Cryotherapy (cryosurgery) — Viva

Q1: Definition and mechanism (2 min)

What is cryotherapy, and at what temperature does liquid nitrogen boil? Walk me through the mechanism by which it destroys tissue. Why is the thaw phase as important as the freeze phase?

Expected answer: Cryotherapy is the controlled destruction of cutaneous tissue by freezing with liquid nitrogen, which boils at -196 C. The lethal target temperature is approximately -20 to -30 C. Four mechanisms converge: extracellular ice formation produces cellular dehydration and solute-effect injury; intracellular ice (the most lethal mechanism) mechanically disrupts the cell membrane; vascular stasis injures the microvasculature, causing thrombosis and ischaemic infarction; and apoptosis occurs at the sub-lethal periphery. A slow thaw allows ice crystals to recrystallise and grow, magnifying membrane injury; this is why a passive thaw is deliberately sought and why heat is never applied to speed thawing.

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Q2: Technique and dosimetry (3 min)

Describe the open-spray technique step by step. How long do you freeze for, and how many cycles? What is the ice-ball halo, and why does it matter? How do the freeze time and cycle number change for a wart versus a superficial BCC?

Expected answer: Clean the lesion; spray from 1-2 cm until an ice ball extends 2-3 mm beyond the visible margin; allow complete thaw; repeat for the required cycles. For a wart, 10-15 seconds per cycle, two cycles, sessions every 3-4 weeks. For actinic keratosis, 5-8 seconds, single cycle. For Bowen disease, 15-20 seconds, two cycles. For carefully selected low-risk superficial BCC, 30 seconds per cycle, two cycles, with a 5 mm halo. The halo matters because the lethal isotherm sits behind the visible edge of the ice ball — a 2-3 mm halo ensures the -20 to -30 C isotherm encompasses the whole lesion, and for BCC the 5 mm halo ensures clearance of subclinical extension. Two cycles are mandatory for malignancy because the second cycle widens the zone of complete necrosis and clears cells that survived the first.

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Q3: Indications and contraindications (2 min)

When is cryotherapy first-line? When is it acceptable but not first-line? When must you never use it?

Expected answer: First-line: viral warts, actinic keratosis, molluscum contagiosum, seborrhoeic keratosis, solar lentigo, small Bowen disease. Acceptable but not first-line: carefully selected low-risk superficial BCC where surgery is unsuitable; hypertrophic lichen planus; prurigo nodularis; keloids combined with intralesional steroid; cutaneous leishmaniasis in endemic regions. Never: suspected melanoma (excisional biopsy mandatory), invasive SCC, nodular/aggressive BCC, any lesion requiring histology, any lesion of uncertain diagnosis. Relative contraindications: dark skin at cosmetically significant sites (permanent hypopigmentation), cold-intolerance disorders (Raynaud, cryoglobulinaemia, cold urticaria), periungual/digital sites in Raynaud, thin atrophic skin, immunosuppression.

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Q4: Complications and prognosis (3 min)

What complications should the patient be warned about, and in what timeframe? Why is hypopigmentation permanent and worst in skin of colour? What is the recurrence rate for warts and for superficial BCC?

Expected answer: Immediate: pain, erythema, oedema. Early (within 48 h): blister formation, which is expected and may be haemorrhagic — do not deroof, aspirate with a sterile needle leaving the roof as a biological dressing; secondary infection; bleeding. Late: permanent hypopigmentation (the most important long-term complication), transient hyperpigmentation, scarring, atrophy, nail dystrophy (periungual), and nerve damage. Hypopigmentation is permanent because melanocytes are more cold-sensitive than keratinocytes, and in skin of colour (Fitzpatrick IV–VI) the contrast with surrounding skin is greatest. Warts may recur and treated basal cell carcinoma sites need surveillance, which is why structured follow-up with a low threshold for biopsy is mandatory.^[1][3]

Q5: Decision and pitfalls (2 min)

A 60-year-old presents with a dark, irregular, changing pigmented nodule on the back. The GP wonders whether to freeze it off in the clinic. How do you counsel?

Expected answer: Absolutely not. A dark, irregular, changing pigmented lesion is melanoma until proven otherwise, and cryotherapy is destructive — it forfeits the histology specimen, prevents staging, and can be fatal by delaying diagnosis. The correct pathway is dermoscopy now, and excisional biopsy for histology and staging.^[1] The single most dangerous cryotherapy pitfall is treating a melanoma or any undiagnosed pigmented lesion; the discipline that prevents it is: if there is any diagnostic doubt, dermoscopy and biopsy before any destructive treatment.

References3ShowHide

1. [1]Mokbel R, Kodresko A, Mokbel K, et al. Cutaneous Cryosurgery in Dermatology: Evolving Principles and Clinical Applications for Benign, Premalignant, and Malignant Lesions In Vivo, 2025.PMID 40010951
2. [2]Dianzani C, Conforti C, Giuffrida R, et al. Current therapies for actinic keratosis Int J Dermatol, 2020.PMID 32012240
3. [3]Chanal J, Aubin F, Penso-Assathiany D, et al. A multicentre pragmatic randomized controlled trial comparing 50% salicylic acid, liquid nitrogen, 5% 5-fluorouracil cream, and 5% imiquimod cream in previously treated plantar warts. The VRAIE (VeRrues plAntaIres en villE) study Ann Dermatol Venereol, 2025.PMID 40743833