Histopathology
Histopathology is the microscopic examination of tissue to establish a diagnosis. Tissue is fixed, processed, embedded in paraffin wax, cut into sections 3 to 5 microm thick, mounted on glass slides and stained, most often with haematoxylin and eosin. The pathologist describes the cells, their arrangement and their relationship to surrounding structures, and relates the findings to the clinical picture and imaging.
It is the reference method for diagnosing and grading cancer, assessing resection margins and lymph nodes, classifying inflammatory, infectious, degenerative and metabolic disease, and monitoring transplanted organs.
The specimen can be a small biopsy, a core, a curettage, a polypectomy, an excision, a resection or a whole organ. The route from fixation to report involves reception, gross description, tissue sampling, processing overnight, embedding, sectioning, staining and microscopy.
Routine reports take 2 to 5 working days. Urgent cases are reported in 24 hours, and cases needing decalcification, immunohistochemistry, molecular testing or consultation take 1 to 3 weeks.
A large proportion of cancer treatment decisions depend on histology. Accuracy depends on specimen quality, fixation, adequate clinical information and the experience of the pathologist, and discrepancy between pathologists occurs most in borderline lesions such as dysplasia, atypical melanocytic lesions, lymphoid proliferations and grading of some tumours.
Indications.
- Any tissue removed at surgery, biopsy or endoscopy, as a routine for diagnosis or confirmation.
- Suspected cancer, and staging, grading and margin assessment.
- Inflammatory bowel disease, coeliac disease, hepatitis, glomerular disease, and other organ biopsies.
- Skin, lymph node and bone marrow biopsy.
- Transplant biopsies for rejection.
- Infectious disease: granulomas, organisms and viral changes.
- Products of conception and placenta.
- Assessment of response after neoadjuvant treatment.
- Research and biobanking (with consent).
Not indicated for lesions that are clearly normal variants or physiological findings with no clinical question, and for some very small benign lesions in low resource settings where discarding after examination by the surgeon is accepted practice.
Specimen requirements.
- Fixative: 10% neutral buffered formalin, in a volume at least 10 times the volume of the tissue, and 15 to 20 times for large specimens. The specimen must be fully immersed.
- Time: the specimen goes into formalin as soon as possible, preferably within 1 hour (cold ischaemia time). Fixation takes 6 to 72 hours, and breast tissue for receptor testing needs the same window.
- Fresh tissue or special media: frozen section, direct immunofluorescence (Michel's medium or saline), culture (sterile container), flow cytometry (saline or culture medium), and electron microscopy (glutaraldehyde).
- Bone: fixation and decalcification, which can damage nucleic acids and some antigens.
- Labelling: two patient identifiers on the container and the form, with site, laterality, number of pieces and orientation.
- Request form: clinical history, examination and imaging findings, previous histology, suspected diagnosis, urgency and contact details.
