A 38-year-old Indian woman presents with a 18-month history of progressive cognitive decline, personality changes, and intermittent headaches. MRI brain shows a left temporal lobe mass that is isointense to gray matter on T1, hyperintense on T2/FLAIR, with minimal enhancement and no significant edema. Histology reveals uniform, well-differentiated astrocytes with mild nuclear enlargement, sparse mitotic figures, and no necrosis. FISH analysis shows IDH1 mutation (R132H positive). What is the most likely diagnosis?

Explanation

## Diagnosis: Diffuse Astrocytoma, IDH-mutant (WHO Grade II) ### Clinical Presentation **Key Point:** The 18-month history of progressive cognitive decline, personality changes, and intermittent headaches in a young adult (38 years) is classic for a **low-grade diffuse glioma**. Low-grade gliomas typically present insidiously over months to years in younger patients. ### Imaging Features **High-Yield:** The MRI characteristics described are hallmarks of a **low-grade (Grade II) glioma**: - **T1:** Isointense to gray matter (well-differentiated, no necrosis) - **T2/FLAIR:** Hyperintense (infiltrative tumor cells replacing normal parenchyma) - **Minimal enhancement:** Intact blood-brain barrier, no high-grade transformation - **No significant edema:** Consistent with slow-growing, non-aggressive tumor These features are **incompatible** with glioblastoma (Grade IV), which characteristically shows ring-enhancing heterogeneous lesions with central necrosis and significant perilesional edema. ### Histopathological Findings **Key Point:** The histology is diagnostic of WHO Grade II: - **Uniform, well-differentiated astrocytes** — no pleomorphism - **Mild nuclear enlargement** — acceptable for Grade II - **Sparse mitotic figures** — Grade II allows rare mitoses - **No necrosis** — necrosis is a defining criterion for Grade IV (glioblastoma) - **No microvascular proliferation** — another Grade IV criterion absent here Per **WHO 2021 CNS Tumor Classification**, glioblastoma (Grade IV) requires either **microvascular proliferation OR necrosis**. Neither is present here. ### Molecular Marker — The Defining Feature **Clinical Pearl:** **IDH1 R132H mutation** is the single most important molecular marker in glioma classification: - IDH mutation is found in **>80% of WHO Grade II–III astrocytomas** - IDH-mutant astrocytomas arise in **younger patients** (30–50 years) with a **prolonged clinical course** - IDH mutation is the hallmark of the **secondary pathway** of gliomagenesis - IDH wild-type in a Grade II-appearing tumor would prompt upgrading to Grade IV per WHO 2021 ### WHO 2021 Glioma Classification Summary | Feature | Diffuse Astrocytoma (Grade II) | Anaplastic Astrocytoma (Grade III) | Glioblastoma (Grade IV) | |---------|-------------------------------|-------------------------------------|------------------------| | **IDH status** | Mutant | Mutant | Wild-type (primary) or Mutant (secondary) | | **Mitoses** | Rare/sparse | Brisk | Brisk | | **Necrosis** | Absent | Absent | **Present** | | **Microvascular proliferation** | Absent | Absent | **Present** | | **Enhancement** | Minimal | Variable | Ring-enhancing | | **Age** | 30–45 years | 35–50 years | >60 years (primary) | ### Why the Other Options Are Wrong - **Option A (Secondary GBM):** Requires histological evidence of necrosis or microvascular proliferation — absent here. IDH mutation alone does not make a tumor GBM. - **Option B (Primary GBM):** Primary GBM is IDH wild-type, occurs in older patients, and shows aggressive imaging/histology. Completely inconsistent with this case. - **Option D (Anaplastic Astrocytoma, Grade III):** Requires brisk mitotic activity and/or significant nuclear atypia. "Sparse mitotic figures" and "well-differentiated" morphology exclude Grade III. **High-Yield (Harrison's / WHO Blue Book):** The combination of IDH1 mutation + well-differentiated histology + no necrosis + no microvascular proliferation = **Diffuse Astrocytoma, IDH-mutant, WHO Grade II** — the correct diagnosis per current classification.