Radiation therapy with ion beams provides highly conformal dose distributions. Therefore, monitoring the dose delivery within the patient in a non‐ invasive way is desired. The clinically available method based on tissue activation measurements with a PET‐camera shows limitations due to the low induced activities and biological washout of the activated nuclei. The prompt production of secondary ions is supposed to be less influenced by biological processes. This contribution investigates the feasibility of beam range monitoring in a patient‐like geometry containing realistic tissue inhomogeneities.