The diencephalon (“interbrain”) is the region of the embryonic vertebrate neural tube that gives rise to posterior forebrain structures. In development, the forebrain develops from the prosencephalon, the most anterior vesicle of the neural tube that later forms both the diencephalon and the telencephalon. In adults, the diencephalon appears at the upper end of the brain stem, situated between the cerebrum and the brainstem.
Lesions arising at the interface of the diencephalon and mesencephalon have lacked classification specificity. In fact, these neoplasms have been termed “thalamic,” “brainstem,” or “basal ganglia” tumors in the past.