Lipoma

Abstract

Rather than true neoplasms or hamartomas, intracranial lipomas are thought to represent congenital malformations that result from an abnormal persistence and maldifferentiation of the meninx primitiva. This embryologic concept explains the high frequency of callosal and other brain malformations associated with intracranial lipomas. Typically occurring in the midline, the most common locations for these lesions are the corpus callosum and the quadrigeminal cistern. In about 10–15 % of cases, intracranial lipomas are encountered in the suprasellar/interpeduncular region. Although intracranial lipomas are usually asymptomatic and often discovered incidentally on brain CT or MRI, symptoms related to intracranial lipomas have been reported. However, it cannot be excluded that symptoms might be related to an underlying associated brain malformation rather than to this congenital fat pad itself. Thus, so-called osteolipomas of the tuber cinereum have been reported in young girls with precocious puberty, but similar lesions have been found incidentally at autopsy in previously asymptomatic men (Fig. 54.1). In addition, in the rare reported cases of excision of tuber cinereum lipoma, no effect of the surgery was described on the precocious puberty itself. Finally, complete removal of intracranial lipomas by surgery is illusive because such lesions are intimately adherent to the surface of the brain, as explained by their embryogenesis. Diagnosis of lipomas is straightforward on brain imaging. On CT, they exhibit low density typical for fat and may harbor superficial calcification (Fig. 54.1). On MRI, lipomas are well-demarcated homogeneous lesions, hyperintense on T1WI and T2WI, and do not enhance following administration of gadolinium (Figs. 54.1 and 54.3). Of note, dermoid cysts are usually heterogeneous and demonstrate gadolinium uptake of their wall. It is also noteworthy that fat appears with a low-T2* signal intensity and may thus be misdiagnosed as hemorrhage. Additional T1 fat-saturation sequence makes it possible to confirm the presence of fat in the lesion and, subsequently, the diagnosis of lipoma. Such sequence rules out hemorrhage, high-protein content lesion, or any other source of T1 hyperintensity. In the sellar region, suprasellar lipoma may also be mistaken for ectopic pituitary posterior lobe, as observed in dwarfism. In the latter case, this T1 ectopic bright spot is seen at the infundibulum; this T1-high signal intensity is not attenuated on T1 fat-saturated sequence: the sella, anterior pituitary lobe, and stalk are hypoplastic, and overall, normal signal hyperintensity in the posterior aspect of the sella turcica is lacking (Fig. 54.2). If intracranial lipomas are frequently observed in midline, laterosellar lipomas might be encountered along cranial nerves (Fig. 54.3).