A 52-year-old man underwent a magnetic resonance imaging of the sacrum to further study an asymptomatic sacral bone lesion that had been incidentally detected on a computed tomography study performed for other reasons.
Abdominal X-ray shows poor delineation of the upper left neural foramina margins, without other relevant osseous findings (Fig. 1).
CT image depicts an eccentric sacral bone lesion with predominant low attenuation of fat (Fig. 2), dystrophic calcification and prominent osseous trabeculae within the lesion, which is surrounded by a thin rim of sclerotic bone. The lesion causes bone expansion of the sacral ala without disrupting the cortex and extends to the articular surface of the sacroiliac joint without joint involvement (Fig. 3).
MR images show signal intensity isointense to subcutaneous fat on both T1 and T2-weighted images, hypointense bony trabeculae interspersed throughout the lesion, scattered irregular internal hypointense calcifications and a small cystic area consistent with fat necrosis.
STIR and fat-saturated T1-weighted and T2-weighted images show homogeneous suppression of the signal intensity consistent with the adipose composition of the lesion.
No associated extraosseous soft-tissue mass nor bone-marrow oedema are seen on MR images (Fig. 4 and 5).
Intraosseous lipoma is a primary bone tumour composed of mature adipocytes. It has been traditionally considered as a rare lesion although it is probably much more common than previously reported. Its diagnosis has increased in the last years due to the increasing use of MR and CT imaging techniques .
It can appear at any age but is more commonly discovered in middle-aged patients. It is slightly more frequent in males.
The commonest location of intraosseous lipoma is the lower limb , mainly the intertrochanteric region of the femur, tibia and calcaneus, but may also appear in other locations. There are very few reported cases of intraosseous lipomas located in the sacrum.
Intraosseous lipomas can cause symptoms such as pain or swelling, but frequently they are incidental findings , as in our case.
The imaging findings of this tumour depend on its histological composition .
Milgram classified intraosseous lipoma in three categories :
Stage 1: solid lesion of viable lipocytes
Stage 2: areas of fat necrosis and calcification with persistent viable lipocytes
Stage 3: complete or near-complete necrosis of the lesion wit variable amounts of calcification, cyst formation and reactive new bone formation.
The radiographic appearance is that of a well-defined benign-appearing osteolytic bone lesion, often with a thin sclerotic rim . Expansile remodelling of bone might be seen. The tumour may be associated with calcifications or ossification that might be quite extensive. Peripheral osseous ridges may be seen within the lesion and be responsible for a septated appearance.
CT and MR imaging demonstrate the fatty component of the lesion, which is diagnostic for intraosseous lipoma  and distinguishes it from other tumours. The fatty component shows fat attenuation on CT images and fat signal intensity similar to that of subcutaneous fat on MR images. Variable degree of peripheral or central calcification or ossification might be seen . With involution, fibrous proliferation and cystic degeneration may develop and might be the predominant finding. Cystic areas show low attenuation on CT images, fluid signal intensity on MR images and may be surrounded by a rim of ossification.
Our lesion shows typical features of fat tissue on CT and MR images, so it is consistent with the diagnosis of intraosseous lipoma.
Malignant degeneration is rare . Surgical treatment is usually not necessary but it may be required for symptomatic lesions.
Differential Diagnosis List
Intraosseous lipoma of the sacrum
Giant cell tumour
Intraosseous lipoma of the sacrum