Pathology of Brain Tumors: Primary Brain Tumor Grading and Classification

Pathology of brain tumors is an important topic in medical science and neuropathology because it helps doctors diagnose and treat diseases affecting the brain and central nervous system. Brain tumors vary widely in their origin, structure, and behavior. Some grow slowly and remain localized, while others are highly aggressive and spread rapidly.

Understanding the classification of brain tumors allows medical professionals and students to identify tumor types, predict their clinical behavior, and choose appropriate treatment strategies.

Many learners find Pathology of Brain Tumors challenging due to the large number of tumor types and their subtle microscopic differences. Studying their classification, grading system, and diagnostic features simplifies this complexity and improves understanding.

Pathology of Brain Tumors

A brain tumor is an unusual growth of cells within the brain or central spinal cord. These can start in the brain (primary) or spread from other body parts (secondary or metastatic). Brain tumors differ greatly in type, location, and behaviour, ranging from non-cancerous (benign) to cancerous (malignant). Knowing their specific characteristics is vital for proper identification and management.

Primary Brain Tumors: Grading

Brain tumors are graded from 1 to 4 based on specific cellular features, using the A.M.E.N. criteria: Atypia, Mitosis, Endothelial cell proliferation, and Necrosis. Higher grades indicate more aggressive tumors.

• Grade 1: All A.M.E.N. features are absent. These are typically slow-growing.

• Grade 2: Atypia (unusual cell appearance) is present.

• Grade 3: Both atypia and mitosis (cell division) are present.

• Grade 4: Three or four of the A.M.E.N. features are present. These are highly malignant and aggressive.

Classification of Brain Tumors

Brain tumors classify based on the cell type they arise from. This classification helps in determining prognosis and treatment. Here, we explore the main categories and specific examples.

Gliomas

Gliomas originate from glial cells, which support and protect neurons.

Astrocytoma

Astrocytomas arise from astrocytes. They show various grades.

• Grade 1: Juvenile Pilocytic Astrocytoma (JPA), a small tumor common in children, often found in the cerebellum. Subependymal Giant Cell Astrocytoma, linked to tuberous sclerosis, is also Grade 1. JPA is cystic with a mural nodule. Prognosis is usually good.

• Grade 2: Includes Pilomyxoid Astrocytoma (PXA), Diffuse Astrocytoma, and Fibrillary Astrocytoma.

• Grade 3: Anaplastic Astrocytoma.

• Grade 4: Glioblastoma. This is the most common malignant brain tumor in adults, often appearing butterfly-shaped and crossing the midline. Microscopy shows serpentine necrosis with pseudo-palisading tumor cells and endothelial cell proliferation (Glomeruloid Body). Prognosis is very poor.

Oligodendroglioma

These tumors usually start in the frontal lobe and often contain calcium deposits.

• Gross: May show calcification.

• Microscopy: Characterized by a "fried egg" appearance (nucleus with a clear area around it) and a "chicken wire" pattern of blood vessels. Perineuronal satellitosis is also observed.

• Mutation: Commonly involves IDH mutation and 1p/19q co-deletion, which suggests a good response to chemotherapy.

Ependymoma

Ependymomas are often Grade 2 tumors.

• Site: In children, they are typically in the 4th ventricle. In adults, they are found in the spinal cord.

• Genetics: Can be linked to Neurofibromatosis-2.

• Microscopy: Features perivascular pseudorosettes, where tumor cells gather around blood vessels.

• Updates: Myxopapillary Ependymoma, a Grade 1 tumor with myxoid and papillae features, occurs in the filum terminale. Rela Fusion Ependymoma, found in children above the tentorium, carries a poor prognosis.

Neuronal Tumors

These tumors arise from neuronal cells.

• Ganglioglioma: Most common type.

• Gangliocytoma

• Neurocytoma

• Dysembryoplastic Neuroepithelial Tumor (DNET): Features floating neurons within pools of mucopolysaccharide.

Undifferentiated Tumors

These are aggressive tumors with cells that have not fully matured.

Medulloblastoma

A Grade 4 tumor, Medulloblastoma is common in children.

• Site: Primarily found in the cerebellum, it is not cystic.

• Spread: Known for "drop metastases" where it spreads via cerebrospinal fluid (CSF) to the spinal cord.

• Microscopy: Shows Homer Wright pseudorosettes (tumor cells arranged around a central pink material).

• Genetics: WNT pathway activation indicates the best prognosis, while certain Non-WNT non-SHH pathways with N-myc amplification show the worst prognosis.

• Syndromes: Gorlin syndrome (PTCH gene mutation, SHH pathway affected) can lead to Desmoplastic Medulloblastoma. TURCOT syndrome links colon tumors with medulloblastoma.

Meningioma

Meningiomas arise from the meninges, the membranes surrounding the brain and spinal cord.

• Risk Factors: More common in females, during pregnancy, after radiation exposure, and in Neurofibromatosis-2.

• Features: Often progesterone receptor positive. These are dura-based tumors, showing whorling of tumor cells and psammoma bodies (calcified concentric structures). Grading depends on tumor progression.

Miscellaneous Tumors

Lymphoma of Brain

Often seen in HIV-positive individuals with EBV infection. Most common type is Diffuse Large B-cell Lymphoma (DLBCL). It shows an angiocentric pattern.

Schwannoma

Originates from Schwann cells, linked to Nf2. Can occur anywhere, appearing dumbbell-shaped in the spinal cord.

• Microscopy: Displays Antony A (hypercellular) and Antony B (hypocellular) areas, plus Verocay bodies (palisading nuclei with central cytoplasm).

Malignant Peripheral Nerve Sheath Tumor (MPNST)

This is a malignant form of schwannoma. A Triton tumor is an MPNST mixed with rhabdomyoblasts.

Craniopharyngioma

Originates from Rathke's pouch.

• Site: Suprasellar location with calcification.

• Age: Predominantly in children.

• Clinical Features: Causes vision issues and headaches.

• Microscopy: Shows palisading nuclei and stellate reticulum. Grossly, "wet keratin" gives a machine-oil appearance.

Diffuse Midline Glioma

A glial tumor affecting children in the brain's midline. It spreads diffusely and has a poor prognosis, often associated with H3 K27M mutation.

Importance of Studying Brain Tumor Pathology

Studying the pathology of brain tumors is important for both medical students and healthcare professionals. Key benefits include:

• Accurate diagnosis of tumor type

• Understanding tumor behavior and prognosis

• Selection of appropriate treatment strategies

• Preparation for medical competitive examinations

In exams such as NEET PG, students are frequently asked about:

• Tumor grading criteria

• Characteristic microscopic features

• Genetic mutations associated with specific tumors

Mastering these concepts improves both clinical understanding and exam performance.