Equipment and Technique

Liquid nitrogen can be applied with a regular or modified cotton-tipped applicator, cryogen spray device, or cryoprobe, all readily available to providers. The cotton-tipped applicator is the least expensive and can be modified to hold more cryogen or to match the size or shape of the lesion with a cotton ball. Liquid nitrogen can be stored in an insulated device in the office and taken out in small aliquots in a polystyrene cup for individual use with patients.

Handheld cryogen spray devices have recently been improving. Traditional cryotherapy systems provide a reservoir for the liquid nitrogen and an applicator tip to dispense the cryo-gen. The applicator tip can have a varying aperture for altering cryogen dispersion. Some providers use an otoscopy cone to protect surrounding skin and direct the spray onto a small central lesion. The unit is generally held 1 to 2 cm above the lesion and the spray directed at a 90-degree angle and aimed at the lesion center. Newer spray devices use a continuous infrared sensing device to determine the treatment site skin temperature and ensure consistent freeze times. These modifications can improve accuracy and assist in achieving optimum cryotherapy results (Cry-Ac Tracker, www.brymill.com).

Prior to cryotherapy, the depth and diameter of the freeze must be anticipated to minimize injury to surrounding tissues. Mark the skin to ensure adequate treatment in critical cryother-apy sessions. Keratin layers are very resistant to cryotherapy and can be treated for 1 to 2 weeks with topical 40% salicylic acid plaster or mechanically pared away before freezing. Application of salicylic acid alone is equally efficacious to cryotherapy of warts for many patients (Gibbs and Harvey, 2006). A wellhydrated skin lesion can increase cryotherapy success rates.

For superficial benign skin lesions, the cryotherapy applicator should almost cover the lesion and should be applied for 20 to 40 seconds to create an ice ball edge 2 to 3 mm beyond the edge of the lesion. In contrast, deeper or premalignant lesions should be treated with an applicator smaller than the lesion to ensure a depth more equal to the radius of treatment. It should be applied for 40 to 90 seconds to form an ice ball 3 to 4 mm outside the lesion. Superficial malignant lesions also require a smaller applicator, but applied for 1 to 3 minutes for an ice ball 5 to 8 mm beyond the lesion. Malignant cells are more resistant to cryotherapy, and destruction requires temperatures at -40° to -50° C. In most cases the depth of a freeze is similar to the radius of the superficial ice ball formed. The lethal zone for tissue destruction is 2 to 3.5 mm inward from the outer margin of the ice ball (Pfenninger, 2003).

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