Measuring 'radiation' can get very complicated because there are various standards of measurement for the many types of radiation extant. Basically, one rad is equal to 100 grays. However, GK is very localized and cannot be compared to OSHA standards, which are concerned with the unwanted absorption of radiation in the workplace, which far different from a medically controlled radiation treatment specifically designed to destroy the DNA of an organic substance (your AN).
GammaKnife, CyberKnife and FSR have all been around for some time (GK for over 40 years). The radiation 'dose' is carefully calculated by a radiation oncologist, sometimes working with a neurosurgeon, as in my case. They know what they are doing. Although there is a slight risk with any radiation treatment, it is almost infinitesimal and nothing to be stressed about.