Intraosseous lipoma of the iliac: case report.

Lipomas are benign tumors that attack fat cells and most often affecting soft tissues in adulthood. On rare occasions, they may affect bones, preferentially the metaphyses of the long bone. They are generally asymptomatic and radiography shows radiolucent lesions with a thin sclerotic rim or radiodense lesions with a thick sclerotic rim. Malignant transformation of these tumors is rare, as is their recurrence, and there is no need for surgery in most cases. In this report, we present a rare case of intraosseous lipoma in the iliac bone.

Introduction

Lipomas are benign tumors that affect adipose cells. They most commonly affect soft tissues in adulthood and are rare in bones. The incidence of intraosseous lipomas is approximately 0.1% among all primary bone tumors and it is believed that they do not preferentially affect either sex. The age group affected is very wide, and cases can be found both among children and among elderly people. They are most commonly diagnosed in the fourth decade of life. Their etiology remains unknown and is a matter of controversy.

Intraosseous lipomas may affect any part of the skeleton and are most frequently located in the transtrochanteric region of the proximal femur (34%), tibia (13%), fibula (10%), calcaneus (8%), iliac bone (8%) and humerus and ribs (5%). They preferentially affect the metaphyses of long bones and present as single lesions. However, reports of multiple tumors scattered around the entire skeleton have been made.

Lipomas present few symptoms. Pain is the commonest of these, and the absence of specific symptoms may cause difficulty in making the diagnosis. There is a need for the aid of imaging examinations. However, once the lipoma has been found, the prognosis is generally good and a full cure can be achieved. The objective of this study was to report on a rare case of intraosseous lipoma of the iliac.

Case report

The patient was a 45-year-old man who reported having insidious pain in his right hip that had started three months earlier. It was unrelated to trauma and did not have any specific characteristics. The pain score on a visual analogue scale (VAS) was 5/10, and it improved through use of non-steroidal anti-inflammatory drugs and worsened with slight effort.

Physical examination did not show any limitation of movements of the pelvis, lumbar spine or right hip. Radiography was then performed on the pelvis in anteroposterior view. A circumferential osteolytic lesion in the wing of the right iliac, of approximately 3 cm in diameter, with well-defined edges, was observed (Fig. 1A). Because of the nonspecific nature of the image obtained through radiography, tomography with three-dimensional reconstruction was requested. A lesion affecting the posterior cortical bone of the wing of the right iliac could be seen (Fig. 1B).

Fig. 1

Radiograph of the pelvis in anteroposterior view, showing circumferential osteolytic lesions in the wing of the right iliac, of approximately 3 cm in diameter, with well-defined borders (A). Tomography with three-dimensional reconstruction, in which a lesion affecting the posterior cortical bone of the wing of the right iliac is observed (B).

A coronal slice for a bone window (Fig. 2A) and an axial slice for a soft-tissue window (Fig. 2B) showed that the lesion extended through the medullary tissue, from the anteromedial to the posterolateral region of the right iliac bone. In this region, there was fracturing of the cortical bone, of osteolytic and insufflative nature.

Fig. 2

Tomographic slices in coronal view for a bone window (A) and in axial view for a soft-tissue window (B), showing that the lesion extends through the medullary tissue, from the anteromedial to the posterolateral region of the right iliac bone. In this region, there was fracturing of the cortical bone, of osteolytic and insufflative nature.

Bone scintigraphy with technetium was performed and did not show the lesion (Fig. 3A and B), which suggested that the lesion was of benign nature. To complement the evaluation, magnetic resonance was performed on the pelvis. In this, T1 coronal imaging showed a lesion with hyposignal, without invasion of soft tissues (Fig. 4A). T2 coronal imaging of the pelvis (Fig. 4B) showed a lesion with hypersignal in the right iliac.

Fig. 3

Bone scintigraphy with technetium, which did not show the lesion (A and B), thus suggesting that it was of benign nature.

Fig. 4

Magnetic resonance imaging of the pelvis, in which the T1 coronal image showed a lesion with hyposignal, without soft-tissue invasion (A), and in T2 (B) with hypersignal in the right iliac.

Thus, the patient underwent surgical curettage of the tumor in the wing of the right iliac bone, which showed fatty tissue with adipocytes, without atypia, in a firm whitish-brown fragment of 2.8 cm, with bone tissue comprising thickened sclerotic trabeculae, hematopoietic cellular tissue, extensive adipose replacement and absence of signs of malignity in the material. The suspicion of benign tumor formation was thus confirmed and it was diagnosed as an intraosseous lipoma. Around three months after the procedure, the patient no longer presented pain and there was no recurrence of the lesion.

