Tumors of the Central Nervous System

Abstract

Knowledge of normal neuroanatomy and neurodevelopment is critical to proper diagnosis. A few normal structures may be mistaken as tumor particularly on frozen sections.The external granular layer (Fig. 6.1a ) is a layer of small neurons that covers the external surface of fetal/infant cerebellum. It is quite prominent at the time of birth but is almost completely regressed by the ninth postnatal month. It should not be misinterpreted as medulloblastoma or lymphocytic infi ltration in younger infants. The internal granular layer is also composed of small neurons. Without a size comparison, these cells may be mistaken for medulloblastoma or other small blue cell tumors in minute specimens or cytologic preparations. Conversely medulloblastoma cells, particularly those with smaller nuclei, can be mistaken as internal granular cells. Purkinje cells are rather large and should not be mistaken as rhabdoid cells (Fig. 6.1b ). Other normal structures that can be mistaken for neoplastic tissue include the posterior pituitary and the pineal gland which may be mistaken as glial neoplasm and normal choroid plexus being mistaken as choroid plexus papilloma. These misinterpretations are likely to be committed when the specimen is minute and communication between the pathologist and the neurosurgeon is poor. Overview and Classifi cationTumor of the CNS is not uncommon in the pediatric age group and some of these entities occur only rarely in adults (Table 6.1 ). While meningiomas are very common in adults, it is uncommon in children and almost nonexistent in infants. The most common pediatric tumors in descending order of frequency are pilocytic astrocytoma, diffuse astrocytomas, medulloblastomas, and craniopharyngiomas. Ependymal neoplasms and schwannomas are the most frequent infratentorial tumors. Most tumors occur in the brain with a supratentorial-to-infratentorial ratio of approximately 2-to-1. Malignant CNS neoplasms represent 16.6 % of all malignancies in the pediatric age group. In general, the incidence is higher in males than females and among white compared to black children. The incidence is 36.2 per million between infancy and 7 years of age and 21.0 per million between 7 and 10 years of age. As a group, they are the second most frequent malignancy and most common solid tumor in pediatric patients. About 2,200 new cases are diagnosed annually in the United States. Among CNS malignancies, astrocytic tumors account for 52 %, while medulloblastoma and primitive neuroectodermal tumor (PNET), other gliomas, and ependymomas accounted for 21, 15, and 9 %, respectively [ 2 ].In contrast to adults, there is a high occurrence of malignancies in the cerebellum and brain stem in young children. About 40 % of all thalamic tumors arise in patients under 18 years old and account for 2-5 % of all intracranial tumors in children [ 3 ]. About two third or pediatric thalamic tumors are high grade in histology [ 4 ]. Tumor of the spinal cord represents about 9.9 % and extramedullary tumors are slightly more common than intramedullary...