CNS Tumors

Gliomagenesis and Malignant Transformation

• Astrocytomas arise from astrocytic lineage through sequential accumulation of genetic alterations: early lesions harbor TP53 mutations (grade II); progression to anaplasia (grade III) involves additional alterations in PTEN and NF1; grade IV glioblastomas typically harbor EGFR amplification, CDKN2A/B deletion, and PTEN loss
• IDH-mutant gliomas (IDH1 R132H or IDH2 R172K) produce 2-hydroxyglutarate, which competitively inhibits alpha-ketoglutarate-dependent dioxygenases (TET enzymes, histone demethylases), causing aberrant DNA methylation patterns and altered histone modifications that suppress differentiation and promote malignant phenotype
• MGMT methylation silences O6-methylguanine-DNA methyltransferase, reducing capacity for DNA alkylation repair and conferring temozolomide sensitivity, representing key therapeutic predictive marker

Oligodendroglioma Biology

• Hallmark 1p/19q co-deletion (whole-arm deletions) occurs early in tumorigenesis; combined with IDH mutation, defines IDH-mutant, 1p/19q co-deleted oligodendrogliomas with dramatically improved prognosis
• Loss of 1p/19q regions (containing TP53 and PTEN homologs, CIC tumor suppressor) facilitates malignant progression and chemotherapy sensitivity
• Co-deletion creates DNA instability paradoxically associated with better prognosis due to enhanced chemotherapy responsiveness

Glioblastoma Pathogenesis—Primary vs. Secondary

• Primary glioblastomas (>90% of cases) arise de novo in elderly patients with rapid progression; characterized by EGFR amplification, PTEN loss, TP53 mutation, forming mesenchymal molecular subtype with poor prognosis
• Secondary glioblastomas arise through malignant transformation of lower-grade astrocytomas in younger patients; harbor IDH mutations and TP53 mutations as early events, progress through grade progression pathway with accumulating alterations
• IDH-wildtype glioblastoma (molecular class) shows activation of PI3K/AKT/mTOR and RTK/RAS/MAPK pathways with genomic instability

Ependymoma Molecular Genetics

• PFA ependymomas (infratentorial, PosteriorFossa Group A) harbor YWHAE-NUTM1 fusion and RELA fusion with NF1 loss, characterized by neural precursor gene expression
• PFB ependymomas show H3K27M mutations and elevated H3K27me3 marks
• Supratentorial ependymomas demonstrate ZFTA-RELA fusion and enhanced malignant potential

Medulloblastoma Subgroup Biology

• WNT-activated tumors (10-15%): harbor TP53 mutations, CTNNB1 (beta-catenin) activating mutations; activate canonical Wnt signaling with excellent prognosis
• SHH-activated tumors (25-30%): contain PTCH1, SMO, SUFU mutations activating hedgehog signaling in cerebellar granule neuron precursors; PTEN loss, TP53 mutations present; intermediate prognosis
• Group 3 (25-30%): MYC amplification, chromosomal instability, poorest prognosis
• Group 4 (25-30%): KDM6A mutations, TRK fusions, intermediate-poor prognosis
• Molecular subgroups drive genomic instability and aberrant developmental pathway activation

Meningioma Tumorigenesis

• NF2 inactivation (60-70% sporadic meningiomas) leads to loss of merlin/schwannomin, derepressing YAP/TAZ transcriptional activity (hippo pathway) driving proliferation
• TRAF7 mutations (25%) activate NF-κB signaling
• KLF4 mutations and SMO mutations (hedgehog activation) in recurrent tumors
• Hormone receptor expression (progesterone, estrogen) explains female predominance and pregnancy-associated growth

CNS Lymphoma Pathogenesis

• Primary CNS lymphoma (95% DLBCL) develops from transformed B cells; frequently EBV-positive in immunocompromised hosts
• PMBL (primary mediastinal B-cell lymphoma) shows CIITA mutations and constitutive JAK-STAT signaling
• CNS involvement requires crossing blood-brain barrier, selecting for cells expressing VLA-4 (very late antigen-4) integrin and achieving CNS-penetrating drug concentrations

