Catheter-based radiation delivery systems employing both beta-particle and gamma-ray emitters are currently being investigated for their efficacy in addressing restenosis following percutaneous coronary intervention (PCI). The dosimetric consequences of source centering within the arterial lumen and presence of residual plaque are potentially important issues for the uniform delivery of dose to the arterial tissue. In this study, we have examined the effect of source centering on the resulting dose to the arterial wall from clinical intravascular brachytherapy sources containing 32P and 90Sr/Y90. Monte Carlo simulations using the MCNP code were performed for these catheter-based sources with offsets of 0.5 mm and 1 mm from the center of the arterial lumen in homogenous water medium as well as in the presence of residual plaque. Three different positions were modeled and the resulting dose values were analyzed to assess their impact on the resulting dose distribution. The results indicate a variation ranging from -40% to +70% for 32P source and -30% to +50% for 90Sr/90Y at a radial distance of 2 mm from the center of the coronary artery, relative to the dose from a centered source, for a 0.5 mm offset. The variation for a 1 mm offset ranges from -65% to +182% for 32P source and to -50% to +140% for 90Sr/90Y. A concentric residual plaque layer was also modeled so as to assess the combined influence of offset and residual plaque on the dosimetry. Finally the effect of cardiac motion and its potential impact on catheter position and hence the dose distribution is also examined by considering two separate cases of catheter displacement. The results indicate that dose variations range between -28% to +91% when it is assumed that cardiac motion causes catheter movement during coronary lesion irradiation.