Application of Monte Carlo calculation for the virtual calibration of a low-energy in vivo counting system

Internal dose assessment can be derived from the measurement of retained activity in the whole body or in an organ at a given time. In radiation protection, this assessment, so-called in vivo measurement, is performed by an external measurement of the subject with germanium detectors (in most cases). Calibration of these detectors is ensured by anthropomorphic phantoms, which however can only provide, for technical reasons, rough representations of human chest leading to substantial corrections on calibration factors that are particularly crucial and delicate in low-energy in vivo measurement resulting in important systematic errors. In order to improve calibration, former works based on numerical phantoms associated with Monte Carlo computing techniques have already been proved their benefit. To go further, a Graphical User Interface called "OEDIPE", a French acronym for "tool for internal personalized dose assessment", has been developing at the IRSN internal dose assessment laboratory, simulating real measurements using person-specific computational phantoms in association with MCNP calculation code. The study presented here is dedicated to the implementation and validation of a real in vivo monitoring system (COGEMA Marcoule, France) equipped with 4 high purity germanium (HPGe) detectors. After modeling with OEDIPE of the facility and measurement geometry (design and positioning of the detectors...), the validation with different configurations was carried out in two steps: first with point sources (different nuclides, different source-detector distances) and, after that, with the Livermore calibration phantom (different overlay plates, lungs contaminated with /sup 241/Am and /sup 235/U). The final goal is to approach a personalized numerical calibration of the facilities in order to improve dose assessment, as the use of physical phantoms for calibration induces large uncertainties. Such application could be an opening door on a better activity assessment in nuclear medicine, especially in personalized dosimetry in radioimmunotherapy.