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🧠 POSTERIOR FOSSA TUMOURS — COMPLETE NEUROSURGERY VIVA GUIDE

Sources: Bradley & Daroff's Neurology in Clinical Practice, Adams & Victor's Principles of Neurology (12th ed.), Robbins & Kumar Basic Pathology, Harrison's Internal Medicine 22E

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📊 QUICK DIFFERENTIAL DIAGNOSIS AT A GLANCE

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🔵 TUMOR 1: MEDULLOBLASTOMA

1. DEMOGRAPHY

• Most common malignant brain tumor of childhood — accounts for ~20% of all childhood brain tumors, ~9.3% of all pediatric brain tumors
• Males > Females — 3:1 to 3:2 (varies by molecular subtype; SHH-activated has equal sex distribution)
• Peak age: 4–8 years; second smaller peak 18–25 years (adults rare)
• >50% occur in children under age 10
• Incidence stable over 20 years (SEER registry data)

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2. GENETICS / ETIOLOGY

Molecular Subgroups (WHO 2021):

Chromosome 17:

• Isochromosome 17q (i17q) — most common cytogenetic abnormality (>40%)

Familial Syndromes:

• Gorlin syndrome (Nevoid Basal Cell Carcinoma Syndrome) — PTCH gene mutation on 9q → desmoplastic/nodular variant
• Turcot syndrome — APC/WNT pathway mutations (colorectal polyps + brain tumor)
• Li-Fraumeni syndrome — TP53 mutation

Histological Variants:

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3. CLINICAL FEATURES

Symptoms (from hydrocephalus — 4th ventricle obstruction):

• Morning headaches awakening child from sleep (key feature)
• Projectile vomiting (often misdiagnosed as GI disease initially)
• Papilledema (due to raised ICP)
• Ataxia of gait + frequent falls → truncal ataxia (vermis involvement)
• 6th nerve palsy (false localizing sign from raised ICP)
• Diplopia, strabismus
• Mental torpor, deteriorating school performance

Duration before diagnosis: typically 1–5 months

Metastatic Spread:

• CSF/Leptomeningeal seeding — most common (drop metastases to spine)
• Bone mets — rare
• Lymph node — very rare
• Hematogenous — rare

Chang Staging (M-stage for metastasis):

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4. RADIOLOGY

CT:

• Hyperdense (small blue cells, high N:C ratio) midline cerebellar mass
• Homogeneous contrast enhancement
• Hydrocephalus (dilated lateral + 3rd ventricles)
• Calcification in ~20%

MRI (Investigation of Choice):

Whole Spine MRI: MANDATORY for staging before surgery (M-staging for drop mets)

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5. TUMOR MARKERS

• No specific serum tumor marker for medulloblastoma
• CSF cytology: malignant cells may be found (confirm M1 staging)
• LDH: elevated in CSF (nonspecific)
• Molecular markers (tissue): β-catenin nuclear staining (WNT), GLI2/PTCH (SHH)

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6. BIOPSY & PATHOLOGY

Cell of Origin:

• Pluripotential stem cells of the external granular layer OR subependymal matrix of 4th ventricle

How/When to Take Biopsy:

• Preferred: Gross total surgical resection (primary) — tissue obtained at surgery
• Stereotactic biopsy if unresectable (rare for medulloblastoma)
• Avoid CSF sampling before surgery if herniation risk is high

Histology (H&E):

• Small round blue cells — scant cytoplasm, hyperchromatic round/carrot-shaped nuclei
• Homer Wright rosettes (neuroblastic rosettes) — tumor cells around neuropil core (no true lumen) — in ~50%
• Numerous mitoses and apoptotic bodies
• Synaptophysin-positive areas (neuronal differentiation)

IHC Panel:

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7. MANAGEMENT

🔪 Surgery

1. Position: prone, head fixed in Mayfield clamp, neck flexed
2. Suboccipital midline incision
3. Craniectomy/craniotomy at foramen magnum level
4. Dural opening
5. Identify tumor in vermis/4th ventricle
6. Microsurgical resection with ultrasonic aspirator (CUSA)
7. Identify and protect brainstem floor of 4th ventricle
8. Confirm hemostasis, duraplasty if needed
9. Temporary EVD (external ventricular drain) if hydrocephalus pre-op

