## WHO Grade II Astrocytomas — Low-Grade Gliomas ### Definition and Key Characteristics **Key Point:** WHO Grade II astrocytomas are slow-growing, infiltrative tumors that arise from astrocytes. Despite their "low-grade" designation, they are still diffuse infiltrative neoplasms, not circumscribed lesions. ### Pathological and Molecular Features | Feature | Grade II Astrocytoma | |---------|---------------------| | Cellularity | Increased but uniform | | Mitotic activity | Minimal (0–1 per 10 HPF) | | Necrosis | Absent | | Microvascular proliferation | Absent | | Nuclear pleomorphism | Minimal | | IDH1/IDH2 mutations | Present in >80% (favorable prognostic marker) | | TP53 mutations | Common (~60%) | | Growth pattern | **Diffusely infiltrative** | ### Why "Non-Infiltrative with Sharp Demarcation" is INCORRECT **High-Yield:** Grade II astrocytomas are **definitionally infiltrative**. They diffusely invade surrounding brain parenchyma without a clear tumor-brain interface. This is a cardinal feature distinguishing them from benign or circumscribed lesions. **Clinical Pearl:** On MRI, Grade II astrocytomas often appear as T2/FLAIR hyperintense lesions without contrast enhancement. The visible lesion on imaging represents only the most densely cellular portion; microscopic tumor extends beyond the signal abnormality. This infiltrative nature is why complete surgical resection is impossible and why adjuvant therapy is often considered. ### Natural History and Prognosis **Key Point:** Grade II astrocytomas have a relatively favorable but variable prognosis: - Median overall survival: 10–15 years (longer than Grade III/IV) - **Malignant transformation**: ~50% progress to Grade III anaplastic astrocytoma or Grade IV GBM within 5–10 years - Factors predicting progression: older age, large tumor size, incomplete resection, TP53 mutations ### Molecular Markers and Prognosis **Mnemonic: "IDH+ = Better Prognosis"** - IDH1/IDH2 mutations → favorable prognostic marker - Correlates with longer survival and slower progression - Absent in primary GBM (which typically arise de novo) ### Comparison: Grade II vs. Grade III vs. Grade IV Astrocytomas | Feature | Grade II | Grade III | Grade IV | |---------|----------|-----------|----------| | Mitotic rate | Minimal | Moderate | High | | Necrosis | No | No | Yes | | Microvascular proliferation | No | No | Yes | | IDH mutations | >80% | ~70% | ~10% | | Median survival | 10–15 yrs | 2–5 yrs | ~15 months | | Infiltrative pattern | Yes | Yes | Yes | **Clinical Pearl:** All astrocytomas (Grades II–IV) are infiltrative. The grade reflects biological aggressiveness, not the presence or absence of infiltration. [cite:Robbins 10e Ch 28]
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