Radiosurgical treatments of tissues of the heart mitigate arrhythmias and treat other tumerous and non-tumerous disease using an implanted fiducial positioned in or near the heart using cardiac catheterization techniques. The fiducials may be implanted after diagnostic and planning images of the target tissues have been acquired. Fiducial implantation may take place the day of a scheduled radiosurgical treatment. Techniques to accommodate post-planning fiducial implantation may include registration of the implanted fiducial location with the treatment plan, and active fiducials may limit collateral imaging radiation exposure while enhancing tracking accuracy.