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

## TL;DR

This study examines how prostate motion during radiotherapy affects dose distribution and urinary side effects, finding that prolonged motion over 3 mm is linked to adverse events.

## Contribution

The study introduces a method to reconstruct motion-blurred doses and correlates motion with urinary adverse events in MR-guided prostate radiotherapy.

## Key findings

- Most prostate treatment fractions had intrafraction motion under 3 mm.
- Prolonged motion over 3 mm was associated with all cases of urinary adverse events.
- Motion-blurred dose accumulation showed significant interpatient variability in bladder, rectum, and urethra.

## Abstract

•Most prostate treatment fractions (>75%) experienced intrafraction motion < 3 mm.•Intrafraction motion resulted in a maximum dose-volume parameter deviation of 2.5 Gy.•All patients with urinary adverse events experienced prolonged motion > 3 mm.

Most prostate treatment fractions (>75%) experienced intrafraction motion < 3 mm.

Intrafraction motion resulted in a maximum dose-volume parameter deviation of 2.5 Gy.

All patients with urinary adverse events experienced prolonged motion > 3 mm.

To evaluate the impact of intra- and interfractional motion on dose distribution and clinical outcome in prostate magnetic resonance (MR)-guided adaptive radiotherapy.

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.

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.

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.