Liquid nitrogen (–196°C) is now used more often than carbon dioxide snow (‘dry ice’, –79°C). It is effective for viral warts, seborrhoeic keratoses, actinic keratoses and some superficial skin tumours (e.g. intraepidermal carcinoma and lentigo maligna). It is applied either on a cotton bud or with a special spray gun (Fig. 24.8). The lesion is frozen until it turns white, with a 1–2 mm halo of freezing around.
Two freeze–thaw cycles kill tissue more effectively than one but are usually unnecessary for warts and some keratoses (Fig. 24.9). Patients should be warned to expect pain and possible blistering after treatment. Care should be taken when treating warts on fingers as digital nerve damage can occur after overenthusi-astic freezing. Standard freeze–thaw times have been established for superficial tumours but temperature probes in and around deep tumours are needed to gauge the degree of freezing for their effective treat-ment. A crust, including the necrotic tumour, should slough off after about 2 weeks. Melanocytes are very sensitive to cold injury; hypopigmentation at a treated site is common and may be permament.