• Posterior fossa syndrome (cerebellar mutism — 25% of patients)
• CSF leak
• Cranial nerve palsies (6th, 7th)
• Bleeding from vermian veins

☢️ Radiotherapy

• Craniospinal irradiation (CSI) is MANDATORY — medulloblastoma is radiosensitive
• Standard risk: CSI 23.4 Gy + posterior fossa boost 54–55.8 Gy
• High risk: CSI 36–39.6 Gy + PF boost 55.8 Gy
• Infants <3 yr: Avoid RT (neurocognitive effects) → chemotherapy first, delay RT
• Proton beam therapy preferred to reduce integral dose

💊 Chemotherapy

🎯 Targeted Therapy

• Vismodegib (GDC-0449) — Hedgehog pathway (SMO inhibitor) for SHH-activated tumors
  • Dose: 150 mg/day orally
  • MOA: Inhibits Smoothened → blocks SHH signaling
  • Side effects: Muscle cramps, alopecia, teratogenicity, premature growth plate fusion in children
• BET bromodomain inhibitors and CDK4/6 inhibitors — investigational for Group 3/4

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🔑 MANAGEMENT CRUX FLOWCHART — MEDULLOBLASTOMA

Suspected Medulloblastoma (child, morning headache, vomiting, ataxia)
          ↓
MRI Brain + Whole Spine + CSF cytology (post-op)
          ↓
Staging: Standard risk vs High risk
   Standard: M0, GTR or <1.5cm² residual, no LCA histology
   High risk: M1–M4, or subtotal resection, or LCA/anaplastic, or age <3
          ↓
SURGERY (Posterior Fossa Craniotomy + GTR)
          ↓
CSF cytology (lumbar puncture ≥2 weeks post-op)
          ↓
┌─────────────────────────────────────┐
│ STANDARD RISK (>3 yr, M0, GTR)     │ HIGH RISK / <3 yr
│ CSI 23.4 Gy + PF boost 54 Gy      │ CSI 36 Gy + PF boost
│ + Vincristine (concurrent)         │ + Intensive chemo
│ + CCNU + Cisplatin (adjuvant)      │ (if <3 yr → chemo only first)
└─────────────────────────────────────┘
          ↓
Follow-up MRI every 3–6 months × 5 yr, then annually

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8. FOLLOW-UP & PROGNOSIS

• 5-year survival: 70–80% overall; WNT subgroup >95%, SHH-TP53 mutant worst
• Standard risk: 80% 5-year EFS
• High risk: 60–65% 5-year EFS
• MRI brain + spine every 3 months for 2 years, then every 6 months, then annually
• Neuropsychological testing (IQ, memory) — radiation effects
• Audiogram (cisplatin ototoxicity)
• Endocrine surveillance (GH deficiency, hypothyroidism from CSI)

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🟢 TUMOR 2: PILOCYTIC ASTROCYTOMA (PA)

1. DEMOGRAPHY

• Most common glioma in children — ~17.6% of all pediatric brain tumors
• Most common pediatric cerebellar tumor overall
• WHO Grade I — benign, potentially curable
• No clear gender predilection
• Predominantly first two decades of life (but can occur in adults)
• NF1 (Neurofibromatosis Type 1) associated — bilateral optic nerve gliomas; germline NF1 mutation

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2. GENETICS / ETIOLOGY

Molecular Hallmark:

• BRAF-KIAA1549 fusion (~70% of sporadic cases) — constitutively active BRAF → MAPK/ERK signaling
• BRAF V600E mutation — lower percentage; more common in cerebral/diencephalic PAs
• FGFR1 mutations, NTRK2 fusions — rare subsets
• NF1 gene inactivation — NF1-associated cases
• No IDH mutation (distinguishes from diffuse astrocytomas)

Predisposing Conditions:

• NF1 — most important; optic pathway gliomas often indolent in NF1
• No other definite environmental factors

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3. CLINICAL FEATURES

Locations (in order of frequency):

1. Cerebellum (most common) — cerebellar hemisphere
2. Optic pathways (optic nerve, chiasm) — especially in NF1
3. Hypothalamus / 3rd ventricle
4. Brainstem (dorsal exophytic)
5. Cerebral hemispheres
6. Spinal cord (rare)

