to the editor: In his article on cryosurgery,¹ Dr. Andrews mentions the use of a Cryogun from Brymill Cryogenic Systems for treatment of skin lesions. When I first purchased a Cryogun, I found it frustrating to use because it sputtered instead of delivering a constant stream of liquid nitrogen, especially with smaller nozzle apertures. The sputtering seemed to increase the pain experienced by the patient.

The sputtering of the Cryogun is caused by the liquid nitrogen “boiling” from a liquid to gas as it takes heat from the dip tube, valve, and delivery tube on its way to the nozzle.¹ The stream at the nozzle is therefore a liquid full of nitrogen gas bubbles.¹

A device called a Back Vent Adapter (Brymill Model #301) solves this problem by venting the gas bubbles just before the nozzle, so that a constant liquid stream is delivered rather than an intermittent stream. It is screwed in place just behind the nozzle aperture, and it must have its vent tube pointing upright because the gas bubbles rise in the liquid nitrogen. I find that the constant stream is much more efficient at freezing skin lesions.

in reply: I appreciated Dr. Havron’s comments. In my article,¹ space requirements limited the range of refrigerants that I could discuss. That being said, liquid nitrogen remains my cryogen of choice as well as that of a majority of experienced cryosurgeons for several reasons. First, the effective treatment temperature of liquid nitrogen is −196°C (− 320.8°F), and tissue temperatures of −60°C to −80°C (−76°F to − 112°F) can be readily achieved, compared with − 20°C to −30°C (−4°F to −22°F) for canister refrigerants that Dr. Havron has described. These temperature levels are not satisfactory for many deeper or larger benign lesions and certainly not for malignancies where tissue temperature levels down to −50°C (−58°F) are required.² Second, most of the research data on the effectiveness and proper technique of cryogen use in treating a broad range of clinical entities has been based on liquid nitrogen. Third, although there is a constant evaporative loss of liquid nitrogen and a need to have the reservoir tank regularly filled (often biweekly or monthly, depending on usage), it remains inexpensive, with costs running from $3.50 to $4.00 per liter; this cost is readily recouped through charges for services provided. I will agree that for physicians who use cryotherapy infrequently and for only the most superficial lesions, canister refrigerants may be appealing because of shelf life considerations and their ease of portability to nursing homes and satellite clinics.

The comment from Dr. Reynolds’ letter on the use of the Back Vent Adapter (Brymill Model #301) for improving liquid nitrogen spray and obtaining a more constant spray delivery is appreciated. I have found, as Dr. Reynolds comments, that the use of the smaller aperture nozzles particularly the “D” size or smaller) are associated with this problem and that back venting does significantly reduce “sputtering” and provide a more constant liquid nitrogen stream. When I was first using the Cryogun, I often chose the “D” size aperture for smaller lesions to reduce scatter and improve spray accuracy; however, with increasing experience and use of the more standardized spray approaches with fixed-target distances and the timed “spot-freeze” technique, I began favoring the standard larger “B” and “C” size aperture tips for which most of the case series and clinical research has been established. I think this ensures the highest potential for cure based on the bulk of documented clinical experience. I do still use the “D” size tip occasionally, for very small lesions on or near eyelids and ears, and find the Back Vent Adapter a useful adjunct.