Purpose/Objective(s)-Conformal radiation therapy in the post-prostatectomy setting requires accurate setup and localization of the prostatic fossa. In this series, we report prostate bed localization and motion characteristics using data collected from implanted radiofrequency transponders.Materials and Methods-The Calypso 4D Localization System uses three implanted radiofrequency transponders for daily target localization and real-time tracking throughout a course of radiation therapy. We reviewed the localization and tracking reports of 20 patients who received ultrasound-guided placement of Calypso transponders within the prostate bed prior to a course of intensity modulated radiation therapy at Fox Chase Cancer Center.Results-At localization, prostate bed displacement relative to bony anatomy exceeded 5 mm in 9% of fractions in the Anterior-Posterior (A-P) direction, and 21% of fractions in the SuperiorInferior (S-I) direction. The three-dimensional vector length from skin marks to Calypso alignment exceeded 1 cm in 24% of all 652 fractions with available setup data. During treatment, the target exceeded the 5 mm tracking limit for at least 30 seconds in 11% of all fractions, generally in the A-P or S-I directions. In the A-P direction, target motion was twice as likely to move posteriorly, towards the rectum, than anteriorly. 15% of all treatments were interrupted for repositioning, and 70% of patients were repositioned at least once during their treatment course.Conclusion-Set-up errors and motion of the prostatic fossa during radiotherapy are nontrivial, leading to potential undertreatment of target and excess normal tissue toxicity if not taken into account during treatment planning. Localization and real-time tracking of the prostate bed via implanted Calypso transponders can be used to improve the accuracy of plan delivery.