Symptoms by Location:

• Cerebellum: Raised ICP (headache, vomiting), limb ataxia, papilledema
• Optic pathway: Visual loss, proptosis, nystagmus
• Hypothalamus: Diencephalic syndrome (in infants) — emaciation despite normal linear growth, hyperemesis, hyperkinesis, nystagmus (classic viva question!)
• Brainstem: Obstructive hydrocephalus, cranial nerve palsies

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4. RADIOLOGY

MRI — Investigation of Choice:

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5. TUMOR MARKERS

• No specific serum marker
• Molecular testing (tissue): BRAF-KIAA1549 fusion (FISH or RT-PCR), BRAF V600E (IHC or sequencing)

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6. BIOPSY & PATHOLOGY

Cell of Origin: Astrocytes (GFAP-positive glial cells)

When to Biopsy:

• Tissue diagnosis should always be attempted (except NF1 patients with typical optic pathway glioma — may observe)
• Resection specimen is standard; stereotactic biopsy for inaccessible lesions (e.g., hypothalamus)

Histology — KEY FEATURES:

• Biphasic pattern — dense compact pilocytic areas + loose microcystic spongy areas (most important histologic feature)
• Piloid cells — bipolar astrocytes with long hair-like processes
• Rosenthal fibers — bright red/pink, fusiform or corkscrew-shaped intracellular aggregates of GFAP (pathognomonic of low-grade/indolent process)
• Eosinophilic granular bodies (EGBs) — mulberry-shaped pink inclusions
• Low mitotic rate, no necrosis, no microvascular proliferation

IHC:

WHO 2021 Classification:

• Pilocytic astrocytoma, WHO Grade I — all subtypes
• Pilomyxoid astrocytoma — variant (historically Grade II, now ungraded; age <3 yr, hypothalamic)

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7. MANAGEMENT

🔪 Surgery

1. Prone position, Mayfield head clamp
2. Posterior fossa craniotomy (suboccipital)
3. Identify cyst — evacuate cystic fluid
4. Locate mural nodule (wall of cyst — does NOT need full cyst wall excision as cyst wall is not tumor)
5. Excise the mural nodule completely — this is the actual tumor (cyst wall = reactive gliosis, benign)
6. Cyst wall alone does not recur — key surgical principle
7. Duraplasty and closure

☢️ Radiotherapy

• Avoid in children — neurocognitive effects, secondary high-grade gliomas (especially NF1!)
• Radiation is a relative contraindication in NF1 (risk of vasculopathy, moyamoya, secondary tumors)
• Reserved for progressive unresectable disease in adults or refractory cases

💊 Chemotherapy (for unresectable/progressive disease)

🎯 Targeted Therapy (Emerging — Key for Viva!)

• BRAF + MEK inhibitors — powerful second-line options:
  • Dabrafenib + Trametinib (BRAF V600E + MEK inhibitor) — impressive responses
  • Selumetinib (MEK inhibitor) — for BRAF-fused tumors, especially NF1 (FDA approved for NF1 plexiform neurofibromas)
  • Mechanism: MEK inhibition → blocks MAPK/ERK pathway → tumor regression

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🔑 MANAGEMENT CRUX FLOWCHART — PILOCYTIC ASTROCYTOMA

Suspected PA (child, cerebellar symptoms, MRI cyst + nodule)
          ↓
MRI Brain (± Spine if symptomatic)
Molecular testing (BRAF status)
          ↓
          ↓
┌──────────────────────────────────────────┐
│ ACCESSIBLE (cerebellum, cortical)        │ INACCESSIBLE / UNRESECTABLE
│ → GTR = CURATIVE                        │ (hypothalamus, optic pathway, BS)
│ → >90% recurrence-free at 5 yr          │ → Observe (NF1 optic glioma)
│ → No adjuvant therapy needed            │ → Chemo: Carboplatin + Vincristine
└──────────────────────────────────────────┘ → Targeted: BRAF/MEK inhibitor
          ↓
FOLLOW-UP MRI every 6 months × 3 yr
then annually
(Visual testing if optic pathway)

