Spinal Intraosseous Hibernoma: A Case Report and Review of Literature.

Hibernoma is a rare benign tumor that arises from vestiges of brown fat. Spinal intraosseous hibernoma has only recently been described in the literature, and only 12 cases have been reported to date due to its extreme rarity. Here, we report the case of a patient who was incidentally diagnosed with an intraosseous hibernoma in the thoracic spine, following a diverse imaging work-up and pathologic confirmation. We correlate the clinical presentation and imaging features of our case with those of previously reported cases during our review of the literature. Copyrights

INTRODUCTION

Hibernoma is a rare benign tumor consisting of brown fat, the adipose tissue found in hibernating animals, that is commonly observed in the soft tissue of the thigh, followed by the shoulder, back, and neck (1). Intraosseous hibernomas are extremely rare (2), and its typical imaging features are poorly recognized. The few reports that include imaging findings include only a few case reports (2345).

We present the clinical, radiological, and pathological features of a case of spinal intraosseous hibernoma. In addition, we comparatively review the clinical and radiologic features of our case with those of the 12 cases that were previously reported in the literature.

CASE REPORT

A 54-year-old female presented with positive Interferon Gamma Release Assay test results following a tuberculosis screening. At the time of presentation, a low-dose chest computed tomography (CT) exam was performed to evaluate for pulmonary tuberculosis. The patient had experienced no local or systemic symptoms. The chest CT scan revealed a part-solid nodule in the right upper lobe, approximately 1 cm in diameter (Fig. 1A). A well-defined sclerotic lesion with intervening lucency was detected on the left side of the T7 vertebral body (Fig. 1B). A MRI scan of the thoracic spine revealed a well-defined, intraosseous lesion within the T7 vertebral body, approximately 2.6 cm in diameter. This lesion was characterized by heterogeneous hyperintensities on T2-weighted images (T2WI), and intermediate to hypointensities on T1-weighted images (T1WI), as compared with the paraspinal muscle or intervertebral disc. A post-contrast fat-saturated T1WI revealed heterogeneous contrast enhancement of the lesion (Fig. 1C). PET-CT was used to evaluate the lung nodule and the intraosseous mass at the level of T7. During the PET-CT scan, we observed mild fluorodeoxyglucose (FDG) uptake (max standardized uptake value: 1.1) in the right upper lung nodule. In contrast, no hypermetabolism was detected in the sclerotic T7 vertebral lesion (Fig. 1D). Based on these findings, we considered a differential diagnosis of atypical presentation of the hemangioma, or a possible blastic metastasis due to an intrapulmonary lesion.

Fig. 1

A 54-year-old female with a spinal intraosseous mass detected on chest CT.

A. CT images of the right upper lobe demonstrate a partially solid nodule, approximately 1 cm in diameter.

B. Coronal (left image) and axial (right image) CT scans demonstrate a well-defined eccentric area of osteosclerosis with intervening lucencies in the T7 body that extends from the superior to the inferior endplate.

C. An axial T2WI image shows a well-defined and slightly inhomogeneous lesion primarily characterized by hyperintensity (left image). Axial T2WI image shows a hyperintense lesion with interspersed serpentine hypointense lines (left image, arrow). Axial T1WI image shows an intermediate signal intensity with interspersed hyperintense areas (middle image). Image of an axial T1 with fat saturation after contrast media injection; note the heterogeneous and intense enhancement (right image). The T2WI image finding of serpentine hypointense lines corresponds to persistent hypointensities in areas of intense enhancement on the axial T1 image following fat saturation (right image, arrow).

T1WI = T1-weighted image, T2WI = T2-weighted image

D. PET scans show low-grade fluorodeoxyglucose uptake of the part-solid nodule in the right upper lobe with a standardized uptake value of 1.1 (left image, arrow). Note the absent uptake of the sclerotic lesion in the T7 vertebral body (right image).

E. Pathologic features of intraosseous hibernoma. Transparent pale-brown fat cell infiltration is observed in the bone marrow cavity (left image, H&E stain, × 100). The tumor contains a large number of brown fat cells with multi-vacuolation and central nuclei (right image, H&E stain, × 400).