Metastatic Disease Mechanisms

• Epithelial-mesenchymal transition (EMT) and loss of E-cadherin facilitate invasion through dura and establishment of brain microenvironment
• Tumor cells express brain-homing integrins and engage with astrocytic and microglial niches providing growth factors (TGF-β, FGF)
• BBB disruption via vascular permeability and abnormal tight junction proteins enables transvascular migration

Genetic Predisposition Syndromes

• Neurofibromatosis type 1 (NF1): NF1 tumor suppressor inactivation predisposes to optic pathway gliomas (15-20% of NF1 patients), brainstem gliomas, and malignant peripheral nerve sheath tumors; TP53 mutations not required
• Neurofibromatosis type 2 (NF2): NF2 gene mutations cause bilateral vestibular schwannomas (pathognomonic), multiple meningiomas, spinal ependymomas; develops by age 30 in 90%
• Li-Fraumeni syndrome: TP53 germline mutations predispose to gliomas (especially brainstem gliomas in children), medulloblastomas, and secondary malignancies post-radiation
• Cowden syndrome: PTEN haploinsufficiency increases Lhermitte-Duclos disease (dysplastic cerebellar gangliocytoma) and medulloblastoma risk
• Turcot syndrome: APC mutations (familial adenomatous polyposis variant) predispose to medulloblastoma; Lynch syndrome variant associated with glioma

Familial Cancer Predisposition

• BRCA1/BRCA2 mutations: modestly increased glioma risk
• SUFU mutations: predispose to medulloblastoma (SHH-subtype)
• PTCH1 mutations (Gorlin syndrome): SHH medulloblastoma predisposition

Radiation Exposure

• Therapeutic radiation (prior CNS or systemic malignancy treatment): increases glioma, meningioma, and sarcoma risk; latency 5-40 years; dose-dependent relationship; higher risk in childhood exposure (developing neural tissue more radiosensitive)
• Atomic bomb radiation and nuclear accidents (Chernobyl): demonstrated meningioma and glioma increases
• Occupational/environmental: debate regarding cellular phone/radiofrequency exposure remains unresolved; no established causation for residential electromagnetic fields

Environmental & Lifestyle Factors

• Prior malignancy: chemotherapy (alkylating agents, nitrosoureas) and radiation increase secondary CNS tumor risk
• Immunosuppression: EBV-positive primary CNS lymphoma risk 1000-fold elevated in AIDS patients (CD4 <50 cells/μL), post-transplant lymphoproliferative disorder
• Obesity: modest association with meningioma (estrogen-responsive pathways)
• Hormonal factors: menopause status, hormone replacement therapy associated with meningioma in women
• Infection: JC virus associated with polyomavirus-associated nephropathy but not established CNS tumor causation

Molecular Risk Factors

• IDH wildtype status: inferior prognosis in astrocytomas and oligodendrogliomas
• EGFR amplification: glioblastoma aggressive subtype
• MGMT unmethylated: reduced temozolomide responsiveness
• TP53 mutations: predict reduced overall survival across glioma grades
• High proliferation index (Ki-67 >30%): independent adverse prognostic factor

Symptoms Related to Mass Effect and Increased Intracranial Pressure

• Headache: present in 50-60% of CNS tumors; classically morning headache with nausea/vomiting (increased ICP); worse with Valsalva, recumbency; may be unilateral (near tumor) or generalized; caused by mass effect on pain-sensitive dura, blood vessels, or ventricular obstruction
• Nausea and vomiting: results from increased intracranial pressure compressing chemoreceptor trigger zone in fourth ventricle; often projectile and non-bilious (distinguish from GI causes)
• Blurred vision and diplopia: papilledema from chronic elevated ICP causing optic disc swelling; diplopia from cranial nerve involvement (CN III, IV, VI palsies) or mass effect on brainstem; horizontal gaze palsy with CN VI palsy indicates rising ICP
• Altered mental status, cognitive decline: frontal/temporal lobe involvement causes personality changes, memory loss, executive dysfunction, language disorders; progressing dementia-like picture in slow-growing tumors