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8. FOLLOW-UP & PROGNOSIS

• Excellent — 80% 20-year survival (Bradley & Daroff)
• After GTR cerebellum: >90% 5-year recurrence-free survival (Shaw & Wisoff)
• Malignant transformation is rare
• Centrally located tumors (hypothalamus, brainstem) have reduced PFS
• MRI every 6 months × 2–3 years, then annually

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🟡 TUMOR 3: EPENDYMOMA

1. DEMOGRAPHY

• 3rd most common pediatric brain tumor — ~4–10% of all brain tumors; 5.5% of pediatric brain tumors
• Most common in 1st decade of life (infratentorial)
• Males:Females ≈ 2:1 (infratentorial)
• Age <3 years → significantly worse prognosis
• 90% intracranial, 10% spinal (spinal ependymoma more common in adults)
• Of intracranial: 75% infratentorial (posterior fossa, 4th ventricle) in children; supratentorial more common in adults

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2. GENETICS / ETIOLOGY

Molecular Classification (WHO 2021 — DNA methylation based):

• ST-EPN-RELA (ZFTA-RELA fusion) — NF-κB pathway activation
• ST-EPN-YAP (YAP1-MAML2 fusion) — better prognosis

• NF2 mutations (22q deletion) — spinal ependymomas
• Myxopapillary, classic, subependymoma subtypes

Key Points:

• NF2 mutations / 22q deletion — spinal ependymomas
• Chr 22q deletion — common in all groups
• No IDH mutations (unlike adult diffuse gliomas)

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3. CLINICAL FEATURES

Infratentorial Ependymoma (4th Ventricle):

• Headache, vomiting, papilledema (obstructive hydrocephalus)
• Ataxia (cerebellar compression)
• Torticollis / neck pain (extension through foramen of Magendie)
• Hoarseness, dysphagia, lower cranial nerve palsies (extension through foramina of Luschka → CPA cistern)
• Downward beating nystagmus (impending tonsillar herniation)
• Increased head circumference in infants

Symptoms present for 1–2 years before diagnosis (slower onset than medulloblastoma)

Spinal Ependymoma:

• Localized pain worse at night (recumbent → spinal venous congestion)
• Radiculopathy, weakness, sensory loss

Metastatic Spread:

• CSF seeding / leptomeningeal drop mets in ~5% (less than medulloblastoma)
• Associated with poor prognosis

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4. RADIOLOGY

Classic MRI Appearance:

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5. TUMOR MARKERS

• No specific serum markers
• CSF cytology — may detect malignant cells
• Molecular: ZFTA fusion (ST-RELA), H3K27me3 loss (PF-A — IHC loss = PF-A subtype)

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6. BIOPSY & PATHOLOGY

Cell of Origin: Ependymal cells lining ventricles and central canal

Histology — KEY FEATURES:

1. Perivascular pseudorosettes — cells radiating around blood vessels with anuclear zones (most common, most important — always present)
2. True ependymal rosettes — cells around a central lumen (canal) — in ~10% only but more specific
3. Ependymal canals — slit-like structures resembling miniature ventricles
4. Well-circumscribed, compresses rather than infiltrates
5. Uniform cells with round/oval nuclei, moderate chromatin

Electron Microscopy (if morphologically ambiguous):

• Cilia with basal bodies (blepharoplasts)
• Microvilli
• Zipper-like intercellular junctions
• Intracellular lumina (EM is confirmatory when light microscopy is ambiguous)

Grading:

• WHO Grade II — ependymoma (classic)
• WHO Grade III — anaplastic ependymoma (hypercellularity, increased mitoses, microvascular proliferation, necrosis)
• Note: Histological grading may be phased out — molecular classification more prognostic

IHC:

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7. MANAGEMENT

🔪 Surgery

1. Prone positioning, Mayfield head clamp
2. Suboccipital craniotomy ± C1 laminectomy (if tumor extends into cervical canal)
3. Dural opening, gentle retraction of cerebellar tonsils
4. Identify tumor in 4th ventricle
5. Microsurgical debulking with CUSA/bipolar
6. Careful dissection from floor of 4th ventricle (brainstem)
7. If tumor extends through foramina of Luschka → may need additional CPA approach
8. EVD placement if hydrocephalus

