Intraosseous lipoma of the parietal bone

Clinical History

Headache. Unremarkable physical examination and medical history.

Imaging Findings

As the patient suffered from headache, cranial radiographs were taken.
The conventional radiograph in occipitofrontal and lateral projection showed a well defined, radiolucent, oval lesion with a thin sclerotic rim in the left parietal bone (Fig. 1 a,b).
The lesion was oval with a thin sclerotic rim, hypodense and wit a density of -44 Hounsfield units on CT (Fig. 2). There was no calcification within the lesion.
On MRI, the lesion was bright on T1 and T2 weighted images (Fig. 3 a,b,c,d). The patient rejected surgical excision or biopsy of the lesion. Follow up was recommended.

Discussion

Although fat represents one of the normal connective tissue elements within the bone, intraosseous lipomas are exceedingly rare [1]. The infrequency of lipomas of bone may be explained, in part, by the classification of osseous lipomas as other processes, including ischemic necrosis, simple or aneurysmal bone cysts, or fibrous dysplasia on the basis of their radiographic or histologic characteristics. In fact, osseous lipoma has been considered either a neoplasm or a degenerative phenomenon related to trauma, infection, or vascular compromise [2]. As in our case, these tumours may be entirely asymptomatic; however, approximately two thirds of patients with intraosseous lipomas have localized pain and soft tissue swelling [2]. The most common sites of intraosseous lipomas are, metaphyses of the long bones and calcaneus [1,2]. There are very few reported cases of intraosseous lipomas within the skull bones. Arslan et al. in 2000 have reported a case of frontal bone lipoma and mentioned that only 12 cases had been reported [3]. In addition parietal localization is significantly rare. To our knowledge, only two parietal bone lipomas have been previously reported and these lipomas were located frontoparietally [2].
Milgram described a classification of intraosseous lipomas. The roentgenologic features of an intraosseous lipoma are determined by the stage of the lesion.
Stage 1 lesions contain only viable lipocytes within the lesion and appear well defined and radiolucent on plain films and CT.
In stage 2 lesions, in addition to stage 1, there are areas of increased density due to calcification secondary to necrosis.
In stage 3 lesions the involution is completed and original trabecular bone patern is absent. Radiodensity is both peripheric and central (5). Our case is an example of stage 1 lesion, there was no calcification in it, either peripherally or centrally.
CT and MRI are the best radiologic methods for the diagnosis of lipoma. CT is able to identify the fatty component of an intraosseous lipoma with the characteristic low attenuation value of such tissue. In MRI, lipomas demonstrate a characteristic high signal on T1 and T2 weighted images [2].
Our patient refused surgical excision or biopsy of the lesion, so the diagnosis was only made by radiologic methods. In our case, the lesion was a typical lipoma with its CT and MRI characteristics.
As differential diagnosis of intraosseous lipomas: bone infarcts, enchondromas, bone cysts, chondromyxoid fibromas, osteoblastomas and fibrous dysplasia are the pathologies to be considered [1,2]. For enchondromas and bone infarcts there is a different situation. Especially a stage 3 lesion would be difficult to differantiate from enchondroma, as they have the same radiologic appereance. In this situation tissue examination would be requıred.
Pathologically, intraosseous lipomas may have a few trabeculae that may appear necrotic or normal. Histologically it may not be possible to differantiate between a necrotic intraosseous lipoma and a bone infarct [2]. The excision of lipomas in asymptomatic patients is not necessary for the risk of malignant change is very low, the diagnosis would be certain by CT and MRI [5].

Differential Diagnosis List

Final Diagnosis

Intraosseous lipoma of the parietal bone

URL:	https://www.eurorad.org/case/2651
DOI:	10.1594/EURORAD/CASE.2651
ISSN:	1563-4086