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Optimal Planning Target Volume Margins to Account for Intra-Fractional Prostate Motion Relative to Treatment
Masaki Bannai1, Amy Brown1,2,3, Christopher Rumley1,2
1Department of Radiation Oncology, Townsville University Hospital, Townsville, Queensland, Australia.
Journal of Medical Imaging and Radiation Oncology
|January 12, 2025
Summary
Real-time monitoring of intra-fractional prostate motion is crucial for prostate radiotherapy, especially with advanced techniques like stereotactic ablative radiotherapy (SABR). This study shows prostate motion can exceed margins during treatment, necessitating continuous tracking.
Area of Science:
- Radiation Oncology
- Medical Physics
- Oncology
Background:
- Prostate motion during external beam radiotherapy (EBRT) is a known challenge.
- Current management relies on fiducial markers and cone-beam CT (CBCT) for inter-fractional corrections.
- Real-time intra-fractional motion management is less established.
Purpose of the Study:
- To evaluate the extent of intra-fractional prostate motion during EBRT using transperineal ultrasound (TPUS).
- To investigate the impact of treatment duration on prostate motion.
- To inform strategies for motion management in modern radiotherapy.
Main Methods:
- Analysis of 3364 fractions from 122 patients undergoing prostate EBRT with TPUS.
- Continuous 3D intra-fractional prostate motion recorded at 2 frames/sec.
- Motion data modeled to predict prostate position within planning target volume (PTV) margins over time.
Main Results:
- Mean treatment time was 3.8 minutes.
- Prostate remained within 5 mm margins in 97.8% (SI) and 98.4% (LR) of fractions.
- 14.5% of fractions showed 3D vector deviations exceeding 5 mm.
- Motion modeling predicted 99% probability of staying within 3 mm for 2 min, and within 5 mm for 3.4 min.
Conclusions:
- Intra-fractional prostate motion monitoring is vital for radiotherapy, particularly with stereotactic ablative radiotherapy (SABR) using reduced PTV margins.
- Longer treatment times and smaller margins increase the risk of geographical miss.
- Real-time monitoring is essential for accurate prostate cancer treatment delivery.