Focal Neurological Deficits Correlating with Tumor Location

• Motor deficits: contralateral weakness from motor cortex (M1), pyramidal tract, or basal ganglia involvement; spasticity (upper motor neuron signs); gait disturbance
• Sensory loss: contralateral sensory diminution from postcentral gyrus, thalamus, or dorsal columns involvement; proprioceptive loss with posterior fossa tumors
• Language disturbance: Broca's aphasia (expressive) with dominant inferior frontal involvement; Wernicke's aphasia (receptive) with superior temporal involvement; anomia with anterior temporal/angular gyrus lesions
• Visual field defects: homonymous hemianopia (optic tract, lateral geniculate, optic radiations); bitemporal hemianopia with suprasellar tumors (pituitary adenomas, craniopharyngiomas) compressing optic chiasm from below
• Ataxia and vertigo: cerebellar involvement causes truncal ataxia (vermis), limb dysmetria (hemisphere), nystagmus, dysarthria; posterior fossa tumors common in children
• Seizures: cortical involvement in 20-40% of supratentorial tumors; generalized tonic-clonic most common; partial seizures indicate superficial cortical location; status epilepticus may present acutely; gliomas and meningiomas most epileptogenic

Endocrinological Manifestations

• Pituitary adenomas: amenorrhea/galactorrhea (prolactin), erectile dysfunction (hypogonadism), growth abnormalities in children (GH-secreting), Cushing's syndrome (ACTH), hyperthyroidism (TSH-secreting); mass effect causing hypopituitarism with anterior pituitary insufficiency
• Hypothalamic involvement: precocious puberty (hamartomas), growth hormone deficiency, diabetes insipidus (posterior pituitary/infundibulum compression)

Systemic/Constitutional Symptoms

• Fever: present in some CNS lymphomas and infection-related presentations; unusual in typical gliomas
• Weight loss: paraneoplastic syndrome or widespread disseminated disease
• Neuropsychiatric: personality changes, depression, psychosis (frontal/temporal involvement)

Meningeal/Spinal Involvement Presentations

• Leptomeningeal dissemination: headache, meningismus (nuchal rigidity), cranial nerve palsies (CN II-XII), myelopathy with lower extremity weakness/sensory level, radiculopathy; CSF pleocytosis with elevated protein
• Spinal cord compression: segmental sensory level, lower extremity weakness, urinary retention/incontinence, lower back pain

Age-Related Presentations

• Pediatric tumors: developmental regression, increased head circumference (infants), ataxia/posterior fossa syndrome (medulloblastoma), vision loss, precocious puberty
• Geriatric: insidious cognitive decline, gait disturbance, falls; glioblastomas present with rapid clinical deterioration

Lab and Imaging Correlates

• Elevated CSF protein: >100 mg/dL common in meningeal involvement and spinal cord compression; marked elevation (>400 mg/dL) suggests meningeal carcinomatosis
• CSF cytology: positive in 50-90% of leptomeningeal disease (often requires multiple lumbar punctures)
• Tumor markers in CSF: alpha-fetoprotein (AFP), beta-hCG in germinomas; PSA rarely; limited specificity
• MRI enhancement patterns: gadolinium enhancement indicates blood-brain barrier disruption; heterogeneous enhancement in high-grade gliomas; diffuse/infiltrative pattern in low-grade astrocytomas
• Restricted diffusion (DWI/ADC): indicates hypercellular regions, prominent in high-grade tumors and lymphomas
• Elevated perfusion (relative cerebral blood volume): high-grade tumors show rCBV >1.5 relative to contralateral gray matter; prognostically unfavorable
• 1H-MR spectroscopy: elevated choline/creatinine ratio (membrane synthesis), elevated lactate (anaerobic metabolism), inverted NAA (neuronal loss) in high-grade lesions