Discussion

Intraosseous lipoma is a rare benign type of bone tumor. It mainly affects the metaphysis of long bones and is asymptomatic in approximately half of the cases.5, 6 It affects the sexes almost equally, such that it is slightly more prevalent among males. It occurs in all age groups, and it slightly more prevalent in the fourth and fifth decades of life. Involvement of the iliac bone is even rarer.6, 7

Dhalin calculated the incidence of intraosseous lipomas as one in every 1000 bone tumors. However, the incidence may be greater because of the difficulty in diagnosing cases of this type of lipoma. It is common for such diagnoses to be made accidentally through imaging examinations.5, 9

The first report of intraosseous lipoma of the iliac bone was made by Buckley and Burkus in 1988. Since then, due mainly to development of diagnostic techniques, the number of cases of intraosseous lipoma reported has increased. Nonetheless, a location in the iliac bone continues to be extremely rare.

When intraosseous lipomas are symptomatic, they may generate clinical manifestations such as pain, local swelling and pathological fractures.5, 6 The lack of signs and symptoms differs intraosseous lipomas from other bone tumors and this is a difficulty that is found in diagnosing this type of tumor.5, 7 Its etiology is a matter of controversy. However, there are reports in the literature of patients with hyperlipoproteinemia and macrodystrophia lipomatosa who developed multiple intraosseous lipomas.12, 13 Sauer and Ozonoff demonstrated a possible relationship between congenital bone abnormalities and lipomas. Another reason why diagnosing intraosseous lipomas may be difficult, which has been reported in the literature, is that its radiological images may be confounded with bone infarction, osteoblastomas and, more rarely, enchondromas.

According to Milgram's classification, intraosseous lipomas are divided into three stages. Stage I comprises solid tumors with viable adipocytes; stage II comprises cases of focal transition in which fatty necrosis and focal calcification are seen, along with regions with viable adipocytes; and lastly, stage III consists of late-stage cases in which there is fatty necrosis, cyst formation, calcification and reactive formation of a new bone structure. Most of the lesions that have been described are in stage I. These stage changes result from a process of involution and infarction that these lesions undergo with the passage of time. Lesions at the initial stages cause reabsorption of the bone trabeculae that existed previously.

Because of the different presentational stages of intraosseous lipomas, they may appear on radiographs both as radiolucent lesions with a thin sclerotic border and as radiodense lesions with a thick sclerotic border. From radiography, the differential diagnosis also depends on the current stage of the intraosseous lipoma. The main differential diagnoses reported in the literature are: bone pseudocyst, bone infarction, fibrous dysplasia, osteoblastoma, enchondroma, chondroblastoma, chondrosarcoma, non-ossifying fibroma and giant-cell bone tumor.16, 17

It is believed that computed tomography (CT) scans and magnetic resonance imaging (MRI) are the most complete examinations for issuing a diagnosis, since these are capable of revealing stage I lesions and the fat rings that are characteristic of stage II and III lesions. Moreover, because of the large number of differential diagnoses given through radiography, as seen earlier, many authors have recommended that CT and MRI should be used to rule out other hypotheses.

CT is useful in diagnosing intraosseous lipomas because it reveals an attenuation that is characteristic of adipose tissue. The tissue density can be calculated through the “Hounsfield index”. Adipose tissue presents lower density than fibrous tissue and cellular neoplasms and, for this reason, is especially radiolucent. In some cases, CT images may be very typical of intraosseous lipomas, which has led many authors to suggest that biopsy can be done away with.15, 16, 17, 18

On CT, stage I lesions are characterized by reabsorption of the bone trabeculae and by bone expansion. An area of attenuation corresponding to the area of radiolucency of the radiograph can be seen. Stage II is demonstrated through areas of attenuation together with areas of calcification and fatty necrosis. Stage III lipomas are the ones that are most difficult to diagnose because of the ossification, calcification, necrosis and cyst formation.

Use of MRI for diagnosing intraosseous lipomas is important because the density of these lesions is similar to that of subcutaneous tissue, both in T1 and in T2. MRI on stage I lesions shows that they have the same density as the subcutaneous tissue in T1 and shows hyposignal in T2. IN stage II, areas of hyposignal are shown in the central region of T1 and T2, which are consistent with the areas of calcification. In addition, a ring of sclerosis can be viewed. In stage III, a thin ring of fat can be seen, along with an area of central calcification and a wide ring of sclerosis that presents hyposignal in T1 and T2. The areas of fatty necrosis show variable signal in T1 and hypersignal in T2.

Histologically, the lesions are characterized by the presence of mature fatty tissue and atrophied bone trabeculae. Differentiation between neoplastic and non-neoplastic adipose cells is fundamental, although complicated. An increased number of blood vessels may be one of the indicators of malignity. In histological analyses on lipomas, it is also common to observe mucinous degeneration and, because of this, microscopic and macroscopic cysts may also be observed.6, 19

Even histologically, it is difficult to differentiate between lipomas and bone infarctions, but certain characteristics may assist in this. In infarctions, the lack of calcifications can be highlighted. In lipomas, diminution of the trabecular structures, expansion of the cortical bone and possible presence of cysts can be highlighted.

Regarding treatment of intraosseous lipomas, surgery is not indicated in most cases of asymptomatic patients. For symptomatic patients, the tumor can be treated with curettage and implantation of a bone graft.5, 6 Malignant transformation of the tumor is rare, as is recurrence.5, 6, 19

Conflicts of interest

The authors declare no conflicts of interest.