Abstract: A detector response correction arrangement and method is proposed for online determination of correction factors for arbitrary positions from arbitrary incident fluence distributions. As modern radiotherapy utilizes more of the available degrees of freedom of radiation machines, dosimetry has to be able to present reliable measurements for all these degrees of freedom. To determine correction factors online during measurement, Monte Carlo technique is used to precalculate fluence pencil kernels from a monodirectional beam to fully describe the particle fluence in an irradiated medium. Assuming that the particle fluence is not significantly altered by the introduction of a small detector volume, the fluence pencil kernels (212) can be integrated (214), and correction factors (216) determined, e.g. by Cavity Theory, in different positions for the detector material.