I am not aware of any studies that have looked at an associated between TENS usage and an increased incidence of cancer, but for more information please speak with your primary care doctor
. The technology has been around for a long time and it is generally thought to be quite safe for most individuals. You are definitely correct that it produces electrical currents within the body, and these can be a problem for individuals who, or example, have an electrical pacemaker. But for most other individuals, who do not depend on an electrical device for health, this is not an issue.
The comparison between TENS units and power line exposure is not entirely straight forward. This is because the feature of power line exposure that has caused concern and investigation is the magnetic field generated by the power line, not the electrical field. Even so, the evidence that the magnetic field from a power line causes cancer is weak, and most health authorities think that, for most types of cancers and levels of exposure, power lines are probably not a significant factor. For more information on this topic, you can always talk with your primary care doctor. Also, the National Cancer Institute has a great fact sheet on power lines and cancer.