Frozen section

Principle

• Fresh, unfixed tissue is rapidly frozen so that ice acts as the supporting medium, allowing sections to be cut within minutes in a cryostat.
• Provides a rapid intraoperative diagnosis to guide the surgeon.

Specimen

• Fresh tissue sent immediately without fixative, labeled and with clinical information and the question to answer.

Reagents and equipment

• Cryostat (refrigerated chamber with microtome), embedding medium (e.g. OCT compound), chucks, rapid freezing aids (cold bar or freezing spray).
• Fixative for slides (e.g. alcohol or alcohol–formalin), rapid H&E or toluidine blue stains.

Procedure

• Gross the specimen and select the most representative area.
• Mount on a chuck in embedding medium and freeze rapidly to limit ice crystal artifact.
• Cut sections at about 5–8 µm with the cryostat set at a temperature suited to the tissue (commonly about −15 to −25 °C; fatty tissue needs colder settings).
• Pick up on a room-temperature slide, fix immediately in alcohol, stain rapidly and mount.
• The pathologist communicates the result to the operating room; turnaround is commonly targeted at about 20 minutes per specimen.

Quality control and pitfalls

• Ice crystal artifact, folds and thick sections reduce quality compared with paraffin sections.
• Fatty tissue is hard to cut; bone and calcified tissue usually cannot be sectioned.
• Sampling error: only part of the specimen is examined.
• Frozen section diagnosis is confirmed on permanent paraffin sections; compare and monitor discrepancy rates.
• Biosafety: fresh tissue may be infectious (e.g. suspected TB); aerosols form during cutting. Decontaminate the cryostat regularly.
• Use the anti-roll plate and blade guard; cryostat blades are extremely sharp.

Clinical use

• Margin assessment, confirmation of malignancy or lesional tissue, sentinel lymph node evaluation and adequacy of biopsy for further studies.
• Also used for fat stains (Oil Red O), enzyme histochemistry, muscle biopsies and immunofluorescence on kidney and skin biopsies.