☢️ Radiotherapy

• Standard of care post-surgery for all children >1–3 years old
• Local conformal RT to tumor bed: 54–59.4 Gy (NOT craniospinal — unlike medulloblastoma, because CSF dissemination is rare)
• CSI only if metastatic disease confirmed
• Infants <1–3 yr: Chemotherapy first to delay RT
• Proton therapy preferred (posterior fossa — cochlea sparing)

💊 Chemotherapy

• Adjuvant chemo does NOT improve survival (unlike medulloblastoma) — chemotherapy role limited
• Used to delay RT in infants (Baby protocols)
• Regimens: Carboplatin + Vincristine, or Cisplatin + Etoposide
• Re-irradiation used at recurrence

🎯 Targeted/Future Therapy

• EZH2 inhibitors (tazemetostat) — for PF-EPN-A with EZHIP overexpression (investigational)
• VEGF pathway (bevacizumab) — investigational

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🔑 MANAGEMENT CRUX FLOWCHART — EPENDYMOMA

Posterior fossa ependymoma (4th ventricular mass, "plastic" extension)
          ↓
MRI Brain + Spine + CSF cytology
          ↓
SURGERY (GTR primary goal)
If residual: Second-look surgery (<72 hr post-op MRI)
          ↓
Post-op MRI at 48–72 hours
          ↓
Molecular profiling (H3K27me3 IHC, ZFTA fusion, DNA methylation)
          ↓
┌────────────────────────────────────────┐
│ >3 yr, localized (M0)                 │ <3 yr OR M+ disease
│ Local conformal RT 54–59.4 Gy         │ Chemotherapy (delay RT)
│ (No CSI unless M+)                    │ CSI if M+ disease
└────────────────────────────────────────┘
          ↓
MRI every 3 months × 2 yr, then every 6 months
Recurrence → Re-irradiation ± chemo

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8. FOLLOW-UP & PROGNOSIS

• Prognosis worse than pilocytic astrocytoma, better than medulloblastoma high-risk
• Age <3 years: significantly worse prognosis
• PF-EPN-A: poor prognosis regardless of grade
• PF-EPN-B: better prognosis
• GTR vs. subtotal resection: major survival difference
• 5-year OS: ~75% (GTR) vs ~30% (incomplete resection)
• MRI follow-up every 3 months for 2 years, then every 6 months

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🔴 TUMOR 4: HEMANGIOBLASTOMA

1. DEMOGRAPHY

• Most common PRIMARY cerebellar neoplasm in adults
• WHO Grade I — benign
• Age range: 15–50 years, peak at 40 years
• Males > Females
• ~10% associated with Von Hippel-Lindau (VHL) disease (familial); rest sporadic
• Equally common in all races (blacks, whites, Asians)
• Rare in children

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2. GENETICS / ETIOLOGY

VHL Disease (Autosomal Dominant):

• VHL gene on chromosome 3p25-26 — tumor suppressor gene
• Germline VHL mutation ("first hit") → second somatic hit → tumor formation
• VHL protein normally promotes HIF-α degradation → VHL loss → HIF-α accumulation → VEGF/EPO overexpression → angiogenesis
• Presents with CNS hemangioblastomas + renal cell carcinoma (clear cell) + pheochromocytoma + pancreatic cysts + retinal angiomas

Sporadic:

• Somatic VHL inactivation (both alleles)

Associations in VHL:

• CNS hemangioblastomas (cerebellum > brainstem > spinal cord)
• Retinal hemangioblastoma (first sign!)
• Clear cell renal cell carcinoma (major cause of morbidity/mortality)
• Pheochromocytoma
• Pancreatic neuroendocrine tumors/cysts

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3. CLINICAL FEATURES

Symptoms:

• Dizziness and ataxia (unilateral cerebellar hemisphere involvement)
• Signs of raised ICP (hydrocephalus from 4th ventricle compression)
• Polycythemia — (viva pearl!) tumor elaborates erythropoietin → secondary erythrocytosis (Hb elevated)
• Retinal angioma — may cause visual symptoms (often the first finding in VHL)
• Spontaneous hemorrhage (tumor is very vascular)

VHL Syndrome Features:

