Radiation Oncology/Medulloblastoma/Overview

Medulloblastoma Overview

Epidemiology

Most common malignant brain tumor of childhood (20% of CNS tumors in children <19 years old; glioma #1). 550 cases in the US
Median age of presentation is 5-7 years old. 75% medulloblastomas occur in children <15 years old.
Slight male predominance.
Highest risk of CSF+ at presentation (30-35%)

Increased intracranial pressure: Headaches, nausea, vomiting
Cranial nerve deficits
Ataxic gait
In infants: loss of milestones, increased head circumference, head tilt due to CN IV palsy
Clinical exam: papilledema, nystagmus, CN abnormalities (VI most common -> "setting sun" sign with downward gaze)
50-75% have <3 months of symptoms; prolonged symptoms duration multiplies probability of favourable treatment outcome of basic disease

Plain films: often invisible
CT: hyperdense on unenhanced study, enhances with contrast, can appear homogenous
MRI: hyperintense on T1, heterogenous on T2, often compresses 4th ventricle
Get imaging before starting steroids
- Pre-op MRI brain and spinal cord (post-op spine can give false positives)
- Post-op MRI brain within 48 hours
Pre-op CSF before surgery if safe (no increased ICP), else 10-14 days after surgery to avoid false-positive findings from surgical debris
If advanced, metastatic work-up

"Small round blue cell"; classified as primitive neuroectodermal tumor (PNET)
Medulloblasts are derived from cerebellar granule cells (account for almost half of all neurons in CNS), through activation of Sonic Hedgehog Pathway, likely from external granluar layer of the cerebellum
Genetics: Loss of 17p, 10q, 16q, chromosome 11; excess of 17q, chromosome 7

Molecular profiling risk stratification (PMID 21357776)
- Excellent prognosis: Wnt pathway
- Good prognosis: Hh pathway (Hedgehog)
- Fair prognosis: all others
- Poor prognosis: c-MYC amplification
Histological risk stratification
- Histologically poor prognosis: Large-cell anaplastic (LCA) variant, diploid DNA, LOH 17p, p53 mutation and expression, low TrkC (tyrosine kinase that mediates neuronal differentiation; 4.8x risk of death), MYC mRNA (c-myc/MYCC) expression, HER2-neu positive (ErbB2+), HER2+/HER4+ coexpression
- Histologically better prognosis: Desmoplastic variant, high TrkC expression, ErbB2-, β-catenin (beta-catenin) nucleopositivity
Historical risk stratification:
- Clinically poor prognosis: Age <3 years, M2-4 disease, unresectable or subtotal resection (>1.5 cm2)
- Clinically better prognosis: Age >5-7 years, M0 disease (no metastasis), tumor is not visible on the postoperative image

Dissemination: CSF seeding, intracranial, subarachnoid space, mets outside CNS <5%
Most recurrences within two years, at the site of primary tumor (50%)
Medulloblastoma tends to follow Collins' law (see PMID 9213055), which defines the period of risk for recurrence as the patient's age at diagnosis + 9 months of gestation. According to Collin's law, if a patient has no evidence of recurrence within this time period, he is considered cured (e.g. a 3 year old with medulloblastoma is cured when his disease free interval reaches 3 years + 9 months). The premise is that if this tumor was present in utero, then gestation+age determines rate of growth for it to become clinically evident. Therefore, if there is residual disease after treatment, it should become clinically evident again within the same time-frame
10-year survival for all stages: 40-50%
For today considerable part of survivors is living 20-25 years and more from the moment of primary treatment

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