Abstract
A 65-year-old woman presented to clinic with a lump at the dorsal aspect of left distal forearm. On clinical examination an oval, soft tissue mass without pulsation was found at the dorsal aspect of left distal forearm. The diagnosis was a rare osteolipoma found underneath the fascia of the muscles extending all the way down to the interosseous membrane and the periosteum of the radius and ulna. The mass was not connected with the interosseous membrane. Osteolipoma are uncommon but have distinguishing radiological and pathological appearances. It is important to keep this differential in mind when a lesion enclosing fatty tissue with ossification is encountered.
Key words: Case report, Freestanding, Forearm, Osteolipoma, Rare
Ossification of a lipoma was first described in 1959, and it is rarely reported.1,2 Different subclassifications of this type of lesion into “osteolipoma” or “ossifying lipoma” have been made depending on the level of fat component in the lesion. When fat component is predominant, the tumor is named ossifying lipomas. In lesions without a predominant fat component the term osteolipoma is used.3 The osseous metaplasia within a lipoma is a rare variant and normally presents in large lipomas that have been present in the tissue for a long time.4 Based on literature reports, when occurring, most osteolipoma are typically found intraosseous or adjacent to bone, and near the scapula, spine, neck, skull, and suprasellar regions.2
It is rare to find an osteolipoma that is not adjacent and/or connected to bone. When they occur, they are most often in the head and neck area and in the oral cavity.4,5 Osteolipomas in the upper extremities that occur independent of bone are rarely reported.1,6 We present a case of an osteolipoma in the forearm with no connection to underlying bony structures.
Images from ultrasound, computed tomography (CT), and histology included in this article are not identifiable. Written informed consent was obtained from the patient for publication of this manuscript and the accompanying images.
Case Report
A 69-year-old woman comes to clinic complaining of a lump at the dorsal aspect of left distal forearm. The lump was first noticed 1.5 years prior to the visit and was gradually increasing in size and she started having soreness, tingling, and numbness in her left dorsal hand.
On clinical examination an oval, soft tissue mass without pulsation was found at the dorsal aspect of left distal forearm. The mass was firm to the touch and loose, ie, not adhered to any nearby structures. The mass had defined borders. There was minimal tenderness to deep palpation of the mass. The area was tested for the Tinel sign with negative result. Finally, the left wrist, fingers, and thumb had full range of motion.
Ultrasound showed an intermuscular circumscribed fat echogenicity mass between the extensor pollicis longus and brevis muscles. The mass contains several central calcifications with no increased vascularity (Fig. 1).
Figure 1.
Ultrasound showing (arrow) a nonpulsatile atypical lipoma in the dorsal aspect of left distal forearm.
CT of the left upper extremity with contrast revealed a no enhancing circumscribed fatty mass with central tubular calcifications involving the distal forearm suggestive of atypical lipoma at the level of the interosseous membrane and extensor pollicis longus muscle making adjacent mass effect. The fatty mass invaginates dorsally at the level of the distal radius. The lesion measures approximately 1.7 × 1.8 × 6.1 cm in maximal anterior-posterior view, transverse, and longitudinal dimensions, respectively (Fig. 2)
Figure 2.
A CT scan (axial view) showing fatty mass with central tubular calcifications at the level of interosseous membrane. B CT scan (sagittal view) showing fatty mass with central tubular calcifications measuring 60.6 mm (approximately 2.39 in).
No adjacent bony remodeling or periosteal reaction was noticed, and no connection with underlying forearm bones was depicted. The preoperative differential diagnosis included a lipoma with tumoral calcinosis and possibility of liposarcoma cannot be excluded.
Excision of the mass was performed. Intraoperatively, the mass was found to be underneath the fascia of the muscles extending all the way down to the interosseous membrane and the periosteum of the radius and ulna. The mass did not appear to pierce through the interosseous membrane to the opposite volar side and it was above the dorsal aspect of the interosseous membrane. The mass was freed from surrounding tissues using blunt dissection and submitted to pathology. The procedure was uneventful.
Gross examination of the resected specimen demonstrated a 4 × 4 × 2 cm aggregate of disrupted tan-yellow to focally firm tissue with smooth surface. Cut surfaces showed focal calcified areas, and the non-calcified areas showed adipose tissue without necrosis. The specimen was decalcified, serially sectioned (4 mm slices) and tissue was paraffin embedded. The histological sections are cut at 4 microns and stained with routine hematoxylin-eosin stain.
Histological examination revealed that the tumor is comprised of mature adipose tissue (Fig. 3A). The adipocytes show no evidence of hypercellularity, atypia, mitotic activity, or necrosis. Embedded within the tumor are scattered foci of cortical type bone with focal osteoblastic rimming. Some of these bone islands displayed a surrounding rim of vascularized cellular fibrous tissue (Fig. 3B). Mature adipose tissue was noted within the bone without any hematopoietic elements. There was no definite lipoblast or features of malignancy. The pattern was consistent with an osteolipoma.
Figure 3.
A Foci of cortical bone with osteoblastic rim (arrow). B Bone with mature adipose tissue without hematopoietic elements (star).
Discussion
Atypical lipomatous tumors contain areas with other mesenchymal elements such as cartilage (chondrolipoma), fibrous tissue (fibrolipoma), or myxoid tissue, and the tumor may develop in areas other than neck, back, and shoulders.6 Osteolipomas are a rare variant of atypical lipomatous tumors and are typically reported in the literature as small case series; therefore, the true incidence has not been established. The occurrence of osteolipoma within the lipoma classification has been cited as less than 1%.7
Most cases of lipomatous lesions with osseous tissue are connected with bone in some way.5 In the presented case, there was no connection between the osteolipoma and adjacent bone tissue.
The etiology of osteolipoma is still unknown but theories regarding the cause of onset exist. One theory is that because cells in an osteolipoma resembles those of a mesenchymoma, the osteolipoma may originate from multipotent mesenchymal cells.5 An alternative theory is that metabolic changes, trauma, or ischemia can cause the fibrous cells of a lipoma to differentiate to osteoblasts, ie, metaplasia.5 Osteolipoma may initially be detected on imaging with plain film X-ray followed with other imaging modalities.
The osteolipoma typically appears homogenous and well defined on CT.6 On magnetic resonance imaging, lipomas are recognized by homogenous signal intensities like subcutaneous fat, whereas the calcified components may appear diffuse or of low signal intensity.8 Performing a contrast-enhanced fat-suppressed sequence may improve the detection of the ossification.9 The extend of a differential diagnosis for non-homogenous lipoma with ossification depends to some extent on its location. If the lesion is near or in a joint, a broader differential evaluation should be considered as mechanical factors may cause misdiagnosis between calcification and ossification.9 The differential diagnosis for a lipomatous tumor with ossification should include conditions, such as hemangioma, loose bodies, synovial chondromatosis, benign teratoma, ossifying fibroma, myositis ossificans, and others. The malignant maladies liposarcoma, extra skeletal osteosarcoma and synovial sarcomas should also be considered and may be suspected if large size, deep location, thickened septa, nodular nonadipose regions, contrast enhancement or decreased fat composition is detected on imaging.9 It is important to carefully review the results as osteolipoma can appear like a liposarcoma with metaplastic changes.5
The osteolipoma is treated by surgical excision and typically has a similar prognosis as other lipomas. A pathologist histological review of a biopsy from the tumor will result in an unequivocal diagnosis of the lesion.5
Conflicts of Interest
No benefits in any form have been received or will be received related directly to this article.
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