Non-traumatic fat necrosis of the buttock: MRI findings of an unusual entity

Description

First described in the breast in 1920, fat necrosis is a localised, benign, non-suppurative inflammatory disease of the adipose tissue. It can occur in adults due to repeated injections, local trauma or tissue injury, collagen vascular disease, myeloproliferative disorders or as a complication of pancreatic disorders in the form of disseminated fat necrosis.^{1 2}

It is an uncommon, self-limited panniculitis of traumatic origin in newborns and can occur in women after breast surgery. Limbs, which are commonly exposed to repeated microtrauma, are the most frequently affected sites.¹

Histopathologically, necrotic fatty tissue is often compartmentalised inside a fibrous capsule and associated with occasional inflammation and calcification.³

A woman in her late 70s had been feeling for the past 2 months a non-painful, round, hard swelling in her left buttock. The finding was clinically visible at inspection; she did not recall any traumatic events involving the region of interest.

Her medical history was unremarkable.

The patient discovered the swelling while washing in the shower; she reported substantial dimensional stability during the last 2 months. She did not show local or systemic signs of inflammation. The patient performed, in the first place, an ultrasound scan which was inconclusive and raised suspicion for a malignant entity.

To get a better understanding of the lesion, she then underwent a contrast-enhanced MRI.

MRI showed an elliptical formation characterised by mixed content, inhomogeneously hyperintense in T1-weighted and short TI inversion recovery (STIR) weighted sequences (figure 1A,B) and hypointense in T1-weighted sequences (figure 1C). The lesion’s walls showed intense contrast enhancement (figure 2A,B) without intralesional vascularisation or mural nodules and did not seem to infiltrate the surrounding structures.

Figure 1.

1.5 T MRI scan, axial planes. In A, the lesion of the left buttock appears to have a content that is almost isointense to the surrounding fat tissue. The same content seems to be inhomogeneous and markedly hyperintense in the STIR-weighted sequence in B and hypointense in the T1-weighted sequence in C. The lesion has a rather regular wall, hypointense in all sequences. Mild signs of surrounding adipose tissue oedema are seen without evidence of infiltration of the adjacent structures.

Figure 2.

1.5 T MRI scan, contrast-enhanced fat-suppressed T1 sequences, axial (A) And coronal (B) Planes. The lesion shows intense peripheral enhancement of its outer portion, in keeping with a capsule. No internal enhancement is seen, nor does the mass seem to infiltrate surrounding structures.

To confirm the diagnostic query, the patient underwent an ultrasound-guided biopsy with a 16G needle. The histologic findings confirmed our hypothesis ab initio, resulting in steatonecrotic tissue with neutrophilic granulocytic inflammatory infiltration.

The patient was offered to remove the lesion surgically, but since its benign nature she decided not to.

Normal ultrasound findings of fat necrosis are potentially heterogeneous and include a simple cyst, a solid mass, a complex mass with mural nodules or just increased echogenicity of subcutaneous tissues; MRI shows variable appearances of both mass-like and non-mass-like lesions ranging from irregular, oedematous fat lobules to complex cystic lesions.² These lesions are often well-defined, lobular and septated. They can show fluid levels surrounded by a hypointense pseudocapsule characterised by enhancement after gadolinium injection, in keeping with central necrotic tissue.² MRI is the gold standard to evaluate fat necrosis within markedly oedematous subcutaneous tissues when ultrasound cannot be conclusive.⁴

Spontaneous cases of fat necrosis in the adult population are exceedingly rare in the literature so far.

Learning points.

• Fat necrosis can occur even in asymptomatic patients whose medical history is unremarkable—but pay attention to the bias in patient recall.

• Normal ultrasound findings of fat necrosis are potentially heterogeneous and include a simple cyst, a solid mass, a complex mass with mural nodules or just increased echogenicity of subcutaneous tissues

• Contrast-enhanced MRI allows confident evaluation of fat necrosis within markedly oedematous subcutaneous tissues where ultrasound cannot always be conclusive.

Ethics statements

Patient consent for publication

Consent obtained directly from patient(s).