Fibrillary astrocytoma, also known as diffuse astrocytoma (WHO grade II), is a low‑grade primary central nervous system tumor arising from astrocytic glial cells. It is characterized histologically by the presence of elongated, slender, fibrillary processes of neoplastic astrocytes within a relatively normal brain parenchyma.
Classification and Nomenclature
- WHO 2021 classification designates this entity as “Diffuse astrocytoma, IDH‑mutant, grade 2” when an isocitrate dehydrogenase (IDH) mutation is identified; tumors lacking IDH mutation are now classified separately.
- Historically, the term “fibrillary astrocytoma” referred to grade II astrocytic tumors that exhibit a fibrillary cytoplasmic architecture without marked cellular atypia or mitoses.
Epidemiology
- Diffuse astrocytomas constitute approximately 10–15 % of all primary brain tumors.
- They most commonly affect adults between 20 and 40 years of age, with a slight male predominance.
Pathophysiology
- Neoplastic astrocytes retain many functional properties of normal astrocytes, including expression of glial fibrillary acidic protein (GFAP).
- The hallmark histologic appearance includes long, delicate, fibrillary processes that interdigitate with surrounding neuropil, giving a “fibrillary” pattern.
- Genetic alterations frequently involve IDH1 or IDH2 mutations, 1p/19q co‑deletion (rare in pure astrocytoma), and ATRX loss.
Clinical Presentation
- Symptoms depend on tumor location and may include seizures, headache, focal neurological deficits, or cognitive changes.
- Tumors are most often located in the cerebral hemispheres, particularly the frontal lobe, but can arise in any region of the brain.
Diagnosis
- Neuroimaging: Magnetic resonance imaging (MRI) typically shows a poorly defined, non‑enhancing, T2/FLAIR hyperintense lesion without a distinct mass effect.
- Histopathology: Surgical or stereotactic biopsy demonstrates diffuse infiltration of fibrillary astrocytes with low mitotic activity, absence of necrosis, and minimal cellular pleomorphism.
- Molecular testing: Identification of IDH mutation status, ATRX expression, and other molecular markers is now standard for accurate classification and prognostication.
Management
- Surgical resection: Maximal safe resection is preferred when feasible, aiming to reduce seizure burden and improve overall survival.
- Radiotherapy: Adjuvant radiotherapy may be considered, particularly after subtotal resection or in cases of progression.
- Chemotherapy: Alkylating agents such as temozolomide are commonly employed, especially for recurrent or progressive disease.
- Observation: In selected low‑risk patients with complete resection and favorable molecular profile, a “watchful waiting” approach may be adopted.
Prognosis
- Median overall survival ranges from 5 to 10 years, influenced by age, extent of resection, and molecular markers (e.g., IDH mutation confers a better prognosis).
- A proportion of fibrillary astrocytomas undergo malignant transformation to higher‑grade gliomas (WHO grade III anaplastic astrocytoma or grade IV glioblastoma) over time.
Research and Emerging Therapies
- Ongoing studies focus on targeted therapies directed at IDH mutations, epigenetic modulators, and immunotherapeutic approaches.
- Molecular profiling continues to refine risk stratification and guide personalized treatment strategies.