The need to evaluate the effectiveness of safety measures and patient handling procedures is often raised for healthcare and radiotherapy in particular. A major challenge here is the analysis of potential human errors and the estimation of their probabilities, because the available Human Reliability Analysis (HRA) methods are not directly applicable. The work presented in this paper is part of on-going research at the Paul Scherrer Institute of Switzerland to develop an HRA method for external beam radiotherapy. This paper in particular presents the identification and characterization of the Performance Influencing Factors (PIFs), which shall support an analyst in performing a structured analysis of the performance context. The paper presents the information sources and their use in the development of the PIF taxonomy. The taxonomy is then compared with a state-of-the-art PIF hierarchy for HRA application to nuclear power plant probabilistic safety assessment. The comparison suggests insights on how the different application domains influence the set of PIFs and their definitions.