Discussion

Post-traumatic liponecrotic granuloma, also referred to as encapsulated fat necrosis, is a benign lesion resulting from aseptic degeneration of adipose tissue secondary to trauma. Small foci of post-traumatic fat necrosis are frequently encountered in routine clinical practice, particularly in areas exposed to blunt trauma, and are often asymptomatic or incidentally detected. In contrast, encapsulated lesions that progressively enlarge and present as pseudotumoral masses are uncommon and may pose a significant diagnostic challenge because of their resemblance to soft-tissue neoplasms. When histopathological examination is performed**,** this entity is characterized by a chronic nonallergic foreign-body reaction with persistent inflammation surrounding foci of necrotic adipose tissue. It has been described in patients of various ages but is reported more frequently in adolescents and middle-aged women, with a predilection for the lower limbs, although involvement of the trunk, hip, and upper limbs has also been reported [1,2]. The educational value of the present case lies not in its gluteal location itself, but in the unusually large size of the lesion, its prolonged evolution over 5 years, its pseudotumoral presentation closely mimicking a soft-tissue neoplasm, and the contribution of multimodality imaging to establishing a confident presumptive diagnosis.

The pathophysiology of liponecrotic granuloma remains incompletely understood. The most widely accepted mechanism involves local hemorrhage, vascular insufficiency, edema, and adipocyte ischemia following trauma, leading to aseptic fat necrosis with saponification under the action of tissue and blood lipases. Progressive inflammation subsequently induces a granulomatous reaction and fibrosis surrounding areas of necrotic adipose tissue. In some cases, interruption of the vascular supply results in infarction of fat lobules followed by encapsulation. Secondary degenerative changes, including fibrosis, dystrophic calcifications, and pseudomembrane formation, further contribute to the broad spectrum of clinical and imaging appearances [[3], [4], [5]].

Clinically, liponecrotic granuloma usually presents as a firm, slowly growing, palpable subcutaneous mass, although most lesions remain small. Less commonly, they progressively enlarge and simulate a soft-tissue neoplasm, particularly when they reach a considerable size, as observed in our patient. Associated findings may include local deformity, skin retraction, ecchymosis, or skin discoloration. Another characteristic feature is the long interval between the initial trauma and clinical presentation, which may extend over several months or even years. In our patient, the lesion became clinically evident 5 years after the traumatic event. Consequently, the causal trauma is often forgotten. Tsai et al. reported that only 23% of patients recalled a previous traumatic episode [2], further complicating the diagnosis and increasing the risk of confusion with soft-tissue tumors.

The imaging features of post-traumatic liponecrotic granuloma vary according to the stage of evolution, from acute fat necrosis to chronic encapsulation and fibrosis [2]. This temporal evolution explains the marked heterogeneity of imaging findings. In our patient, the lesion's unusually large size and pseudotumoral appearance required a systematic multimodality assessment, which proved essential for lesion characterization and exclusion of malignancy.

On ultrasound, fat necrosis usually appears as a heterogeneous lesion with variable echogenicity. It may present as peripheral hyperechoic areas surrounding central hypoechoic or heterogeneous components. In some cases, the lesion is iso- or hypoechoic, reflecting different stages ranging from acute inflammation to chronic fibrosis. Color Doppler typically demonstrates absent or minimal internal vascularity. In our patient, ultrasound demonstrated a large, well-circumscribed avascular isoechoic mass with posterior acoustic shadowing caused by peripheral calcifications.

Computed tomography is particularly useful for demonstrating the fatty nature of the lesion, dystrophic calcifications, and the presence of a thin fibrous capsule, findings that strongly support a benign diagnosis [4]. In our patient, CT demonstrated a well-defined lobulated fat-containing mass with internal calcifications, consistent with chronic encapsulated fat necrosis.

Magnetic resonance imaging provides optimal tissue characterization. Fat necrosis typically demonstrates high signal intensity on T1-weighted images, with complete signal suppression on STIR or other fat-suppressed sequences, reflecting its fatty composition, whereas the fibrous capsule appears hypointense on all sequences. Enhancement, when present, is usually limited to the capsule or inflammatory components and varies according to the stage of evolution [3]. In our patient, MRI clearly demonstrated these characteristic features and, together with the clinical history of trauma, strongly supported a presumptive diagnosis while excluding imaging findings suggestive of lipomatous malignancy.

Although no single imaging feature is pathognomonic, the combination of a characteristic history of trauma and concordant ultrasound, CT, and MRI findings can strongly suggest the diagnosis of encapsulated fat necrosis and reliably differentiate it from malignant soft-tissue tumors in typical cases. While histopathological examination remains the reference standard in diagnostically challenging cases or when malignancy cannot be confidently excluded, it is not systematically required. In patients with characteristic clinical and imaging findings, a confident presumptive diagnosis can often be established, allowing appropriate conservative management or therapeutic excision without prior biopsy.

A limitation of the present case is the absence of histopathological confirmation and clinical follow-up, as the patient was lost to follow-up after imaging evaluation. Consequently, the diagnosis should be regarded as presumptive, based on the characteristic clinical history and concordant multimodality imaging findings.

The differential diagnosis is broad and includes both post-traumatic and nontraumatic lesions. Among post-traumatic conditions, Morel-Lavallée lesion, chronic organized hematoma, and early-stage myositis ossificans should be considered [2]. Morel-Lavallée lesion is a closed degloving injury that typically develops within the potential space between the subcutaneous tissue and the deep fascia following blunt trauma. In contrast, the lesion in our patient was confined to the subcutaneous adipose tissue and demonstrated homogeneous fat signal intensity, a thin fibrous capsule, complete fat suppression, and no significant enhancement, findings that favored encapsulated fat necrosis over a Morel-Lavallée lesion [2,4]. Chronic organized hematoma was considered less likely because of the absence of blood degradation products, fluid–fluid levels, or progressive peripheral enhancement [2,4]. Other differential diagnoses include seroma, lipoma, epidermal inclusion cyst, dermatofibrosarcoma protuberans, and well-differentiated liposarcoma [2,4]. Well-differentiated liposarcoma was considered because of the lesion's large size and fat-containing appearance; however, the absence of thick or nodular enhancing septa, nonadipose soft-tissue components, internal vascularity, or other aggressive imaging features, together with the characteristic history of trauma, strongly favored post-traumatic encapsulated fat necrosis [2,4], described in several anatomical locations, including the breast [1], and the lower limbs [6].

The present case illustrates not the rarity of the anatomical location itself, but the diagnostic challenge posed by an unusually large encapsulated lesion with a pseudotumoral appearance closely mimicking a soft-tissue neoplasm. It further emphasizes the value of careful clinicoradiological correlation and multimodality imaging in supporting a confident presumptive diagnosis and helping to avoid unnecessary invasive procedures.

Regarding management, treatment is usually conservative. Small asymptomatic lesions may remain stable, regress, or resolve spontaneously [2]. Surgical excision is generally reserved for large symptomatic lesions or when diagnostic uncertainty persists despite imaging. In patients presenting with a characteristic clinical history and typical multimodality imaging findings, conservative management may be appropriate without prior histopathological confirmation.

Nevertheless, the absence of histopathological confirmation and imaging follow-up represents a limitation of the present report. Therefore, the diagnosis should be interpreted as presumptive, based on the characteristic clinical history and highly concordant multimodality imaging findings.

In most cases, no specific treatment is required, reflecting the benign and often self-limiting nature of this condition [2].