====== WHO Grade 2 Meningioma ======

**WHO Grade 2 meningiomas**, also called **atypical meningiomas**, represent an intermediate grade between benign (Grade 1) and malignant (Grade 3) tumors. They exhibit increased proliferative activity, brain invasion, or atypical histological features, and have a **higher risk of recurrence and progression** compared to Grade 1.

===== Definition =====
A **WHO Grade 2 meningioma** is defined by one or more of the following:
  * **Brain invasion** (since WHO 2016, retained in 2021)
  * **Mitotic index ≥4 mitoses per 10 high-power fields (HPF)**
  * **At least 3 of the following 5 histological features**:
    - Increased cellularity
    - Small cells with high nuclear-to-cytoplasmic ratio
    - Prominent nucleoli
    - Sheet-like growth pattern
    - Foci of spontaneous necrosis

===== Histological Subtypes =====
  * **Atypical meningioma (most common)**
  * **Clear cell meningioma** (molecularly defined)
  * **Chordoid meningioma**

===== Histopathological Features =====
  * Mitotic activity ≥4 per 10 HPF
  * Increased nuclear atypia and hypercellularity
  * Brain invasion (any degree)
  * Possible necrosis or architectural distortion
  * May be diagnosed purely on molecular subtype (e.g. clear cell, chordoid)

===== Molecular Markers =====
  * Loss of chromosome 1p, 14q, or CDKN2A/B deletions associated with worse prognosis
  * DNA methylation profiling may further stratify recurrence risk

===== Clinical Behavior =====
  * **Aggressive behavior** compared to Grade 1
  * **Higher recurrence rate**, even after complete resection
  * Often present in younger patients or in non-skull-base locations

===== Treatment =====
  * **Maximal safe surgical resection** is primary treatment
  * **Simpson grade I–III** resection is preferred
  * **Postoperative radiotherapy** is often recommended, especially if:
    - Resection is subtotal (Simpson grade IV–V)
    - Brain invasion is present
    - Tumor is recurrent
  * Adjuvant RT improves progression-free survival but may not always be necessary after gross total resection

===== Prognosis =====
  * **5-year recurrence-free survival**:
    - ~50–70% after gross total resection
    - Worse with subtotal resection or brain invasion
  * **Regular MRI follow-up** is essential, typically every 6–12 months for the first 5 years

===== References =====
  * WHO Classification of Tumours Editorial Board. *WHO Classification of Tumours of the Central Nervous System*. 5th ed. IARC, 2021.
  * Goldbrunner R, et al. EANO guideline on meningiomas. *Lancet Oncol*. 2016.
  * Sahm F, et al. DNA methylation-based classification and grading of meningiomas. *Acta Neuropathol*. 2017.