Radiation Protection during the Change of Sources for the Gamma Knife System in the General Hospital of Vienna

The Leksell Gamma Knife of the General Hospital of Vienna is a complete system for radiosurgery. The Gamma Knife delivers highly accurate external irradiation of intracranial structures from an array of collimated beams of ionising radiation targeted in compliance with a predeviced treatment protocol. The ionising radiation is emitted by cobalt-60 sources in the radiation unit. The sources are heavily shielded within a cast iron body with shielding doors which provide protection for the patient and the staff. Each of the 201 radiation sources located in the radiation unit is composed of cobalt-60 pellets which are encapsulated in double stainless steel capsules with welded closures. At the time of calibration the specific activity of each source was approximately 925 GBq/g (250 Ci/g) and the total activity was 222 TBq. After six years of operation without any problems and incidents, due to the decrease of activity the 201 old sources had to be changed. The cobalt sources were delivered to the site in specially designed protective casks. With a special loading machine, which is a shielded cell, designed for handling radiation sources with full radiation safety, the 201 old sources were unloaded and the loading procedure with the new sources was done. The paper describes all the radiation protection measures foreseen from the arrival of the new sources, the unloading of the old sources, the loading of the Gamma Knife with the new sources and the retransport of the old sources, especially the dose and dose-rate measurements and the results of the smear and wipe-test.