The Calypso system has previously been described in detail.10 , 17 For this study, three Calypso beacon transponders were inserted into a radiolucent foam cylinder representing the prostate gland. The transponders were positioned in the shape of an equilateral triangle with side length approximately 3 cm, in accordance with the manufacturer's recommendations. The foam cylinder was subsequently placed inside an anthropomorphic pelvic phantom (CIRS pelvic phantom) (Fig. 1 ). The phantom was scanned on a Siemens SOMATOM Emotion CT scanner (Siemens Medical Systems, Forchheim, Germany) with 1 mm slices.
Images were transferred to Varian's Eclipse™ treatment planning system (Version 11.0.42) (Varian Medical Systems, Palo Alto, California). Following standard planning procedures for Calypso patients, a treatment plan was created with the isocenter at the center of mass of the transponders. CBCT and orthogonal kV setup fields were added to the plan, to allow radiographic acquisition at the linear accelerator. Standard departmental procedures were used to transfer the treatment plan and associated imaging fields to Varian's Aria record and verify system (Version 13.6) and the Calypso tracking station (Version 3.0).
Images were transferred to Varian's Eclipse™ treatment planning system (Version 11.0.42) (Varian Medical Systems, Palo Alto, California). Following standard planning procedures for Calypso patients, a treatment plan was created with the isocenter at the center of mass of the transponders. CBCT and orthogonal kV setup fields were added to the plan, to allow radiographic acquisition at the linear accelerator. Standard departmental procedures were used to transfer the treatment plan and associated imaging fields to Varian's Aria record and verify system (Version 13.6) and the Calypso tracking station (Version 3.0).