Impact of intra- and interfractional motion on the dose distribution and urinary adverse events for magnetic resonance-guided prostate cancer radiotherapy

Purpose: To evaluate the impact of intra- and interfractional motion on dose distribution and clinical outcome in prostate magnetic resonance (MR)-guided adaptive radiotherapy. Methods: Seventy-six patients were treated with five-fraction stereotactic radiotherapy on a 1.5 T MR linear accelerator. Cine MR images were acquired during treatment and exported to a motion monitoring research package, to generate motion traces in the anterior-posterior (AP), superior-inferior (SI) and left-right (LR) directions. Planned dose distributions were shifted according to motion trace to reconstruct intrafraction motion-blurred dose. Intrafraction motion-blurred doses were deformably registered to fraction one motion-blurred dose to obtain interfraction accumulated dose. The impact of motion on prostate, bladder, rectal and urethral dose-volume parameters were assessed. Correlation between patients experiencing prolonged motion > 3 mm and urinary adverse events was evaluated. Results: The average intrafraction prostate motion was -0.0 ± 0.2 mm (LR), 0.3 ± 0.5 mm (AP), and -0.3 ± 0.6 mm (SI). Intrafraction motions were associated with dose-volume parameter deviations up to 2.5 Gy. During 24 months of follow-up, 3 patients experienced ≥ Grade 2 acute adverse events, and 4 patients reported ≥ Grade 2 long term adverse events evaluated with common terminology criteria for adverse events. All patients with clinically significant urinary adverse events experienced prolonged large intrafraction motion > 3 mm. Interfraction accumulated motion-blurred dose demonstrated substantial interpatient variabilities for bladder, rectum and urethra. Conclusion: Prolonged intrafraction motion > 3 mm may result in dose difference and clinically significant urinary adverse events. However, combined intra- and interfraction motion might average out fraction-specific variations.