Histological Findings—Gliomas

Astrocytoma (IDH-mutant)

• Fibrillary background: abundant fibrillary glial processes creating characteristic fibrous matrix; cells lack significant pleomorphism in low grades
• Nuclear features: oval to elongated nuclei with fine chromatin, minimal mitotic activity (grade II); hyperchromatic nuclei, brisk mitoses (grade III/IV)
• Immunohistochemistry: GFAP-positive (glial fibrillary acidic protein) in astrocytic processes; IDH1 R132H mutant-specific antibody (clone H09) highlights tumor cells; p53 overexpression in many cases (TP53 mutations)
• Grade III (anaplastic astrocytoma): increased mitotic activity (≥4 mitoses/10 hpf), regional necrosis, vascular proliferation WITHOUT perinecrotic spread
• **Grade IV

Immediate stabilization (mass effect / herniation)

• Corticosteroids: dexamethasone is preferred (minimal mineralocorticoid effect, long half-life); it restores blood-brain barrier integrity and reverses vasogenic edema within hours. Symptomatic peritumoral edema is the indication — not the tumor itself.
• Hyperosmolar therapy: mannitol or hypertonic saline plus head-of-bed elevation and normocapnia for impending herniation; emergent external ventricular drain for obstructive hydrocephalus from posterior fossa or intraventricular tumors.
• Anticonvulsants: levetiracetam is first-line for tumor-associated seizures (few enzyme-inducing interactions with chemotherapy). Per the American Academy of Neurology, prophylactic antiepileptics in seizure-naive brain tumor patients are not recommended.

Definitive management (NCCN Central Nervous System Cancers guideline)

• Maximal safe resection: tissue for integrated histologic + molecular diagnosis (IDH, 1p/19q, MGMT) and cytoreduction; extent of resection correlates with survival in gliomas. Intraoperative awake mapping preserves eloquent cortex.
• Glioblastoma (IDH-wildtype): the Stupp regimen — fractionated external-beam radiotherapy (60 Gy/30 fractions) with concurrent daily oral alkylating agent (temozolomide) followed by adjuvant cycles; alternating electric field therapy (tumor-treating fields) is added for newly diagnosed disease. MGMT-methylated tumors derive the greatest temozolomide benefit; elderly/poor-performance patients receive hypofractionated radiotherapy.
• IDH-mutant, 1p/19q co-deleted oligodendroglioma: radiotherapy followed by PCV (procarbazine, lomustine, vincristine) — the co-deletion predicts chemosensitivity.
• Primary CNS lymphoma: high-dose methotrexate-based, CNS-penetrant induction; steroids should be withheld until biopsy because lymphocytolysis obscures histology.
• Meningioma: observation with serial MRI for small asymptomatic lesions; resection (Simpson grade predicts recurrence) or stereotactic radiosurgery otherwise.
• Prolactinoma: dopamine agonist (cabergoline) first, not surgery — the Endocrine Society reserves transsphenoidal resection for medical failure or apoplexy.
• Recurrence: re-resection, re-irradiation, or anti-VEGF antibody (bevacizumab), which relieves edema but has not shown overall survival benefit.

Disease-related — emergencies

• Herniation syndromes (EMERGENCY): expanding mass plus vasogenic edema exceeds compensatory CSF/venous displacement (Monro-Kellie). Uncal herniation signals with an ipsilateral blown pupil (CN III) and contralateral hemiparesis; tonsillar herniation with bradycardia, hypertension, and irregular respirations (Cushing reflex). Requires hyperosmolar therapy and decompression.
• Obstructive hydrocephalus (EMERGENCY): fourth-ventricle or aqueductal compression by medulloblastoma, ependymoma, or pineal tumors; morning headache, vomiting, upgaze palsy (Parinaud), and enlarging ventricles on imaging.
• Status epilepticus (EMERGENCY): cortical irritation by supratentorial gliomas and meningiomas.
• Intratumoral hemorrhage: fragile microvascular proliferation in glioblastoma or hypervascular metastases (melanoma, renal cell, choriocarcinoma); abrupt deficit with hyperdensity on non-contrast CT.
• Venous thromboembolism: gliomas release tissue factor; unilateral leg swelling or hypoxemia — anticoagulation is generally permissible after weighing hemorrhage risk.
• Leptomeningeal dissemination: multilevel cranial neuropathies with a hypoglycorrhachic, high-protein CSF.
• Pituitary apoplexy (EMERGENCY): hemorrhage into an adenoma — thunderclap headache, ophthalmoplegia, and adrenal crisis requiring stress-dose hydrocortisone.

