Portal imaging of implanted fiducial markers has been in use for image-guided radiotherapy (IGRT) of prostate cancer, with ample attention to localization accuracy and organ motion. The geometric uncertainties in point-based rigid-body (PBRB) image registration during localization of prostate fiducial markers can be quantified in terms of a fiducial registration error (FRE). Statistical process control charts for individual patients can be designed to identify potentially significant deviation of FRE from expected behaviour. In this study, the aim was to retrospectively apply statistical process control methods to FREs in 34 individuals to identify parameters that may impact on the process stability in image-based localization. A robust procedure for estimating control parameters, control limits and fixed tolerance levels from a small number of initial observations has been proposed and discussed. Four distinct types of qualitative control chart behavior have been observed. Probable clinical factors leading to IGRT process instability are discussed in light of the control chart behaviour. Control charts have been shown to be a useful decision-making tool for detecting potentially out-of-control processes on an individual basis. It can sensitively identify potential problems that warrant more detailed investigation in the IGRT of prostate cancer.