(a) Purely intrasellar-infradiaphragmatic
(b) Intra- and suprasellar, infra- and supradiaphragmatic
(c) Supradiaphragmatic parachiasmatic, extraventricular
(d) Intra- and extraventricular
(e) Paraventricular in respect to the third ventricle
(f) Purely intraventricular
Purely intraventricular type may lead to the misunderstanding that the tumor originated from nervous tissues
Many tumors are classified into “giant” type which is not suitable for guiding the surgical approach
(I) Intrasellar or infradiaphragmatic
(II) Occupying the cistern with/without an intrasellarcomponent
(III) Lower half of the third ventricle
(IV) Upper half of the third ventricle
(V) Reaching the septum pellucidum or lateral ventricles
Tumors classified as IV and V may have a good prognosis, which is not in line with common sense as the complicated surrounding structure
Grade 0, a tumor of subdiaphragmatic origin with competent diaphragma sellae growing in the pre-chiasmatic direction
Grade 1, a tumor of subdiaphragmatic origin with incompetent diaphragma sellae growing through the diaphragmatic aperture in the retro-chiasmatic direction
Grade 2, a tumor of supradiaphragmatic origin growing retrochiasmatically in the direction of the third ventricular floor or into adjacent cerebrospinal fluid spaces
Plenty of tumors growing pre-chiasmatically
This classification may lead to the misunderstanding that the tumor originated from nervous tissues
Q, originating beneath the diaphragma
S, originating from the extra-arachnoidal and intra-arachnoidal segments of the pituitary stalk
T, originating at the tubero-infundibulum
Based on embryology and originating sites
Based on surrounding membranous structures
However, most classification systems do not emphasize the Rathke’s pouch cell origin and the peripheral structures, especially the membrane structure on the growth pattern of the tumor, which leads to the following two situations. First, some classification systems violate the embryology perspective, for example, intraventricular craniopharyngioma. Second, some classification systems are only based on the relative position of the tumor and anatomical structure, which does not allow for the classification of some large tumors, thus losing the guiding meaning of treatment.
The QST scheme supports Erdheim’s embryological theory regarding the origin of these lesions from remnants of incompletely involuted hypophyseal duct/Rathke’s pouch remnants along the pituitary-hypothalamic axis. In particular, Erdheim found the presence of squamous cells, the presumed remnants of Rathke’s pouch migratory pathway, at two specific sites, that is, the dorsal surface of the pituitary gland (which may well correspond to the development of type Q) and the distal end swelling of the pars tuberalis just beneath the chiasm-infundibulum junction. This second location matches the high rate of T-type lesions found in this study. Curiously, Erdheim found small nests of squamous cells along the middle extension of the pars tuberalis wrapping the pituitary stalk, between the pituitary gland and the infundibulum, and the S type developing around the middle area of the pituitary stalk was the least often found (when CPs are erroneously categorized as “suprasellar” lesions in most textbooks and monographs).
The QST scheme incorporates analysis of the meningeal relationships at the tumor boundaries, that is, the arachnoid and pia mater, which can affect the growth patterns of tumors. Morphological heterogeneity among CPs may well correspond to the pattern of meningeal covering proposed in the QST scheme. Multiloculated lesions could develop when ASPS allows the expansion of the tumor throughout the intra-arachnoidal spaces of the chiasmatic cistern for the S and T types.
In the QST system, the tumor origin determines the tumor type, but the surrounding membrane structure can affect the tumor growth pattern. Understanding the pathological relationship between tumors and peripheral structures is of great significance for identifying and protecting important structures during surgery.
4.2 Some Case of Craniopharyngiomas Difficult to Be Classificated
This is a case of giant type Q craniopharyngioma. In other classification systems, it is not possible to classify this tumor because the tumor is so large that it is difficult to discern the relationship between the tumor and surrounding structures. At the same time, the tumor occupied the space of the third ventricle. In previous classification systems, there was no suitable type for the tumor. In the QST system, we classified this tumor as type Q because the tumor originated in the pituitary fossa and grew upward; although it occupied the space of the third ventricle, there was a multilayer membrane structure between the tumor and the third ventricle floor. Previous typing methods couldn’t classify this huge tumor. This case will be classified as level 4 if applying the previous typing method, which implicated the worst prognosis according to their statement. But as we know that this is a tumor of type S and will result optimistic outcome regularly (Fig. 4.1).
This is a type S tumor. This shape of tumor is irregular and cannot be classified in other systems. On MRI, we can see that the pituitary fossa is not enlarged and the pituitary gland is visible, indicating that the tumor does not have a subsellar origin. This tumor originates from the remnants of the Rathke’s’ pouch precursor cells around the pituitary stalk. For some part of the ASPS and Liliequist membrane is not strong enough, the tumor could expand around the pituitary stalk, even into the space of the cervical spinal cord. The third ventricle floor is pushed upward by the tumor, but there remains membrane structure separating the tumor and the third ventricle floor (Fig. 4.2).
This is a case of type T tumors. Although the morphology of this tumor is similar to Q-type tumor, they have a completely different origin and growth pattern. This tumor originates from the remnants of Rathke’s pouch precursor cells near tuberalis. The cystic part occupies the space of the third ventricle, and some of the tumor break through the inner arachnoid and expanded into the subarachnoid space. Some other tumors break through the pia mater of the pituitary stalk, and grow into the stalk, and then expand into the pituitary fossa. Therefore, the pituitary stalk and pituitary gland are often not visible on the MRI. Previous typing methods couldn’t classify this huge tumor. This case will be classified as level 4 if applying the previous typing method, which implicated the worst prognosis according to their statement. But as we know that this is a tumor of type Q and will result optimistic outcome regularly (Fig. 4.4).