Hibernoma: A case report of a rare soft lipoid tissue in the neck

Abstract

Hibernoma is rare and benign soft tissue tumor, arising from persistent fetal brown tissue. Although radiologic features are not specific, certain imaging characteristics, common locations and patient demographics may suggest hibernoma as a differential diagnosis of soft tissue tumors that have fatty components. Total resection is required due to the possibility of sampling errors in myxoid and well differentiated liposarcoma tissue components that resemble hibernoma. We report the case of a young adult with a right supra-clavicular swelling that had been present for 10 years, with no other associated signs. The patient underwent a CT scan and cervical MRI, with surgical removal, confirming the diagnosis of a hibernoma. This article also reviews the current imaging literature of hibernomas.

Introduction

Hibernoma is a rare, benign soft tissue tumor that originates from brown adipose tissue, which typically retrogress during infancy but in some cases may persist into adulthood, withing the neck, interscapular regions, mediastinum, and retroperitoneum [1]. These tumors are usually asymptomatic [2]. Cervical Hibernomas are rare condition are rare and pose a diagnostic challenge due to their similarity to neck lesions, such as lipomas, lymphadenopathy, and malignant soft tissue tumors [3].

Imaging is considered as the fundamental cornerstone in the diagnostic management of cervical masses especially for particularly for exploring complex anatomical regions like the neck. The preliminary suspicions are established by the clinical examination, the radiological investigations refine the differential diagnosis while evaluating anatomical relationships and preparing for surgery. Ultrasound together with CT scans and MRI scans provide unique benefits but MRI stands out as the superior choice for soft tissue tumor characterization [3]. The definitive diagnosis is confirmed by histopathological examination, which remains essential and underscores the importance of a multidisciplinary approach.

The curative approach for hibernomas, remains as complete surgical resection, serving both as a diagnostic and therapeutic measure. The strategy has positive results by maintaining low recurrence rates and supporting excellent long-term patient prognosis [4]. This case discusses clinical features alongside imaging and histopathological details of neck hibernomas while providing essential diagnostic insights and therapeutic advice.

Case presentation

A 32-year-old female patient who attended to our tertiary care center with history of ten-year evolution, of a prominent slowly-growth soft tissue swelling in the right supraclavicular region. The mass was painless with no other symptoms such, dysphagia, dyspnea, or neurological deficits. The patient's medical history was unremarkable for trauma, infections, or systemic illnesses that might have contributed to the mass development.

Physical examination showed a well-circumscribed, mobile subcutaneous tumor measuring approximately 9 cm in length and 3 cm in width, with a soft consistency. The encasing skin appeared normal without erythema, warmth, or ulceration. There was no palpable cervical lymphadenopathy detected during the comprehensive head and neck examination.

The initial diagnostic evaluation included a cervical ultrasound, which demonstrated a well-defined, ovoid, hyperechoic mass with heterogeneous internal echotexture. Doppler imaging revealed prominent intralesional vascularity, raising initial suspicion for an atypical lipomatous lesion or vascular tumor. Subsequent contrast-enhanced CT imaging of the neck confirmed the presence of mass with fat-density areas, associated with thin, enhancing septations and a complete pseudocapsule (Fig. 1). The mass demonstrated no evidence of calcifications, necrosis, or infiltrative margins. MRI examination provided superior soft tissue characterization, showing T1 isointense signal, incomplete fat suppression on T2-weighted sequences with fat saturation, and mild delayed enhancement of the fibrous septations. Notably, the lesion contained prominent serpentine vessel best appreciated on postcontrast T1-weighted images with fat suppression (Fig. 2). These imaging characteristics were highly suggestive of a hibernoma, though differential diagnoses including lipoma, well-differentiated liposarcoma, and angiolipoma were considered.

Fig. 1.

Computed tomography without and with contrast. Demonstrate, an oval soft tissue mass in the right lower cervical region, will defined, with mass effect on trapeze and sterno-cleido-mastoid muscles. This mass is hypodense, with some areas of fatty structures (A), contrast enhanced CT shows minimal heterogeneous enhancement of the tumor, with large intratumoural blood vessels (B-D: Arrow).

