Posterior Fossa Tumors In Children
Epidemiology And General Overview
Posterior fossa (infratentorial) tumors represent the majority of pediatric central nervous system (CNS) malignancies.
- Comprise 60-70% of childhood brain tumors.
- Predominate in children aged 1-10 years.
- Pathogenesis involves diverse histologic subtypes; primitive embryonal tumors and low-grade gliomas remain most prevalent.
Relative Incidence And Prognostic Summary
| Tumor Type | Relative Incidence (%) | Clinical Presentation | Diagnostic Imaging | Prognosis |
|---|---|---|---|---|
| Medulloblastoma | 35-40 | 2-3 months headache, emesis, truncal ataxia | Homogeneously enhancing 4th ventricular mass | 65–85% survival; poorer in infants (20–70%) |
| Cerebellar Astrocytoma | 35-40 | 3-6 months limb ataxia, secondary headache, emesis | Cystic mass with solid mural nodule | 90–100% survival with total resection |
| Brainstem Glioma | 10-15 | 1-4 months diplopia, weakness, cranial nerve dysfunction | Diffusely expanded, minimally enhancing mass (diffuse type) | >90% mortality (diffuse); better in localized types |
| Ependymoma | 10-15 | 2-5 months ataxia, headache, diplopia, facial asymmetry | Enhancing 4th ventricular mass, cerebellopontine predilection | >75% survival with total resection |
| Atypical Teratoid/Rhabdoid (AT/RT) | 5-10 (infantile malignant tumors) | Strabismus, facial weakness, macrocephaly (infants) | Laterally extended mass | ≤20% survival in infants |
Clinical Manifestations
Clinical presentation correlates with anatomic location, growth rate, and obstruction of cerebrospinal fluid (CSF) drainage.
Signs Of Raised Intracranial Pressure (ICP)
- Morning headache: Exacerbated by recumbency; manifests as irritability in young children.
- Emesis: Predominantly early morning.
- Visual disturbances: Intermittent blurred vision, diplopia, papilledema.
- Cranial nerve VI palsy.
- Macrocephaly: Occurs in infants with open cranial sutures.
- Cushing triad: Hypertension, bradycardia, altered respiratory pattern (indicates acute ICP elevation).
Cerebellar And Brainstem Dysfunction
- Disorders of equilibrium: Ataxia, dysmetria, broad-based gait.
- Torticollis: Secondary to cerebellar tonsil herniation.
- Brainstem deficits: Gaze palsy, multiple cranial nerve palsies, hemiparesis, hyperreflexia, clonus.
Diagnostic Evaluation
Immediate neurologic assessment and neuroimaging required.
Neuroimaging
- Magnetic Resonance Imaging (MRI): Gold standard; performed with and without gadolinium contrast.
- Computed Tomography (CT): Utilized acutely to detect hydrocephalus, hemorrhage, or calcifications.
- Spinal MRI: Essential to evaluate leptomeningeal spread (drop metastases).
Cerebrospinal Fluid (CSF) Analysis
- Lumbar puncture for cytologic evaluation.
- Contraindicated in newly diagnosed hydrocephalus with CSF obstruction prior to shunt/ventriculostomy to prevent fatal brain herniation.
- Polyamine assays: Detect tumors approximating CSF pathways (medulloblastoma, ependymoma, brainstem glioma).
Specific Tumor Profiles
Medulloblastoma
Most common pediatric malignant CNS tumor; arises within cerebellum. Represents ~62% of embryonal tumors.
Pathology And Staging
- Histopathology: Small round blue cells, Homer Wright rosettes, synaptophysin immunopositivity.
- Extent of Disease: Chang staging system evaluates CSF spread, nodular leptomeningeal disease, and rare extraneural metastases.
Molecular Subgroups
Classification dictates prognosis and treatment intensity.
| Subgroup | Genetics / Mutations | Histology | Prognosis |
|---|---|---|---|
| WNT | CTNNB1 mutation, 6q loss | Classic | Very good (5-year survival 97%) |
| SHH | PTCH1, SMO, GLI2, MYCN amplification | Desmoplastic / Nodular | Good in infants; Intermediate in older |
| Group 3 | MYC amplification, i17q | Classic, Large cell/anaplastic | Poor (survival <50% with MYC amp) |
| Group 4 | CDK6 amplification, i17q | Classic, Large cell/anaplastic | Intermediate |
Management
- Maximal safe surgical resection (gross total or near-total resection <1.5 cm² residual).
