Sphenoid wing meningioma: Neuroradiology
Meningiomas are the most common primary orbital brain tumour, with 20% centred on the sphenoid wing (SWM). There are two main growth patterns: the nodular or 'en-masse', and the more diffuse 'en-plaque' patterns. They are classified according to their site of origin along the sphenoid ridge: lateral, middle or clinoidal. Hyperostosis and sclerosis are common and are usually due to tumour infiltration rather than a reactive phenomenon.MRI gives excellent contrast and spatial resolution, particularly with intravenous gadolinium. SWM is characterised as a well-circumscribed durally based mass, with variable T1 and T2 signal characteristics. On T2 they are usually iso- to hyperintense, and a dural tail occurs in about three quarters of cases. Up to 50% of tumours have elicited parenchymal vasogenic oedema, although this doesn't correlate with tumour size. Vessels that occur in a meningioma are visualised as serpiginous flow-void structures and can give an indication of tumour vascularity.Diffusion-weighted sequences can be helpful and rely on free random “Brownian” motion of water molecules between the intracellular and extracellular space. In lesions with increased cellularity, such as SWM, that restriction of water molecule motion is depicted as hyperintensity on the diffusion-weighted trace image. This is quantified on the corresponding apparent diffusion coefficient (ADC) map, and gives a measure of cellularity, with restricted diffusion depicted as low signal. It can also help to differentiate between tumour mass and vasogenic oedema in SWM.Meningiomas have a typical spectrum on MR spectroscopy (MRS) with an alanine peak and sometimes a glutamine peak, and although MRS is not routinely used in clinical practice, it may be helpful in atypical meningiomas. Meningiomas also express somatostatin receptors, hence the value of 68-Gallium Dotatate CT/MR imaging: the radio-labelled gallium attaches to the somatostatin receptor thus is avidly taken up thes tumours. This modality is useful where there is uncertainty about the diagnosis, where subtle recurrence is suspected or to clarify whether associated bony changes are reactive or due to infiltration.The differential diagnoses for SWM include metastasis and fibrous dysplasia, amongst other lesions, but CT can differentiate the latter due to the presence of preservation of the cortex and typical regions of intra-diploic “ground-glass” matrix despite the additional bony expansion and sclerosis.