Histopathology
Histopathology tests the laboratory pathway from fixation to diagnosis, the special stains used to demonstrate specific substances, immunohistochemistry panels and the difference between grading and staging.
Tissue processing
- Fixation in 10% neutral buffered formalin, dehydration through graded alcohols, clearing in xylene, paraffin embedding, sectioning at 3–5 µm and staining with haematoxylin and eosin.
- Frozen section is used intraoperatively for margin assessment or to confirm diagnosis; it takes minutes but morphology is inferior, and adipose tissue sections poorly.
Special stains
- PAS: glycogen and fungi.
- Ziehl-Neelsen: acid-fast bacilli.
- Congo red: amyloid, apple-green birefringence under polarised light.
- Prussian blue: iron.
- Masson trichrome: collagen and fibrosis.
- Giemsa: Helicobacter pylori and mast cells.
Immunohistochemistry
Cytokeratin marks carcinoma, vimentin sarcoma, CD45 lymphoid cells, S100 melanoma and neural tissue, desmin muscle, and chromogranin or synaptophysin neuroendocrine tumours.
Grading and staging
Grading describes differentiation; staging describes anatomical spread, and stage usually predicts prognosis better.
Worked example
A thyroid fine-needle aspirate is reported as papillary carcinoma. Confirmatory features are optically clear Orphan Annie nuclei, nuclear grooves and psammoma bodies; the lesion is graded and staged, with stage driving prognosis.
Traps
Confusing grade with stage, ordering frozen section for a diagnosis needing well-preserved morphology, and misinterpreting Congo red without polarisation.