H&E = hematoxylin and eosin

Bone biopsy was performed with a 14-gauge core needle using the left transpedicular approach under fluoroscopic guidance. Histopathological examination of the core biopsy specimen confirmed the presence of transparent pale-brown fat cell infiltration in the bone marrow cavity. A large number of brown fat cells with multi-vacuolation and central nuclei were observed in the tumor mass (Fig. 1E). The morphological features of the tissue were consistent with brown fat, which indicated that the lesion was an intraosseous hibernoma. CT-guided core biopsy of the part-solid right lung nodule confirmed the diagnosis of adenocarcinoma and the patient was referred for lobectomy.

DISCUSSION

Spinal intraosseous hibernoma is an exceedingly rare entity. Its characteristic clinical, radiological, and pathologic features are reported within only 12 case reports. The clinical and radiologic features of these cases are summarized in Table 1. Patients with spinal intraosseous hibernomas tend to be female (male-to-female ratio of 3:9), with an age range of 49–84 years. Reported lesion locations include the sacrum (n = 5), thoracic spine (n = 4), and lumbar spine (n = 3). All lesions were detected incidentally during clinical workups for unrelated musculoskeletal disorders, and were mostly unrelated to the patients' symptoms.

Table 1

Clinical Summary of Patients with Spinal Intraosseous Hibernoma that Have Been Previously Reported in the Literature

Reference	Age/Sex	Site	Reason for Omaging	Clinical Presentation	CT Finding (X-Ray)	MR Finding	Other Imaging
This case	54/F	T7 VB	Tuberculosis screening	Incidentally detected	Sclerotic lesion with intervening lucency	T1: heterogeneous intermediate to hypointensity	PET-CT: no hypermetabolism
						T2: heterogeneous hyperintensity
						CE: heterogeneous enhancement
Bonar et al. (5)	48/F	T5 VB	Swallowed glass	Incidentally detected	Sclerosis	T1: heterogeneous hypointensity	PET: mild hypermetabolism (max SUV 3.3)
						T2: hyperintensity
						CE T1: peripheral enhancement	Bone scan: mild increased uptake
	50/F	T12 VB	Staging of breast cancer	Incidentally detected	Sclerosis	T1: isointensity to muscle	Bone scan: subtle uptake
						T2: hyperintensity
Hafeez et al. (4)	67/F	L3 VB	Pressure sore	Incidentally detected	Speckled sclerosis with intervening lucency	T1: diffuse hypointensity
						T2: diffuse hyperintensity
Jerman et al. (9)	67/F	Left sacrum	Low back pain	Incidentally detected	PET-CT: well-defined sclerotic lesion	T1: hypointensity	Bone scan: increased metabolic activity
						STIR: heterogeneous, hyperintense peripheral rim	PET: mild hypermetabolism
						CE T1WI: moderate enhancement throughout the lesion and in the peripheral rim
Kumar et al. (2)	57/F	Left sacral alar	Low back pain	Incidentally detected	Lucent center, sclerotic rim	T1: heterogenously hyperintense; STIR/T2 fat-sat: heterogenously hypointense,
						CE-FS T1: mild heterogenous enhancement
Ringe et al. (10)	70/F	Left mass lateralis of sacral bone	Low back pain	Low back pain	Sclerotic, well-demarcated	T1: Hypointensity
						T2: Hyperintensity
Song et al. (3)	65/M	T12 VB	Hepatocelluar carcinoma	Incidentally detected	Sclerosis	T1: heterogeneous hypointensity	Bone scan: increased uptake
						T2: hyperintensity
	71/F	L3 VB	Low back pain	Three patients had other cause of pain on imaging, one patient only showed intraosseous lesion, but relieved with analgesic	Sclerosis	T1: heterogeneous hypointensity	PET: mild hypermetabolism
						T2: hyperintensity
	49/M	T12 VB	Low back pain		X-ray: sclerosis	T1: heterogeneous hypointensity
						T2: hyperintensity
	68/M	Sacral ala	Low back pain		Osteolysis with peripheral sclerosis	T1: heterogeneous hypointensity
						T2: hyperintensity
	56/F	L3–4 VB	Low back pain		Sclerosis	T1: heterogeneous hypointensity
						T2: hyperintensity
Westacott et al. (6)	84/F	Sacrum	Right hip and low back pain	Cause of pain Relieved with analgesics	Lytic bone lesion	N/A