• Hepatic/pancreatic cysts
• Renal cell carcinoma
• Pheochromocytoma (episodic hypertension, sweating, palpitations)

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4. RADIOLOGY

MRI — Investigation of Choice:

• Age <40 years
• Multiple lesions
• Family history
• Retinal angioma

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5. TUMOR MARKERS

• Erythropoietin (EPO) — elevated serum EPO → polycythemia (Hb >18 g/dL, Hct >52%)
  • Normal: Men 4.3–17.7 mIU/mL; Women 4.3–9.9 mIU/mL
  • In hemangioblastoma: often >20–30 mIU/mL
  • Secondary erythrocytosis resolves after tumor removal
• VHL genetic testing — germline mutation analysis

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6. BIOPSY & PATHOLOGY

Cell of Origin: Stromal cells (neoplastic component) — uncertain histogenesis (mesenchymal origin)

Histology — KEY FEATURES:

• Abundant capillaries throughout tumor mass (rich vascular network)
• Foamy lipid-laden stromal cells — the actual neoplastic component (interspersed between capillaries)
• Solid portions are dark red (vascular)
• Sharply demarcated from surrounding tissue

IHC:

Distinguishing from Metastatic Renal Cell Carcinoma (VHL context!):

• Inhibin-α +, RCC antigen −, CD10 − = hemangioblastoma
• RCC: EMA+, PAX8+, CD10+, inhibin-α −

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7. MANAGEMENT

🔪 Surgery

• En-bloc excision (do NOT piecemeal excision — risk of torrential hemorrhage)
• Open the cerebellar cyst → evacuate fluid → identify mural nodule on wall
• Excise the nodule en-bloc — cyst wall alone does not need resection
• Pre-operative embolization (endovascular) — controversial; used for large vascular nodules; does not clearly reduce recurrence
• Complete removal = cure (high recurrence if nodule left)

• Hemorrhage (highly vascular tumor)
• Cranial nerve injury
• Recurrence if incomplete

☢️ Radiosurgery

• Stereotactic radiosurgery (SRS/Gamma Knife) — for:
  • Multiple lesions (VHL)
  • Surgically inaccessible lesions
  • Small asymptomatic lesions
  • Recurrent lesions
• Series show good control rates (~90% at 5 years for small lesions)

No standard chemotherapy

🎯 Targeted Therapy (Investigational):

• Bevacizumab (anti-VEGF) — for VHL-associated hemangioblastomas (VHL pathway drives VEGF)
• HIF-2α inhibitors (belzutifan) — FDA approved for VHL-associated tumors (RCC, CNS hemangioblastomas)
  • Dose: 120 mg orally daily
  • MOA: Inhibits HIF-2α → reduces VEGF/EPO transcription
  • Major breakthrough for VHL disease management (2021 FDA approval)

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🔑 MANAGEMENT CRUX FLOWCHART — HEMANGIOBLASTOMA

Adult, cerebellar cyst + mural nodule, ataxia ± polycythemia
          ↓
MRI Brain + Spine (for multiple lesions)
CBC (polycythemia?), EPO level
VHL gene testing + ophthalmic exam (retinal angioma)
Abdominal MRI (renal, pancreatic involvement in VHL)
          ↓
┌──────────────────────────────────────┐
│ SYMPTOMATIC / GROWING               │ ASYMPTOMATIC / SMALL
│ → Surgical excision (en-bloc)       │ → Observe (VHL: serial MRI 6-12 mo)
│ → OR Gamma Knife SRS               │ → SRS if growing/asymptomatic
│ (if small/multiple/inaccessible)    │
└──────────────────────────────────────┘
          ↓
VHL disease? → Multidisciplinary: 
Screen family, Renal surveillance, 
Ophthalmology, Endocrine (pheo)
          ↓
Follow-up MRI every 6–12 months
New lesions: treat as they become symptomatic

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8. FOLLOW-UP & PROGNOSIS

• Excellent with complete excision of nodule
• High recurrence if nodule not completely removed
• VHL patients: new lesions form throughout life → lifelong surveillance
• Children of affected parents should be tested for VHL mutation and screened for retinal angioma
• Retinal lesions cause blindness if untreated
• Polycythemia resolves after tumor removal
• MRI annually for VHL; every 1–2 years after sporadic complete excision

