Dose calibrators quality controls in Switzerland: six years of experience

In Switzerland, the legal use of open radioactive sources in nuclear medicine and the general requirements for quality controls are defined in a federal ordinance. The metrological traceability is guaranteed through a directive of the Swiss metrological office (M.E.T.A.S.) that requires each instrument to be monitored at least once a year through either a verification or an intercomparison. The verification is performed onsite by an accredited laboratory with a set of three gamma sources (Co-57, Cs-137 and Co-60) and - if applicable - a beta source (Sr-90/Y- 90). The intercomparison is made through conventional mail. A source of I-131 or Tc- 99 m is measured both in the nuclear medicine department and in an accredited laboratory. The maximum tolerated error is 10% for gamma sources and 20% for beta sources. This methodology guarantees that the instruments have a correct response for most of the energy range used in practice. Not all nuclides are systematically probed and manufacturers are ultimately responsible for the calibration factors. The precision of the measurements performed in Switzerland is satisfactory with only about 6% of the measurements out of the tolerances. This monitoring also allowed us to improve the skills of the personnel and update the park of instruments by getting rid of dose calibrators displaying old units. (authors)