This patient noticed a swelling on the medial aspect of left knee after blunt trauma. On examination, a soft lobulated painless swelling was noted on the medial aspect of left knee. Plain radiographs of the knee and all laboratory tests were normal.
The patient noticed a swelling on the medial aspect of left knee after blunt trauma. On examination, a soft lobulated painless swelling was noted on the medial aspect of left knee. Plain radiographs of the knee and all laboratory tests were normal.
An ultrasound (Fig. 1) showed well-defined homogeneous hyperechoic lesion within the subcutaneous fat.
MRI of the left knee included axial(a) and sagittal(b) T1-W; coronal(c) T2-W; axial STIR(d) and T1-W post Gadolinium fat saturated (e) images (Fig. 2). All these images show a well-defined homogeneous lobulated mass in the medial subcutaneous tissue of the knee measuring approximately 4 x 5x 1.5 cm. On T2-W, the lesion displays a hyperintense signal intensity (SI) greater than that of subcutaneous fat and on T1-W, the lesion also shows hyperintense signal in-between the intensities of subcutaneous fat and muscle. On the STIR sequence, the lesion displays a similar signal intensity to muscle. Following intravenous contrast, the post-Gadolinium DTPA fat saturated T1-W image displays no significant enhancement.
Histology revealed a lipoma with hibernoma-like features.
Hibernoma is a rare benign tumour of brown fat most commonly found in the peri-scapular region and to lesser extent in the thigh, neck and chest wall soft tissue. There is a wide range for age at presentation but the majority present in their third and fourth decades with a slight male prevalence. Pure hibernomas are rare, most hibernomas contain a mixture of white and brown (hibernomatous) adipose tissue. Lesions usually present with a slow growing painless mass. They are generally well-circumscribed, encapsulated lesions that range in size from 1 – 30cm with a mean of 9.3cm. Unlike usual lipomas that are composed of large fat cells with a single cytoplasmic fat vacuole and a peripheral nucleus, hibernomas vary in their histological appearance depending on the relative amount of multivacuolated, mitochondria-rich, brown fat cells, associated small capillary proliferation and stromal background (Fig.3). Six histological variants are described (Mietinnen et al., 2002) that are not of prognostic value although they may show different MR signals. The most characteristic variant of hibernoma (eosinophilic variant) contains large numbers of multivacuolated cells with abundant granular, eosinophilic cytoplasm and a small central nucleus. Some cases are composed entirely of pale cells - the pale variant, and others have a mixture of pale and eosinophilic cells, the mixed variant. Most commonly, hibernomas contain small clusters of brown fat amidst ordinary white fat cells - the lipoma-like variant. Myxoid and spindle cell variants are rare. Mitoses are not seen and cytological atypia is most unusual.
The MR imaging characteristics of hibernoma reflect the variable histology and depend on the relative amounts and distribution of brown and white fat, capillary proliferation and stromal background.
The MR images demonstrate well-defined, lobulated lesions. The lipoma-like hibernomas have a more homogeneous appearance with similar or slightly reduced signal intensity (SI) on T1-WSE compared with subcutaneous fat and similar or slightly increased signal on T2-WSE. Fine internal low intensity fibrous strands are common and easily depicted against the surrounding fat. On STIR, lesions display an intermediate signal intensity higher than that of subcutaneous fat and similar to muscle. Mixed lesions (pale and eosinophilic brown fat cells) display a more inhomogeneous appearance with smaller amounts of slightly altered fatty tissue and more non-specific linear or mass-like components displaying low SI on T1-W and high SI on T2-W and STIR.
Hibernomatous tissue enhances avidly but the overall enhancement of the lesion will depend on the amount of hibernomatous tissue within the lesion. In lipoma-like hibernomas the relative amount of hibernomatous tissue is small, therefore enhancement may be negligible or absent as in this case.
Hibernoma
A well-defined, lobulated mass-like lesion is visible in the medial soft tissue of the patient’s left knee. Ultrasound images show that the main echogenicity within the lesion is fatty, with relatively well-defined margins and homogeneous internal echoes. Fine linear hypoechoic bands are observed, dividing the internal structure of the mass. On transverse and coronal MRI, the signal intensity of the lesion is mostly similar to, or slightly differs from, the surrounding subcutaneous fat (slightly decreased signal on T1WI, mildly increased signal on T2WI or STIR sequences), with local fine band-like low-signal structures. The degree of enhancement inside the lesion varies with the proportion of brown fat, but in this case there is essentially no distinct enhancement or only mild enhancement. No significant bone destruction or bone marrow edema signal is observed in the surrounding area, and there are no obvious abnormalities in the articular surfaces or surrounding soft tissue.
Based on the patient’s age, symptoms (a painless, slowly growing soft tissue mass), and imaging characteristics, the possibility of a fatty tumor is high. Although hibernoma is rare, it is worth considering due to its characteristic partial fat content, potential mild enhancement, and unique signal features.
Combining the patient’s medical history (recurrent occurrence, soft texture, slow longitudinal enlargement), clinical symptoms (discovered incidentally after a mild trauma, with no significant pain), imaging features (primarily a fatty signal with possible slight enhancement), and histological findings (presence of brown fat cells, etc.), the most likely diagnosis is hibernoma (suspected lipoma-like subtype). If there is a need to exclude other rare pathological types, surgical excision and pathological examination are recommended for a definitive diagnosis.
Treatment Strategy:
Rehabilitation and Exercise Prescription:
Note: If the patient has osteoporosis, compromised cardiopulmonary function, or other chronic conditions, it is necessary to individualize exercise intensity and frequency to ensure safety.
Disclaimer:
This report provides a reference-based analysis based on the existing medical history and imaging data. It does not replace in-person consultation or individualized medical advice from a professional physician. If there are any questions or changes in symptoms, please promptly consult a reputable hospital for further examination and treatment.
Hibernoma
