Preliminary report of a gas conditioner to improve operational reliability of cryotherapy in developing countries

Performance of cryotherapy devices using nitrous oxide and carbon dioxide

OBJECTIVE

To compare temperatures reached by 4 different cryotherapy devices commonly used to treat precancerous cervical lesions in low-resource countries using nitrous oxide (N(2)O) and carbon dioxide (CO(2)) as well as with and without a gas conditioner.

METHODS

Bench testing was conducted using 4 different cryotherapy devices and locally obtained N(2)O and medical- and industrial-grade CO(2) refrigerant with and without a gas conditioner. A thermocouple was used to continuously measure the temperature of the cryotherapy tip. Comparison across the cryotherapy devices was based on the mean and lowest temperatures.

RESULTS

Without the gas conditioner, all of the devices tested reached mean temperatures colder than -50°C with N(2)O, although 2 devices reached warmer temperatures in a proportion of N(2)O tests. Only 2 of the devices reached mean temperatures colder than -50°C with CO(2). One-way analysis of variance identified the device as the dominant factor for the temperature differences, while the gas was not a determinant of temperature variation. The gas conditioner hindered the performance of 2 of the devices, and only improved the performance of 1 device.

CONCLUSION

Both N(2)O and CO(2) reach appropriate freezing temperatures with some cryotherapy devices. Performance of some cryotherapy devices is suboptimal.

Effect of cough technique and cryogen gas on temperatures achieved during simulated cryotherapy

BACKGROUND

Cryotherapy is often used to treat cervical precancer in developing countries. There are different methods and cryogen gases used for cryotherapy, including the freeze-flush-freeze (cough) technique employed to minimize gas blockage. However, there is limited information to compare their effectiveness.

METHODS

Using a tissue model, we compared temperature-time curves for four cryotherapy methods: uninterrupted freezing with nitrous oxide (N2O) and carbon dioxide (CO2), and two methods using a standard and extended version of the cough technique with CO2.

RESULTS

Uninterrupted freezing with both N2O and CO2 produced tissue temperatures less than -20 degrees C (-40 degrees C and -30 degrees C respectively). CO2 cryotherapy procedures using the two cough techniques produced temperatures greater than -20 degrees C in the model tissue.

CONCLUSION

CO2 cryotherapy using the cough technique may not achieve sufficiently low temperatures to produce the desired therapeutic effect. Other alternatives to the prevention of gas blockage should be developed.

Comparison of depth of necrosis achieved by CO2- and N2O-cryotherapy

OBJECTIVE

To compare the depth of necrosis achieved using CO2- and N2O-cryotherapy.

METHOD

We treated 20 women with CO2-cryotherapy and 20 with N2O-cryotherapy 24 h prior to undergoing total hysterectomy for reasons unrelated to cervical cancer. Depth of necrosis in the ectocervix was measured on surgically removed tissue.

RESULTS

The mean depth of necrosis for the anterior lip achieved by N2O-cryotherapy was 5.3 mm compared with 3.4 mm by CO2-cryotherapy (P<0.001). We found similar results for the posterior lip (5.0 vs 3.1 mm; P<0.001). N2O-cryotherapy was more likely than CO2-cryotherapy to achieve a depth of necrosis of >or=4.8 mm for the anterior lip (75% vs 15%; P<0.001) and posterior lip (60% vs 5%; P<0.001).

CONCLUSIONS

Our study showed that CO2-cryotherapy may not achieve the depth of necrosis necessary to completely destroy cervical precancer and therefore may be less robust than N2O-cryotherapy.

Human papillomavirus and cervical cancer

Cervical cancer is the second most common cancer in women worldwide, and knowledge regarding its cause and pathogenesis is expanding rapidly. Persistent infection with one of about 15 genotypes of carcinogenic human papillomavirus (HPV) causes almost all cases. There are four major steps in cervical cancer development: infection of metaplastic epithelium at the cervical transformation zone, viral persistence, progression of persistently infected epithelium to cervical precancer, and invasion through the basement membrane of the epithelium. Infection is extremely common in young women in their first decade of sexual activity. Persistent infections and precancer are established, typically within 5-10 years, from less than 10% of new infections. Invasive cancer arises over many years, even decades, in a minority of women with precancer, with a peak or plateau in risk at about 35-55 years of age. Each genotype of HPV acts as an independent infection, with differing carcinogenic risks linked to evolutionary species. Our understanding has led to improved prevention and clinical management strategies, including improved screening tests and vaccines. The new HPV-oriented model of cervical carcinogenesis should gradually replace older morphological models based only on cytology and histology. If applied wisely, HPV-related technology can minimise the incidence of cervical cancer, and the morbidity and mortality it causes, even in low-resource settings.