The general criteria and the scientific approach adopted for an 'International comparison on Monte Carlo modelling for in vivo measurement of Americium in a knee phantom' that is being organised within the EU Coordination Action CONRAD (Coordinated Network for Radiation Dosimetry) are described her. Detection system and a knee voxel phantom based on a computerised axial tomography of the Spitz anthropometric knee phantom with a homogeneous distribution of 241Am in bone have been considered for the simulation of three specific situations: (a) a single Low Energy Germanium detector for a point 241Am source in air; (b) the calculation of photon fluence spectra in air around the voxel phantom; and (c) the calculation of the energy distribution of pulses and peak detection efficiency in the real detection system geometry.