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⚖️ DIFFERENTIAL DIAGNOSIS OF POSTERIOR FOSSA TUMORS

Comprehensive Comparison Table

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Quick Radiological Differentiators

POSTERIOR FOSSA MASS
        ↓
Is it MIDLINE (vermis/4th ventricle)?
├── YES → Medulloblastoma (child) OR Ependymoma
│   ├── Restricted diffusion (low ADC) → Medulloblastoma
│   ├── Plastic extension through foramina, calcification → Ependymoma
│   └── Floor attachment (brainstem floor) → Ependymoma
└── NO → Hemispheric mass
    ├── Cyst + mural nodule, CHILD → Pilocytic Astrocytoma
    ├── Cyst + mural nodule, ADULT ± polycythemia → Hemangioblastoma
    ├── Solid, enhancing, ADULT → Metastasis (lung, breast, melanoma)
    └── Multiple lesions → Metastases or VHL hemangioblastomas

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Additional Posterior Fossa Tumors to Know

Atypical Teratoid/Rhabdoid Tumor (ATRT)

• Age: <3 years (most common CNS malignancy in infants)
• SMARCB1/INI1 deletion/mutation (chromosome 22q11.2) — IHC: INI1 loss
• Heterogeneous MRI, restricted diffusion, hemorrhage
• Extremely aggressive, poor prognosis
• Chemotherapy-intensive, RT if tolerated

Choroid Plexus Tumors

• Papilloma (WHO I) vs. Carcinoma (WHO III)
• Location: 4th ventricle (childhood) and lateral ventricle
• Presents with hydrocephalus (overproduction of CSF)
• MRI: frond-like enhancing mass in ventricle
• Surgery curative for papilloma

Brainstem Glioma / DIPG

• Diffuse Intrinsic Pontine Glioma (DIPG) — H3K27M mutation (histone)
• Expands the pons (>50% pons involved = diagnostic)
• Clinical: cranial nerve palsies (VI, VII), long tract signs, ataxia
• Biopsy: Now recommended (for molecular diagnosis, clinical trials)
• Treatment: RT (54 Gy) — palliation; ONC201/suramin (targeted for H3K27M)

Dermoid / Epidermoid Cysts

• Epidermoid: CPA angle, young adults; "pearly tumor"; DWI shows restriction (NOT enhancement)
• Dermoid: Midline, children; skin appendages; fat signal on T1
• Chemical meningitis if ruptured

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📋 PEDIATRIC BRAIN TUMOR FREQUENCY TABLE (from Bradley & Daroff)

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🔬 OVERALL IHC COMPARISON CHEATSHEET

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💉 DRUG SUMMARY CHEATSHEET

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🎯 HIGH-YIELD VIVA POINTS

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Sources: Bradley & Daroff's Neurology in Clinical Practice | Adams & Victor's Principles of Neurology 12th ed. | Robbins & Kumar Basic Pathology | Harrison's Principles of Internal Medicine 22EThat's your complete posterior fossa tumour viva guide! Here's what's covered:

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What's included:

1. Medulloblastoma — most common malignant pediatric brain tumor, full WHO molecular subgrouping (WNT/SHH/Group 3-4), Chang staging, CSI protocol, Packer regimen doses
2. Pilocytic Astrocytoma — most common pediatric glioma, BRAF-KIAA1549 fusion, biphasic histology with Rosenthal fibers/EGBs, BRAF/MEK targeted therapy, GTR is curative
3. Ependymoma — plastic/toothpaste foramen extension, PF-EPN-A/B molecular classification, H3K27me3 IHC, local RT (not CSI), EZH2 inhibitors
4. Hemangioblastoma — most common primary cerebellar tumor in adults, VHL/3p25-26, polycythemia from EPO, inhibin-α IHC, belzutifan (FDA 2021)

• Demography → Genetics → Clinical features → Radiology (with when to do/not do) → Tumor markers → Pathology/IHC → Full management with drug doses/MOA/side effects → Management flowchart → Follow-up & prognosis

• Full differential diagnosis comparison (including ATRT, DIPG, dermoid)
• IHC cheatsheet across all tumors
• Drug summary with doses, MOA, side effects
• High-yield viva Q&A at the end