1. Recent Updates in WHO Classification of CNS Tumors (WHO CNS5, 2021) — 10 Marks

Introduction

Major General Changes

• Over 40 tumor types/subtypes are now defined chiefly by their molecular features rather than histology alone.
• Layered/integrated diagnosis is mandated: histological classification, WHO grade, and molecular information are combined into a single integrated diagnostic line, with each parameter listed below it.

• CNS tumor grading is now done within tumor types (e.g., astrocytoma, IDH-mutant can be grade 2, 3 or 4) rather than across entirely separate entities.
• Arabic numerals (1, 2, 3, 4) replace Roman numerals (I, II, III, IV).
• The term "anaplastic" is discouraged for grade 3 designation (e.g., oligodendroglioma grading no longer uses a strict mitotic count cut-off).

• "NOS" (Not Otherwise Specified) used when molecular testing has not been/cannot be done.
• "NEC" (Not Elsewhere Classified) used when testing was done but the result doesn't fit any WHO-defined type.

• DNA methylome profiling is incorporated for defining some novel tumor types and subtypes.

Key Specific Changes by Category

Clinical/Academic Significance

• Improves reproducibility of diagnosis and prognostication.
• Guides targeted therapy and clinical trial stratification (e.g., IDH-inhibitors for IDH-mutant gliomas).
• Requires close integration between neuropathology and molecular diagnostic laboratories, which has resource implications especially in low-resource settings.

2. Crush and Squash Preparation — 10 Marks

Definition

Indications

1. Neurosurgical intraoperative consultation (most important use) - rapid diagnosis of brain/spinal cord lesions when a decision on extent of resection is needed on-table.
2. Lymph node and soft tissue rapid cytology.
3. Parasitology - crush preparation of muscle for Trichinella spiralis larvae, cysticercus, or of tissue/scrapings for Donovania granulomatis (Donovan bodies) and Schistosoma ova.
4. Superficial nodules/lesions when frozen section facility or time is limited.

Procedure (Technique)

1. A 1-2 mm representative, fresh fragment of tissue is taken (avoid necrotic/hemorrhagic areas; take from the periphery/interface of the lesion where cellularity is best).
2. The fragment is placed on one clean glass slide; a second slide is placed over it at a right angle or directly on top.
3. Gentle, even pressure is applied (never a shearing/dragging motion) to compress and spread the tissue into a thin monolayer film - excessive force causes crush artifact and nuclear distortion.
4. The two slides are pulled apart smoothly, giving two smears.
5. One slide is immediately wet-fixed in 95% ethanol (for Hematoxylin & Eosin or Papanicolaou stain); the other may be air-dried for rapid Toluidine blue/Diff-Quik stain, or a portion is submitted for simultaneous frozen section.
6. Staining and reporting are completed typically within 10-20 minutes, and results are communicated to the surgeon while the patient is still on the table.

Advantages over Frozen Section

• Preserves nuclear cytomorphological detail (chromatin pattern, nucleoli) far better - no ice-crystal artifact.
• Technically simple, fast, and inexpensive - no cryostat required.
• Very useful for soft, gliotic, or highly cellular CNS tissue that fragments poorly on freezing (e.g., lymphoma, glioma, medulloblastoma, pituitary adenoma).
• Better demonstration of cell dissociation patterns that help distinguish glial vs neuronal vs mesenchymal tumors.

Limitations

• Architectural/histoarchitectural detail is lost (cannot assess true tissue pattern, invasion, or vascular arrangement well) - frozen section or paraffin section still needed for definitive architecture-based diagnosis.
• Poor performance for fibrous, calcified, or firm tumors (e.g., meningioma with heavy collagen, chordoma) which do not smear well - "crush resistant" tissue.
• Cannot always distinguish tumor grade reliably.
• Operator-dependent; overzealous pressure produces crush artifact mimicking hyperchromasia/malignancy.
• Best used as a complement to frozen section, not a total replacement, especially in ambiguous or low-grade lesions.

Typical Cytomorphological Patterns Seen (CNS)

• Astrocytoma: fibrillary background, elongated glial processes, uniform oval nuclei.
• Glioblastoma: pleomorphic cells, necrosis, endothelial proliferation, high N:C ratio.
• Meningioma: whorled clusters, psammoma bodies, nuclear pseudoinclusions.
• Medulloblastoma/PNET: small round blue cells with high N:C ratio, "carrot/comet-shaped" nuclei, apoptosis.
• Schwannoma: spindle cells with wavy/buckled nuclei (Antoni A/B pattern).
• Pituitary adenoma: monomorphic round cells in acinar/trabecular pattern, salt-and-pepper chromatin.

Conclusion