Fig. 2.

Neck MRI: Axial image, T2 sequence with fat saturation (A and B), T1 sequence (C) Axial image, postcontrast T1 sequence (D). Demonstrate an oval mass, clearly defined in right cervical area, displaying a consistent response akin to fat (red arrow), in different sequences, (A/B) Axial T2-weighted MRI, isointense mass to fat tissue. (C) Axial T1-weighted MRI, isointense mass to fat tissue. (D) Axial image postcontrast, presents mild and late enhancement, with large intratumoral vessels.

The patient underwent complete surgical excision of the mass via a transcervical approach under general anesthesia. Intraoperative findings revealed a well-encapsulated, lobulated tumor with a distinct plane of dissection from the surrounding supraclavicular soft tissues and brachial plexus. The mass demonstrated a rich vascular supply, requiring careful hemostasis during resection. The excised specimen measured 10 × 5 × 3 cm in its greatest dimensions and weighed 85 grams. Gross examination showed a tan-brown, soft, and spongy cut surface. Multiple sections were submitted for histopathological analysis to confirm the diagnosis and rule out malignant transformation.

Microscopic examination demonstrated a proliferation of large, polygonal cells with abundant granular, eosinophilic cytoplasm containing multiple small vacuoles. The tumor cells exhibited uniform, small, centrally located nuclei with finely dispersed chromatin and inconspicuous nucleoli. The characteristic multivacuolated brown fat cells were interspersed with univacuolated white adipocytes and delicate vascular channels. Immunohistochemical staining revealed strong positivity for S-100 protein in the multivacuolated cells, while CD31 highlighted the prominent vascular network. The histological features, combined with the imaging and macroscopic findings, were diagnostic of a classic hibernoma. The tumor margins were free of neoplastic involvement, confirming complete excision (Fig. 3).

Fig. 3.

(A) proliferation is composed of brown fat adipocytes with microvacuolar cytoplasm (HESx400). (B) Tumor proliferation composed of adipocytes.

The patient recovered well postoperatively with no complications and remained disease-free at 6-month follow-up, consistent with the typically excellent prognosis of these benign neoplasms which, when properly managed, have an excellent long-term prognosis and insignificant recurrence risk.

Discussion

Hibernomas are uncommon benign tumors originating from remnants from fetal brown fat tissue [3]. Despite their rarity, they present significant diagnostic difficulties due to their variable clinical and imaging features. These tumors also mimic malignant soft tissue masses. This discussion emphasizes pivotal findings, highlighting epidemiological facts, clinical approaches, and the most important imaging features, to guide the treatment strategies for cervical hibernoma.

Hibernomas account for approximately 1.6% of all lipomatous tumors, with a predilection for adults in their third to fifth decades of life [5]. These tumors can occur in various anatomical regions, though cervical hibernomas are particularly rare, with less than 20 documented cases in the literature [6]. A slight female predominance has been reported [7]. Recent studies, aided by positron emission tomography's technological developments, suggests that the true incidence of brown fat in adults is between 30% and 100%, indicating a higher risk of hibernoma development [8]. This tumor usually arises in the lower extremities (33%), trunk (23%), upper extremities (22%). Head and neck hibernoma (13%), is not that common [8].

