Historically, staging was based on an intraoperative evaluation of both the size and extent of the tumor, coupled with postoperative neuroimaging of the brain and spine and cytological evaluation of cerebrospinal fluid (CSF) (the Chang system). Intraoperative evaluation of the extent of the tumor has been supplanted by neuraxis imaging prior to diagnosis and postoperative imaging to determine amount of primary site residual disease. The following tests and procedures are now used for staging:
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M2 —gross nodular seeding in cerebellar-cerebral subarachnoid space and/or lateral or third ventricle.
M3 —gross nodular seeding in spinal subarachnoid space.
M4 —extraneural metastasis.
Postoperative degree of residual disease is designated as:
Gross-total resection/near-total resection—no or minimal (not measurable) evidence of residual disease after diagnosis.
Subtotal resection—residual disease after diagnosis; this is conventionally further subdivided into less than, greater than, or equal to 1.5 cm2 of measurable residual disease.
Biopsy—no tumor resection; only a sample of tumor tissue is removed.
Since the 1990s, prospective studies have been performed using this staging system to separate patients into average-risk and high-risk medulloblastoma subgroups.[2,3,4]
The presence of diffuse (>50% of the pathologic specimen) histologic anaplasia has been incorporated as an addition to staging systems. If diffuse anaplasia is found, patients with otherwise average-risk disease are up-staged to high-risk disease.
Staging of CNS Primitive Neuroectodermal Tumors (PNETs)
Patients with CNS PNETs are staged in a fashion similar to that used for children with medulloblastoma; however, the patients are not assigned to average-risk and high-risk subgroups for treatment purposes (refer to the Staging of Medulloblastoma section of this summary for more information).
Staging of Medulloepithelioma and Ependymoblastoma
Dissemination of both medulloepitheliomas and ependymoblastomas frequently occurs.[5,6] The tumors are staged in the same way as medulloblastoma; however, the patients are not assigned to average-risk and high-risk subgroups for treatment purposes (refer to the Staging of Medulloblastoma section of this summary for more information).