90% MVAs were predominantly slightly hypointense to subcutaneous fat. One lesion was isointense with subcutaneous fat, but this lesion only contained from 80-90% MVAs. All non-lipoma-like lesions were slightly hyperintense on STIR but so too were two of the four lipoma-like lesions. Four of the six non-lipoma-like lesions showed marked or moderate inhomogeneity due to thick septa and prominent vessels. Of the two cases that received intravenous contrast, both showed enhancement corresponding to regions of >90% MVAs or prominent vessels within fibrous septa.

CONCLUSIONS

MR imaging has shown some distinguishing features between lipoma-like and non-lipoma-like hibernomas. Lipoma-like hibernomas are usually isointense with subcutaneous fat on T1-weighting, are either homogeneous or slightly inhomogeneous and may contain thin tortuous vascular structures. Non-lipoma-like hibernomas are pre-dominantly slightly hypointense to subcutaneous fat on T1-weighting, often display marked or moderate inhomogeneity with prominent septa and vessels and enhancement is typical. The appearances of non-lipoma-like hibernomas are not diagnostic and may be mimicked by lipoma variants and by well-differentiated liposarcoma or atypical lipoma.

Recurrence and bleeding in hibernomas

Hibernomas are rare, benign, soft tissue tumors of brown fat. Despite being benign, we have experienced recurrent hibernomas and cases of significant bleeding at our institution. The current literature contains case reports of recurrence, and one large series that describes pathologic variants but contains little clinical data. We could not find data on bleeding. We hypothesized that recurrence is related to the resection type and/or the pathologic variant. We also postulated that atypical histologic findings correlate with excessive bleeding during excision. We sought to confirm that the MRI features of hibernomas could distinguish them from lipomas yet place them among lipomatous tumors. We retrospectively reviewed all hibernomas at our institution over 10 years. There were six hibernomas. Two were removed with a marginal excision and four with an intralesional excision. Two recurred after intralesional surgery. Histologic findings were typical for all patients. An MRI was available on only one patient and showed a fatty tumor that was distinguishable from a lipoma. Recurrences occurred after intralesional but not after marginal excisions. Histology could not predict recurrence. Hibernomas have the potential for significant bleeding during surgical excision, as seen in two of our cases.

LEVEL OF EVIDENCE

Prognostic study, Level IV (case series). See the Guidelines for Authors for a complete description of levels of evidence.

Fine-needle aspiration characteristics of hibernoma

BACKGROUND

Hibernoma is a rare, benign lipomatous tumor with features of brown fat. The preoperative diagnosis of hibernoma is difficult at times because its clinical, radiographic, and fine-needle aspiration (FNA) characteristics overlap with those of liposarcoma.

METHODS

The preoperative FNA findings of eight surgically excised hibernomas from seven patients (three men and four women, ages 24-60 years) were reviewed. The cytologic features were compared with the histologic features of the corresponding surgical specimens as well as lipomatous tumors and other lesions that may cause confusion in the differential diagnosis.

RESULTS

The FNA cytologic features of the hibernomas were found to correspond well with their histologic appearance. The FNA findings included small, round, brown fat-like cells with uniform, small cytoplasmic vacuoles and regular, small, round nuclei; delicate branching capillaries; and variable numbers of mature fat cells.

CONCLUSIONS

The FNA cytologic features of hibernoma are characteristic and useful in the preoperative investigation of lipomatous tumors, particularly with regard to excluding a diagnosis of liposarcoma.

Intradural spinal hibernoma: case report

BACKGROUND

Hibernoma is a rare, benign tumor arising from vestigial remnants of brown adipose tissue. It usually affects muscle and subcutaneous tissue and is asymptomatic and slow growing.

METHODS

A review of the worldwide literature contains about 110 cases of hibernomas. However, only two cases (including our case) of hibernoma located in the intradural spine have been reported. We present an extremely rare case of intradural extramedullary spinal hibernoma and summarize the magnetic resonance imaging (MRI) findings of the lesion.

CASE DESCRIPTION

A 35-year-old woman suffered from motor weakness and numbness of the left upper extremity. Cervical spinal MRI revealed an intradural extramedullary mass at C7. She underwent surgery via the posterior approach and the lesion was completely removed. Histopathologic examination showed multivacuolated cells with centrally placed nuclei, and the diagnosis of hibernoma was made.

CONCLUSIONS

Because the CT and MRI findings of hibernoma are almost same as those of other lipomatous tumors, a surgical specimen of this tumor is necessary to establish the correct diagnosis. Early diagnosis and total resection are necessary as some cases showed malignant features or rapid growth. Etiologically, the tumor may have developed by the ectopic growth or migration of adipose tissue.

Mediastinal lipoblastoma with intraspinal extension: MRI demonstration

Lipoblastomatous lesions are mesenchymal tumors of embryonal white fat and are classified into two forms: a superficial, well-defined mass (lipoblastoma) or a deep, infiltrative lesion (lipoblastomatosis). We report an unique case of mediastinal lipoblastoma in a 17-month-old boy which harbored a dual nature and exhibited the characteristics of both forms, a large well-encapsulated intrathoracic main tumor with focal infiltrative features at the thoracic inlet and transforaminal intraspinal extension forming a long-segment extradural mass. In addition to specific signal characterization of a fatty mediastinal mass with intratumoral streaks and whorls corresponding to the fibrovascular network, magnetic resonance (MR) imaging offered clear demonstration of the chest wall, lower neck and intraspinal extension, which was important for preoperative planning.