Acetabular intraosseous lipoma

Case contributed by Ashesh Ishwarlal Ranchod

Presentation

Lower back and right hip pain. Negative trauma. Poor response to analgesia.

Patient Data

Age: 40 years
Gender: Female
x_ray

A cystic, bubbly, expansile, well-defined lesion involving the right acetabulum and inferior pubic ramus is identified. It has a narrow zone of transition with benign features. There is endosteal scalloping. There is no intralesional or soft tissue calcification. There is cortical thinning however no pathological fracture or cortical break is present. The right femoral head is uninvolved. The bony pelvis is otherwise unremarkable.

Multiplanar and multiaxial contrast-enhanced MRI of the right hip confirms an expansile, multiseptated, mixed cystic and solid (fatty) intraosseous lesion within the right quadrilateral plate, posterior acetabulum column extending into the right ischial tuberosity and inferior pubic ramus. There is a peripheral ring-like enhancement relating to the cystic components of the lesion. The solid fatty component is non-enhancing with fibrous septations. There is cortical thinning however no disruption or pathological fracturing. There is no extraskeletal or soft tissue involvement. There is no periosteal reaction.

Incidental bilateral hip effusions. MRI pelvis is otherwise unremarkable.

pathology

Histopathological confirmation of a benign intraosseous lipoma with no malignant transformation.

Follow up study

mri

Follow-up MRI after 13 months, confirms the persistent, histologically proven, intraosseous lipoma within the right acetabulum, ischial tuberosity, and inferior pubic rami.

There is a linear cystic component seen best on sagittal sequences extending to a cortical defect consistent with the tract/site of the previous bone biopsy. There is stability of the lesion with mild improvement, reduced cystic component, and no evidence of progression. There is no new soft tissue involvement or pathological fracture.

Case Discussion

This case demonstrates the plain film and MRI appearance of a histopathologically proven intraosseous lipoma within the right acetabulum, ischial tuberosity and inferior pubic ramus. Based on Milgram and co workers' proposed categories, this is a stage 3 heterogeneous lesion containing fluid equivalent cavities with ring-like enhancement on contrast administration and solid fatty components with septa. There is however no calcification or ossification. The broad differential diagnosis based on the plain film appearance includes a unicameral bone cyst in an adult patient, fibrous dysplasia, Langerhans cell histiocytosis, giant cell tumor of the hip/pelvis, and non-ossifying fibroma, and an aneurysmal bone cyst.

MRI dramatically reduces this differential diagnosis to fat-containing lesions.

Case courtesy of Dr Amaresh I. Ranchod

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