Noninvasive inter- and intrafractional motion control in ultrahypofractionated radiation therapy of prostate cancer using RayPilot HypoCath™—a substitute for gold fiducial-based IGRT?

Purpose In ultrahypofractionated radiation concepts, managing of intrafractional motion is mandatory because tighter margins are used and random errors resulting from prostate movement are not averaged out over a large number of fractions. Noninvasive live monitoring of prostate movement is a desirable asset for LINAC-based prostate stereotactic body radiation therapy (SBRT). Methods We prospectively analyzed a novel live tracking device (RayPilot HypoCath™; Micropos Medical AB, Gothenburg, Sweden) where a transmitter is noninvasively positioned in the prostatic urethra using a Foley catheter in 12 patients undergoing ultrahypofractionated intensity-modulated radiation therapy (IMRT) of the prostate. Gold fiducials (Innovative Technology Völp, Innsbruck, Austria) were implanted to allow comparison of accuracy and positional stability of the HypoCath system and its ability to be used as a standalone IGRT method. Spatial stability of the transponder was assessed by analyzing transmitter movement in relation to gold markers (GM) in superimposed kV image pairs. Inter- and intrafractional prostate movement and the impact of its correction were analyzed. Results A total of 64 fractions were analyzed. The average resulting deviation vector compared to the GM-based position was 1.2 mm and 0.7 mm for inter- and intrafractional motion, respectively. The mean intrafractional displacement vector of the prostate was 1.9 mm. Table readjustment due to exceeding the threshold of 3 mm was required in 18.8% of fractions. Repositioning reduced the time spent outside the 3‑mm margin from 7.9% to 3.8% of beam-on time. However, for individual patients, the time spent outside the 3‑mm margin was reduced from to 49% to 19%. Conclusion the HypoCath system allows highly accurate and robust intrafractional motion monitoring. In conjunction with cone beam CT (CBCT) for initial patient setup, it could be used as a standalone image-guided radiation therapy (IGRT) system.