CE = contrast-enhanced, FS = fat-saturated, N/A = not available, RFA = radiofrequency ablation, STIR = short T1 inversion recovery, SUV = standard uptake value, T1WI = T1-weighted image, VB = vertebral body

The published cases share several imaging features. The majority of cases showed a sclerotic lesion with a variable definition on CT or radiograph. Eleven cases showed sclerotic masses on CT or radiograph, and only one case presented with an osteolytic mass (6). MRI revealed intermediate to hypointensities on T1WI–relative to skeletal muscle–and hyperintensities on T2WI or fluid-sensitive images. Contrast-enhanced MRI examinations were conducted with three cases. These studies revealed peripheral rim enhancement (n = 1), heterogeneous enhancement (n = 1), and moderate enhancement throughout the lesion and peripheral rim (n = 1). Nuclear medicine tests, such as a bone scans or PET scans, showed variable uptake or hypermetabolism.

Our case exhibited CT and MRI findings similar to those of previously reported cases. CT revealed the presence of a sclerotic mass and characteristics of intermediate to hypointensities on T1WI, and heterogeneous hyperintensities on T2WI with a heterogeneous enhancement pattern. However, unlike previously reported cases, our case did not show hypermetabolic features on PET-CT. Such variable findings may be explained by the fact that spinal intraosseous hibernoma may not show FDG uptake during PET-CT examination.

Histopathological examination of the core biopsy revealed infiltration of the tumor cells within the marrow space between the bony trabeculae with preservation of overall intact anatomy, despite slight hypertrophy of the lamellar bony trabeculae within the lesion and mild sclerosis. This pathologic feature correlated with observed imaging features, including a sclerotic lesion with an intervening lucency. In contrast, our findings stand in contrast with reported features of intraosseous lipomas, which are typically well-defined osteolytic intramedullary lesions with thin sclerotic rims.

Kumar et al. (2) reported that the MR findings of intraosseous hibernomas were similar to those of soft-tissue hibernomas. Soft-tissue hibernomas are vascular tumors that contain fibrovascular septa. These features may contribute to heterogenous nature of MR signals within the lesion (7). Song et al. (3) reportedly observed small- to medium-sized vessels within an intraosseous hibernoma. Our case showed serpentine and persistent hypointense lines on MRI, which indicated vascular elements (Fig. 1C).

For the imaging finding of a solitary sclerotic bone lesion, the differential diagnosis should include benign notochordal cell tumor (BNCT), atypical hemangioma with intramedullary sclerosis, or blastic metastasis. BNCTs present as sclerotic lesions on CT and show hypointensities on T1WI. These tumors are usually located at midline and do not show contrast enhancement on MRI. Atypical hemangioma with intramedullary sclerosis and an intraosseous hibernoma have similar conventional MR features: hyperintensities on T2WI and hypointensities on T1WI. Unlike usual hemangiomas, atypical hemangiomas have lower signal intensities on T1WI due to the increased vascular components and intramedullary sclerotic portion. These features contribute to possible signal differences, in comparison with usual hemangiomas. Furthermore, both atypical hemangioma and intraosseous hibernomas have various enhancement patterns. Therefore, as in this case, if both tumors appear as sclerotic lesions, it may be challenging to distinguish between these two lesions using MR features. This is also true in cases of metastases. Usmani et al. (8) reported that atypical hemangiomas might vary in appearance and may present as sclerotic lesions on CT and hypointensities on T1WI, potentially resembling metastatic lesions. Metastatic lesions usually show lower signal intensities than intervertebral discs; however, in many cases, hibernomas show diverse imaging characteristics. This makes it challenging to distinguish hibernomas from metastases. Although PET is useful for differentiating these lesion types, in our case, there was no uptake in the lesion under PET. As with our case, it can be difficult to differentiate between various conditions based only on uptake values. In such situations, biopsy and pathologic diagnosis play a critical role.

In conclusion, hibernomas demonstrate common imaging features, but may also demonstrate considerable heterogenicity. The growing number of radiological investigations of the spine is expected to increase the rate with which spinal intraosseous hibernomas are detected. Radiologists should consider spinal intraosseous hibernoma as a differential diagnosis in patients with sclerotic spinal intraosseous lesions.