Technique

Specimen frozen rapidly to ~−20 °C in a cryostat; 4-8 μm sections cut, stained with H&E (or rapid IHC where available).
Turnaround typically 10-20 minutes per specimen from receipt.
Operator must communicate clearly with pathologist — orientation marker (suture / ink), defect map, clinical question, prior treatments.

Indications in skin oncology

Intra-operative margin clearance at non-Mohs centres for high-risk BCC and cSCC at sensitive anatomical sites (periocular, nose, ear, lip).
Complex / staged reconstruction — confirm margin clearance before flap inset or graft.
Salvage surgery for previously irradiated or recurrent disease.
DFSP wide local excision — slow Mohs / en-face permanent sections preferred but frozen section used at centres without Mohs.
Microcystic adnexal carcinoma, EMPD — slow Mohs / permanent en-face is preferred (frozen-section accuracy limited).

Melanocytic lesions: frozen section is unreliable for melanoma in situ / lentigo maligna; freezing artefact, fragmentation and lack of IHC reduce sensitivity. Permanent en-face sections (Mohs-by-permanent-paraffin) or staged excision with rush paraffin is preferred — supported by BAD and ACMS guidance.
Sclerosing / morphoeic BCC and infiltrative cSCC: thin strands may be missed in vertical frozen sections.
Perineural invasion: hard to identify reliably on frozen section.
Freezing artefact, ice crystals and section curling can mimic or hide tumour.
Tumour budding and grade assessment less reliable than permanent.

Mohs: complete circumferential and deep-margin assessment of 100% of the resection margin, en-face frozen sections, integrated surgical / pathological role. Highest cure rates for BCC and cSCC at high-risk sites.
Conventional frozen section: typically vertical sections from a partial set of margins; less of the true margin is examined; risk of sampling error.
If Mohs unavailable, slow-Mohs / 3D-histology with permanent paraffin sections delivers near-Mohs results without freezing artefact.

Plan operating time and reconstruction strategy assuming frozen-section may upstage and require re-excision.
Send selected margins as fresh, orientated specimens with clearly drawn defect map.
If margin equivocal: ink and proceed with conservative reconstruction (e.g. delayed graft) pending permanent paraffin.
Record all frozen-section reports in the MDT minutes; cross-check against final paraffin report.
Audit your frozen-section / paraffin concordance and false-negative / false-positive rates annually as part of clinical governance.