Magnetic Resonance Guided Radiation Therapy (MRgRT) Prostate Motion and Margins.

[INTRODUCTION] Systematic and random errors in radiation dose delivery necessitate the use of planning target volume (PTV) margins to ensure adequate clinical target volume (CTV) treatment. Advances in magnetic resonance-guided radiation therapy (MRgRT) have enabled improved imaging with possible margin reduction; however, the optimal PTV margins remain uncertain. This study aimed to evaluate the adaptive radiotherapy component of intra-fractional prostate movement in MRgRT for prostate cancer (PCa) patients and determine appropriate PTV margins.

[METHODS] This study retrospectively analyzed 18 PCa patients treated using a 1.5 T MR-Linac. The initial fusion MR and verification MR scans were registered offline to assess prostate displacement between the two scans in the anterior-posterior (AP), left-right (LR) and superior-inferior (SI) directions. Random and systematic errors were calculated, and the PTV margins were determined using the Van Herk formula.

[RESULTS] The average time between MR scans was 22 min (range 9-54 min) compared to an average beam-on time of 6 min (range 2-11 min). Mean and standard deviation of translational displacement was 1.2 ± 0.9 mm in the AP, 0.6 ± 0.5 mm in the LR, and 1.1 ± 0.8 mm in the SI directions. The calculated PTV margin was 3.2 mm in AP, 1.7 mm in LR, and 3.2 mm in SI directions. There was an observed trend of increased prostate motion with increased treatment duration.

[CONCLUSION] MRgRT facilitates PTV margin reduction for PCa; however, our findings suggest that increased on-couch time may be associated with greater prostate motion. Future studies with larger patient cohorts and real-time motion monitoring are recommended to optimise margin strategies.