The majority of clinically diagnosed cervical hibernoma patients have no symptoms. They may occasionally exhibit "pressure"-related pain or discomfort associated with the mass effect, in addition to being usually mobile and pliable, hibernomas can occasionally manifest with compression of nearby nerves and related pain [9]. However, atypical presentations, such as rapid growth, pain, or compressive symptoms may raise suspicion for malignancy [10]. The medical history and physical examination should look for signs that may suggesting more alarming alternate diagnoses, such as liposarcoma or others malignant tumors. In our patient, there were no alarming signs, such as an alteration in general condition or an associated tumor syndrome. Among the diagnostic procedures, CT, MR imaging plays a key role in evaluating hibernomas, by providing helpful information. On CT, hibernoma usually show a hypodense, heterogeneous characteristic, with well-defined margins, and frequent intertumoral septa. Diffuse enhancement is typically seen in hibernomas following contrast administration [11]. The magnetic resonance imaging of hibernomas display an intermediate intensity, most likely an intermediate signal intensity, between muscle and subcutaneous fat due to their high mitochondrial concentration of fat. Furthermore, because of their strong vascularization, they show contrast medium enhanced intensity. A drop in signal intensity is ascribed to the fat content of sequences that use fat suppression. Taking into consideration this radiological semiology, fat necrosis, angiolipoma, giant cell tumor, pediatric rhabdomyosarcoma, and atypical lipoma/well-differentiated liposarcoma are among the differential diagnoses for hibernomas [12]. Hibernoma exhibit market activity on FDG PET/CT, due to their hypermetabolic character, as demonstrated by Ciappuccini and Bardet, where propranolol pretreatment significantly reduced FDG uptake [13]. In addition to FDG, hibernomas have been unpredictably detected using others radiotracers, such [68Ga] Ga-DOTATATE (targeting somatostatin receptors) [14] and [68Ga] Ga-FAPI (targeting fibroblast activation protein), highlighting their diverse molecular expression profiles [15]. These findings suggest that hibernomas exhibit not only metabolic hyperactivity but also aberrant receptor patterns, implicating pathways beyond adrenergic signaling. The variability in tracer uptake underscores the need for further research into the pathophysiology and diagnostic implications of these rare tumors [13].

For the microscopic examination, these tumors are distinguished by cells with varied levels of differentiation [16]. Multivacuolar adipocytes are interspersed with univacuolar adipocytes, associated with brown fat cells with granular eosinophilic cytoplasm. The characteristic color of hibernoma is attributed to hypervascularity and abundant mitochondria. We distinguish 4 histologic variants (typical, myxoid, lipoma-like, and spindle cell), all of them with a benign course [17]. Typical hibernoma described as an eosinophilic, pale, and mixed cell types. While spindle cell hibernoma exhibited characteristics of spindle cell lipoma and the lipoma-like variant only included scattered cells, the myxoid variant had a loose basophilic matrix [16]. Some cytology features (admixture of multivacuolated and univacuolated fat cells, a rich, delicate, capillarylike vasculature) are known to cause a misdiagnosis of liposarcoma with regard to the histopathological differential diagnosis with liposarcoma, and pathologists take these similar characteristics into consideration [17].

In order to address the patient's clinical symptoms and, most importantly, to confirm that the tumor is benign, surgical treatment for hibernomas involves removing the tumor entirely serving both diagnostic and therapeutic purposes [18]. The tumors are typically well-encapsulated, allowing for straightforward resection with clear margins. The prognosis for hibernomas is excellent with exceedingly rare recurrences, and no cases of malignant degeneration or secondary localization described [19].

Conclusion

Cervical hibernoma is an uncommon and challenging diagnosis to make, particularly when a potentially malignant tumor, such as liposarcoma, is suspected. We report the occurrence of a cervical hibernoma notable for its uncommon location. When it comes to making a differential diagnosis, MRI is crucial, particularly when dealing with benign tumors. However, it is still not very useful when it comes to malignancy. The differential diagnosis of lipomatous soft-tissue tumors is broad and comprises benign (lipoma, hemangioma, angiolipoma) and malignant (liposarcoma) lesions. The radiologic identification of hibernomas is challenging, and even after biopsy, it can be challenging to distinguish them from well-differentiated liposarcomas, even with the use of multimodal imaging. As a result, total surgical resection is strongly advised.

Patient consent

I, the author of the article « Hibernoma: a case report of a rare soft lipoid tissue in the neck», declare that informed written consent was obtained from the patient for publication of the Case Report and all imaging studies in RADIOLOGY CASE REPORTS.