A 52-year-old man undergoes MRI for progressive headaches and cognitive decline. A diffuse infiltrative tumor is noted in the frontal lobe with T2 hyperintensity. Biopsy shows cells with branching processes and perivascular rosettes. Which finding best distinguishes this ependymoma from a diffuse astrocytoma?

Explanation

## Distinguishing Ependymoma from Diffuse Astrocytoma ### Histopathological Hallmarks **Key Point:** True ependymal rosettes with a central lumen lined by ciliated columnar cells are the pathognomonic feature of ependymoma. Perivascular rosettes alone are non-specific but are prominent in ependymoma. | Feature | Ependymoma | Diffuse Astrocytoma | |---------|-----------|--------------------| | **Perivascular rosettes** | Prominent, characteristic | May be present but not defining | | **True ependymal rosettes** | Present (cells around central lumen) | Absent | | **Cilia/microvilli** | Present on luminal surface | Absent | | **GFAP expression** | Negative to weak | Strongly positive | | **Genetic marker** | RELA fusion, H3K27M mutation (subependymoma) | TP53, ATRX mutations | | **Location** | Typically 4th ventricle, spinal canal | Cerebral hemispheres | ### Clinical Pearl **High-Yield:** Ependymomas are WHO grade II–III tumors arising from ependymal lining of ventricles or spinal canal. The presence of true ependymal rosettes with cilia is diagnostic. Diffuse astrocytomas lack this feature and instead show GFAP+ fibrillary background. ### Immunohistochemical Distinction - **Ependymoma:** GFAP−/weak, EMA+, S100+, vimentin+ - **Astrocytoma:** GFAP++, S100+, EMA− **Mnemonic:** **TRUE ROSETTES = EPENDYMAL LUMEN** — Only ependymomas have true rosettes with a central lumen lined by ciliated cells. **Warning:** Perivascular rosettes alone are not diagnostic of ependymoma; they can be seen in other gliomas. The presence of **true ependymal rosettes** is the discriminating feature. [cite:Robbins 10e Ch 28]