A 35-year-old woman from Mumbai presents with a 6-month history of progressive focal weakness of the left arm and hand. MRI brain reveals a well-demarcated, non-enhancing lesion in the right motor cortex with minimal surrounding edema. Histopathology shows uniform cellularity with moderate nuclear enlargement, scattered mitotic figures (3–4 per 10 HPF), and no necrosis. GFAP and IDH1 R132H mutation are positive. What is the most likely WHO grade of this tumor?

Explanation

## Diagnosis: WHO Grade III Anaplastic Astrocytoma (IDH-mutant) ### Clinical Presentation **Key Point:** A 35-year-old with 6 months of progressive focal weakness is consistent with a diffuse glioma. While the clinical course appears subacute, histopathological features — not clinical tempo alone — determine WHO grade under the 2021 classification. ### Imaging Characteristics | Feature | Grade II | Grade III | Grade IV | | --- | --- | --- | --- | | Enhancement | Absent | Variable | Marked (ring-like) | | Edema | Minimal | Moderate | Significant | | Necrosis | None | None | Present | | Demarcation | Well-demarcated | Infiltrative | Infiltrative | **High-Yield:** Non-enhancing appearance does not exclude Grade III; up to 30–40% of anaplastic astrocytomas may lack contrast enhancement on MRI. Imaging alone cannot reliably distinguish Grade II from Grade III — histopathology is definitive. ### Histopathological Criteria (WHO 2021) **Key Point:** The critical distinguishing feature between Grade II and Grade III astrocytoma is **mitotic activity**: | Feature | Grade II | Grade III | | --- | --- | --- | | Cellularity | Mild–moderate | Moderate–high | | Nuclear atypia | Mild–moderate | Moderate–marked | | Mitoses | 0–2 per 10 HPF | ≥3–4 per 10 HPF | | Necrosis | Absent | Absent | | Microvascular proliferation | Absent | Absent | **This case shows 3–4 mitoses per 10 HPF**, which crosses the threshold for anaplasia (Grade III). The WHO 2021 classification (and prior 2016 criteria) defines anaplastic astrocytoma by the presence of **significant mitotic activity** as the key anaplastic feature, even in the absence of necrosis or microvascular proliferation. - Uniform cellularity with **moderate nuclear enlargement** ✓ (consistent with Grade III) - **3–4 mitoses per 10 HPF** ✓ (exceeds Grade II threshold; diagnostic of anaplasia) - **No necrosis** ✓ (excludes Grade IV) - **No vascular proliferation** (not mentioned; excludes Grade IV) ### Molecular Markers **High-Yield:** **IDH1 R132H mutation-positive** + **GFAP-positive** confirms this is an IDH-mutant astrocytoma. Under WHO 2021: - IDH-mutant astrocytomas are graded 2, 3, or 4 based on histological features - Grade 3 = IDH-mutant astrocytoma with significant mitotic activity (≥3–4/10 HPF) - Grade 4 = IDH-mutant astrocytoma with necrosis and/or microvascular proliferation (absent here) - IDH mutation does NOT cap the grade at II; it simply defines the lineage **Mnemonic — Grade III vs IV:** **"Necrosis = IV"** — absence of necrosis and microvascular proliferation keeps this at Grade III despite IDH mutation. ### Why NOT Grade II? Grade II (diffuse astrocytoma, IDH-mutant) requires: - Mitotic activity of **0–2 per 10 HPF** (this case: 3–4 — exceeds threshold) - Absent or minimal nuclear pleomorphism The mitotic count of **3–4 per 10 HPF** is the decisive feature elevating this tumor to Grade III. The explanation that "3–4 mitoses are within acceptable range for Grade II" is incorrect per WHO 2021 criteria. ### Why NOT Grade IV? Glioblastoma (Grade IV) requires **necrosis** and/or **microvascular proliferation** — both absent in this case. ### Clinical Pearl **Key Point:** Per WHO Classification of CNS Tumors 2021 (Louis et al.), IDH-mutant astrocytomas are graded 2–4. The presence of ≥3 mitoses per 10 HPF in an IDH-mutant astrocytoma without necrosis or microvascular proliferation is diagnostic of **Grade 3 (Anaplastic Astrocytoma, IDH-mutant)**. IDH mutation confers better prognosis than IDH wild-type GBM but does not override histological grading criteria. [cite: WHO Classification of CNS Tumors 2021 (5th ed.), Louis et al.; Robbins & Cotran Pathologic Basis of Disease, 10e, Ch. 28]