Localization accuracy of two electromagnetic tracking systems in prostate cancer radiotherapy: A comparison with fiducial marker based kilovoltage imaging

•RayPilot transmitters may migrate within the prostate between treatment fractions.•Localization accuracy of the RayPilot suffers from instability of the transmitters.•Localization accuracy of Calypso is comparable to kV imaging of fiducials. The aim of this study was to evaluate the localization accuracy of electromagnetic (EM) tracking systems RayPilot (Micropos Medical AB) and Calypso (Varian Medical Systems) in prostate cancer radiotherapy. The accuracy was assessed by comparing couch shifts obtained with the EM methods to the couch shifts determined by simultaneous fiducial marker (FM) based orthogonal kilovoltage (kV) imaging. Agreement between the methods was compared using Bland-Altman analysis. Interfractional positional stability of the FMs, RayPilot transmitters and Calypso transponders was investigated. 582 fractions from 22 RayPilot patients and 335 fractions from 26 Calypso patients were analyzed. Mean (± standard deviation (SD)) differences between RayPilot and kV imaging were 0.3 ± 2.2, −2.2 ± 2.4 and −0.0 ± 1.0 mm in anterior-posterior (AP), superior-inferior (SI) and left-right (LR) directions, respectively. Corresponding 95% limits of agreement (LOA) were ±4.3, ±4.7 and ±2.1 mm around the mean. Mean (±SD) differences between Calypso and kV imaging were −0.2 ± 0.6, 0.1 ± 0.5 and −0.1 ± 0.4 mm in AP, SI and LR directions, respectively, and corresponding LOAs were ±1.3, ±1.0 and ±0.8 mm around the mean. FMs and transponders were stable: SD of intermarker and intertransponder distances was 0.5 mm. Transmitters were unstable: mean caudal transmitter shift of 1.8 ± 2.0 mm was observed. Results indicate that the localization accuracy of the Calypso is comparable to kV imaging of fiducials and the methods could be used interchangeably. The localization accuracy of the RayPilot is affected by transmitter instability and the positioning of the patient should be verified by other setup techniques. The study is part of clinical trial NCT02319239.