Frozen Section
Frozen section is a rapid microscopic examination of tissue during an operation, so the surgeon can decide what to do next while the patient is still under anaesthesia. Fresh tissue is frozen, cut into thin sections in a cryostat, stained and examined by the pathologist, who reports to the operating room by telephone or intercom.
It answers a focused question: is this lesion benign or malignant, are the margins clear, is this lymph node involved, is this tissue adequate for diagnosis, or is this tissue the intended structure (for example, parathyroid tissue). The result changes the extent of the operation in real time.
The tissue is snap frozen in a medium at about -20 to -30°C, cut at 5 to 10 microm, and stained with haematoxylin and eosin or toluidine blue. Processing takes 10 to 20 minutes, and the report is usually given in 15 to 30 minutes of receipt.
Overall agreement with the final paraffin diagnosis is about 95 to 98%. The deferral rate (cases where a diagnosis is postponed to the permanent sections) is about 2 to 5%, and the error rate is 1 to 3%, mostly from sampling and interpretation. Final diagnosis always rests on the permanent sections.
Indications.
- Diagnosis of an unexpected mass or lesion found at operation, when the result will change surgery.
- Margin assessment of a tumour resection (breast, skin by Mohs surgery, head and neck, gastrointestinal, bladder and ureter, prostate).
- Sentinel and regional lymph node assessment (breast, melanoma, gynaecological cancer, with node-by-node judgement).
- Confirmation that tissue is adequate and representative for diagnosis (for example, lesional tissue in a brain biopsy).
- Identification of tissue type (parathyroid versus thyroid or lymph node).
- Assessment of ovarian masses for borderline or malignant disease.
- Detection of infection, with tissue sent for culture.
- Triage of fresh tissue for special studies (lymphoma needing flow cytometry, tumour banking).
- Intraoperative evaluation of Hirschsprung disease (ganglion cells), and of donor organs for transplantation (steatosis, fibrosis).
- Evaluation of cancer extent when it determines operation type.
Not indicated when the result will not change management, for diagnosis of small lesions where all the tissue is needed for permanent sections, or for suspected lymphoma needing complete fixed tissue.
Contraindications and precautions.
- Very small specimens (under 3 mm) are best kept for permanent sections, since freezing uses up tissue.
- Fatty tissue and bone freeze poorly. Bone needs decalcification and is not suitable.
- Suspected infection with highly transmissible organisms (tuberculosis, prion disease, hepatitis, HIV): extra precautions for staff and cryostat decontamination. Prion disease (Creutzfeldt-Jakob) specimens are not frozen in a shared cryostat.
- Melanocytic lesions, pigmented lesions and thyroid follicular lesions are difficult, and frozen section is avoided or used with caution.
- Tissue needed for ancillary tests (molecular testing) must not be wasted.
Preparation and technique.
The surgeon sends the fresh specimen, unfixed and without formalin, in a labelled container or on a saline moistened gauze, with a request form giving the clinical question, relevant history, imaging and the contact number. The specimen is oriented, with sutures or ink for margins.
The pathologist selects the area of interest, trims it, and freezes it in embedding medium on a chuck at low temperature. Sections are cut at 5 to 10 microm, mounted, fixed briefly in alcohol, and stained. The pathologist examines the slide and phones the result, stating the diagnosis or that it is deferred, and writes the report.
The remaining tissue is fixed in formalin and processed, and the frozen block is kept. The final report compares the frozen and the permanent diagnosis, and any discrepancy is documented.
