The typical pathological feature of pCP is the stratified papillary squamous epithelium. At the center of the squamous epithelium, the fibrous vascular core is visible. The cells around the fiber core are called the basal cells. In the tumor parenchyma, new blood vessels of different sizes and densities can be seen. In pCP, calcification and wet keratin are rare, and cystic cavities are still visible in some tumor parenchyma (Fig. 3.2).

Previously, we found that the suprasellar arachnoid arising from the basement membrane of the arachnoid envelops the pituitary stalk to form an arachnoidal sleeve (ASPS). The pituitary stalk was divided into four segments in accordance with the folds of the ASPS, namely, the infradiaphragmatic, extra-arachnoidal, intra-arachnoidal, and subarachnoidal segments (Fig. 3.3).

On the basis of the tumor origin and on the presence of an arachnoid envelope around the pituitary stalk (ASPS), we established a QST typing system for craniopharyngioma (Fig. 3.4). Type Q tumors originate below the diaphragma, and the pituitary gland is therefore most likely to be involved. Type S tumors originate from pars tuberalis adherent to the extra-arachnoidal and intra-arachnoidal segments of the pituitary stalk, and this type of tumor is more likely to involve the pituitary stalk. The pituitary gland and the third ventricle floor remain almost normal or may be slightly displaced by the tumor. Type T tumors originate at the top of pars tuberalis. These tumors are located in the subarachnoid space, outside the pia mater, and usually occupy the third ventricle cavity, involving the third ventricle floor and the proximal pituitary stalk; the distal pituitary stalk and pituitary gland are usually normal.