CNS tumors have a wide variety of histological appearances and cytological origins, reflecting the complexity of the tissue from which they are derived. The successive diagnostic classifications for different brain tumors have arbitrarily emphasized one histological characteristic or another so as to provide simultaneous information on the cellular origin of the tumor, its etiology, and patient prognosis.
The CNS tumor classification systems described above ignored the special characteristics of pediatric CNS tumors, particularly the histological variation present in the same mass and the differences in how the tumors evolve. Considering the above characteristics, as well as the variability of tumor evolution depending on its location, the age of the patient, and the different histological features, in 1985 Gilles proposed the need for a classification system that could predict prognosis, i.e., survival and response to treatment (4, 29). He proposed that the classification of pediatric CNS tumors should consider, rather than solely histogenetic criteria, other criteria with prognostic significance, such as localization of the tumor mass, and, consequently, the possibility for total resection; age at presentation; the histological variability/variation within the mass; and the evaluation of certain histological characteristics that might have a prognostic significance that differed from that in adults (4). These thoughts influenced the subsequent evolution in tumor classification and resulted in the classification systems that are in current use.