Treatment-related

• Corticosteroid toxicity: hyperglycemia, proximal steroid myopathy, psychosis, and lymphopenia predisposing to Pneumocystis jirovecii pneumonia — trimethoprim-sulfamethoxazole prophylaxis is given during concurrent chemoradiation per NCCN.
• Temozolomide myelosuppression: nadir thrombocytopenia and lymphopenia; monitor counts each cycle.
• Pseudoprogression: enlarging enhancement within roughly the first three months after chemoradiation, more common in MGMT-methylated tumors; mimics true progression — perfusion MRI or short-interval repeat imaging rather than immediate regimen change.
• Radiation necrosis: delayed coagulative necrosis with a Swiss-cheese enhancing lesion months to years later; managed with steroids or bevacizumab.
• Posterior fossa (cerebellar mutism) syndrome: after vermian medulloblastoma resection — mutism, ataxia, emotional lability.
• Post-sellar surgery: transient diabetes insipidus, SIADH, panhypopituitarism, CSF rhinorrhea.
• Late radiation effects: neurocognitive decline, hypopituitarism, growth failure, cavernomas, and secondary meningioma or sarcoma decades later.

• ***Pseudopalisading necrosis* plus microvascular (glomeruloid) proliferation** = glioblastoma, WHO grade 4; a butterfly lesion crossing the corpus callosum with heterogeneous ring enhancement is the classic image. GFAP-positive, IDH-wildtype in the elderly.
• Best next step for any suspected brain mass is MRI brain with and without gadolinium; tissue diagnosis follows. Do not perform lumbar puncture first when mass effect is present — tonsillar herniation is the tested consequence.
• The buzzword–tumor pairs examiners recycle: fried-egg cells with chicken-wire capillaries and calcification → oligodendroglioma (IDH-mutant, 1p/19q co-deleted, best glioma prognosis); Rosenthal fibers and eosinophilic granular bodies in a cystic cerebellar mass with a mural nodule → pilocytic astrocytoma (BRAF fusion, WHO grade 1, cured by resection); perivascular pseudorosettes → ependymoma; Homer Wright rosettes in small blue cells → medulloblastoma; psammoma bodies and whorls with a dural tail, EMA-positive → meningioma; Antoni A/B with Verocay bodies, S100-positive at the cerebellopontine angle → schwannoma; motor-oil cyst fluid with wet keratin and calcification → craniophar1yngioma.
• The one association to know: bilateral vestibular schwannomas = NF2; hemangioblastoma with polycythemia (tumor erythropoietin) = von Hippel-Lindau.
• MGMT promoter methylation predicts temozolomide benefit — the mechanism is loss of alkyl-guanine repair. IDH mutation is prognostic across gliomas; both are now part of the WHO integrated diagnosis.
• Common distractor 1: the most frequent intracranial neoplasm overall is metastasis (multiple lesions at the gray-white junction), not a primary tumor; among primaries, meningioma is the most common benign and glioblastoma the most common malignant in adults.
• Common distractor 2: enlarging enhancement soon after chemoradiation is often pseudoprogression, not treatment failure — repeat imaging rather than abandoning therapy.
• Common distractor 3: withhold steroids before biopsy of suspected primary CNS lymphoma; a vanishing periventricular lesion after dexamethasone is the giveaway.