Case report

A 40-year-old woman with no significant medical or surgical history presented with a progressively enlarging left gluteal mass that had evolved over 5 years following trauma. The patient reported being struck by a car as a pedestrian, with the point of impact involving the left buttock. Immediately after the accident, she experienced local pain associated with skin erythema. Over time, she gradually noticed the development of a painless, partially firm mass that progressively increased in size.

Physical examination revealed a well-defined, relatively firm bulky mass located in the left gluteal region, without overlying inflammatory skin changes (Fig. 1). The remainder of the physical examination was unremarkable.

Fig. 1: Clinical photograph of the left gluteal region showing a bulky, well-defined subcutaneous mass (red arrow) without overlying inflammatory skin changes.

Soft-tissue ultrasound revealed a large, well-circumscribed subcutaneous mass with lobulated margins and heterogeneous isoechoic echotexture. Color Doppler examination showed no internal vascularity. Peripheral calcifications producing posterior acoustic shadowing were also identified (Fig. 2).

Fig. 2: Transverse ultrasound image of the left gluteal soft tissues demonstrating a large, well-defined subcutaneous mass with lobulated margins and heterogeneous isoechoic echotexture, without internal vascularity on color Doppler. Peripheral calcifications (yellow arrow) produce posterior acoustic shadowing.

A pelvic computed tomography (CT) scan was subsequently performed, demonstrating a large, well-defined lobulated fat-containing mass with a few internal calcifications within the subcutaneous soft tissues of the left gluteal region (Fig. 3).

Fig. 3: CT scan of the pelvis in soft-tissue window, axial (A), coronal (B), and sagittal (C) planes, showing a lobulated, well-circumscribed mass of fat density with internal calcifications, located within the subcutaneous soft tissues of the left gluteal region (yellow arrow).

For further tissue characterization, pelvic magnetic resonance imaging (MRI) was performed. MRI revealed a well-circumscribed lobulated lesion confined to the subcutaneous adipose tissue of the superolateral quadrant of the left buttock, confined to the subcutaneous adipose tissue, measuring 122 × 109 × 89 mm (craniocaudal × anteroposterior × transverse). The lesion was surrounded by a thin capsule that appeared hypointense on all sequences and enclosed a central fatty component demonstrating high signal intensity on T1-weighted images, with complete signal suppression on STIR and other fat-saturated sequences. No significant enhancement was observed after gadolinium administration (Fig. 4).

Fig. 4: Pelvic MRI including coronal T1-weighted (A), axial T1 fat-saturated (B), coronal STIR (C), axial diffusion-weighted imaging (D), and contrast-enhanced coronal T1 fat-saturated (E) images, demonstrating a well-defined lobulated lesion confined to the subcutaneous tissue of the superolateral quadrant of the left gluteal region. The lesion is surrounded by a thin hypointense capsule visible on all sequences and contains a central fatty component that is hyperintense on T1-weighted images, with complete signal suppression on fat-saturated sequences, including STIR. No internal enhancement is observed after gadolinium administration (yellow arrow).

The unusually large size of the lesion, its slow progression over 5 years, and its pseudotumoral appearance initially raised concern for a soft-tissue neoplasm. However, the patient's characteristic history of blunt trauma combined with the concordant multimodality imaging findings was highly suggestive of post-traumatic encapsulated fat necrosis (liponecrotic granuloma), making this the most likely diagnosis.

Given the characteristic clinical history and the typical imaging features demonstrated on ultrasound, CT, and MRI, a confident presumptive diagnosis of post-traumatic liponecrotic granuloma was established. As the imaging findings were considered highly characteristic and no radiological features suggestive of malignancy were identified, histopathological confirmation was not obtained.

Unfortunately, the patient was subsequently lost to follow-up; therefore, information regarding further management, histopathological confirmation, or imaging follow-up was not available.