Is there a differential?
This is pretty characteristic but one could consider a CSF cyst (arachnoid / ependymal / neuroepithelial cyst), an very large pineal cyst or at a stretch an epidermoid cyst.
Non contrast CT demonstrates a pyramidal (triangular in axial section) CSF density structure located in the midline, separating the thalami and extending as far forward as the foramen of Monro.