- Post-operative Craniospinal Irradiation (CSI): Highly radiosensitive; 23.4 Gy for average-risk, 36 Gy for high-risk, with tumor bed boost (54-55.8 Gy).
- Chemotherapy: Cisplatin, lomustine, vincristine, cyclophosphamide regimens.
- High-dose chemotherapy with autologous stem cell rescue replaces CSI in children <3 years to prevent devastating neurocognitive late effects.
Cerebellar Astrocytoma
Predominantly Juvenile Pilocytic Astrocytoma (JPA, WHO Grade I).
Pathology And Genetics
- Microscopic features: Biphasic appearance (compact fibrillary tissue interspersed with loose microcystic areas), Rosenthal fibers, low mitotic potential.
- Molecular genetics: MAPK pathway overactivation; predominantly KIAA1549-BRAF fusion or BRAF V600E point mutations.
Management
- Gross total surgical resection often curative; 90-100% survival.
- Subtotal resection managed with observation; second resection upon progression.
- Chemotherapy (carboplatin, vincristine) utilized for progressive, unresectable disease.
- Targeted therapy: MEK inhibitors (trametinib) for BRAF fusions; dabrafenib for BRAF V600E mutations.
Ependymoma
Accounts for 10-15% of posterior fossa tumors; originates from ependymal lining of the 4th ventricle. Peak incidence at 6 years.
Pathology And Subgroups
- Histology: Perivascular pseudorosettes, ependymal rosettes, monomorphic nuclei.
- Molecular subgroups:
- Posterior Fossa EPN-A: Typical in young children (median 3 years); dismal outcome (10-year OS 50%).
- Posterior Fossa EPN-B: Typical in older adolescents; better prognosis.
Management
- Gross total surgical resection is the primary prognostic determinant.
- Focal radiation therapy (54-60 Gy) essential post-resection to prevent local recurrence.
- Adjuvant chemotherapy lacks established benefit, though utilized in infants to delay radiation.
Brainstem Glioma
Constitutes 10-15% of posterior fossa tumors. Outcomes strictly depend on tumor location and imaging characteristics.
Classification
- Diffuse Intrinsic Pontine Glioma (DIPG): 70-85% of brainstem tumors.
- Focal, Dorsally Exophytic, Cervicomedullary variants: 15-30% of brainstem tumors.
Pathology And Genetics
- DIPG characterized by diffuse infiltrating grade II-IV glioma.
- Genetics: ~80% harbor H3K27M mutation in histone H3.3 or H3.1; ~20% harbor ACVR1 mutations.
Management
- DIPG: Not amenable to surgical resection. Standard therapy is palliative focal radiation (54 Gy); median survival <12 months.
- Focal/Dorsally Exophytic: Surgical resection viable; favorable outcome as these are typically low-grade gliomas.
Atypical Teratoid/Rhabdoid Tumor (AT/RT)
Highly aggressive embryonal tumor representing 10-20% of CNS tumors in children <3 years.
Pathology And Genetics
- Histology: Heterogeneous cell pattern including rhabdoid cells expressing epithelial membrane and neurofilament antigens.
- Cytogenetics: Partial or complete deletion of chromosome 22q11.2; characteristic loss of INI1/SMARCB1 gene.
Management
- Highly refractory to conventional medulloblastoma therapy; uniformly fatal without intensive multimodal protocols.
- Favorable responses observed with high-dose alkylating agents, high-dose methotrexate, focal radiation, and consolidation with triple autologous stem cell rescue.
Complications And Late Effects
Over 50% of childhood brain tumor survivors experience chronic late effects secondary to tumor infiltration and multimodal therapy.
Neurologic And Neurocognitive Sequelae
- Radiotherapy and intrathecal/high-dose chemotherapy (methotrexate, cytarabine) induce neurotoxicity.
- White matter destruction (leukoencephalopathy) leads to memory impairment, visual-motor integration deficits, lowered IQ, and seizures.
- Mineralizing microangiopathy: Dystrophic calcification in basal ganglia causes gait abnormalities and memory deficits.
- Somnolence syndrome: Subacute radiation toxicity presenting with lethargy, hypersomnolence (up to 20 hours/day), and transient cognitive dysfunction.
Endocrine Dysfunction
- Craniospinal and cranial irradiation induce hypothalamic-pituitary axis damage.
- Deficits include growth hormone deficiency, delayed or precocious puberty, hypogonadism, and central hypothyroidism.
Secondary Neoplasms
- Ionizing radiation establishes significant risk for secondary malignancies, notably secondary high-grade gliomas or meningiomas occurring years to decades post-treatment.