Optimal Planning Target Volume Margins to Account for Intra-Fractional Prostate Motion Relative to Treatment Duration: A Study Using Real-Time Transperineal Ultrasound Guidance

Masaki Bannai; Amy Brown; Christopher Rumley; Timothy Squire; Alex Tan

doi:10.1111/1754-9485.13831

Optimal Planning Target Volume Margins to Account for Intra-Fractional Prostate Motion Relative to Treatment Duration: A Study Using Real-Time Transperineal Ultrasound Guidance

J Med Imaging Radiat Oncol. 2025 Jan 12. doi: 10.1111/1754-9485.13831. Online ahead of print.

Authors

Masaki Bannai¹, Amy Brown^{1

2

3}, Christopher Rumley^{1

2}, Timothy Squire^{1

2}, Alex Tan^{1

2}

Affiliations

¹ Department of Radiation Oncology, Townsville University Hospital, Townsville, Queensland, Australia.
² James Cook University, Townsville, Queensland, Australia.
³ Queensland University of Technology, Brisbane, Queensland, Australia.

PMID: 39800351
DOI: 10.1111/1754-9485.13831

Abstract

Introduction: Prostate motion during external beam radiotherapy (EBRT) is common and typically managed using fiducial markers and cone beam CT (CBCT) scans for inter-fractional motion correction. However, real-time intra-fractional motion management is less commonly implemented. This study evaluated the extent of intra-fractional prostate motion using transperineal ultrasound (TPUS) and examined the impact of treatment time on prostate motion.

Methods: Patients undergoing prostate EBRT with TPUS at a single institution from August 2016 to August 2021 were analysed. Pre-treatment daily CBCT corrected inter-fractional prostate shift. Continuous intra-fractional prostate motion was recorded at two frames per second in three dimensions, with three-dimensional (3D) displacement calculated as a vector. Motion data were modelled to determine the probability of the prostate remaining within pre-specified PTV margins relative to treatment delivery time.

Results: The study analysed 3364 fractions delivered to 122 patients. The mean treatment delivery time was 3.8 min. The prostate remained within a 5 mm margin with high frequencies in the superior-inferior (SI) and left-right (LR) directions, 97.8% and 98.4% of fractions respectively while 5.5% of fractions had deviations greater than 5 mm in the anterior-posterior (AP) direction. By contrast, the 3D vector exceeded a 5 mm margin in 14.5% of fractions. Drift motion modelling indicated a 99% probability of the vector staying within a 3 mm margin for 2 min, while for a 5 mm margin, the duration extended to 3.4 min.

Conclusions: Intra-fractional prostate motion monitoring is increasingly important as SABR with reduced PTV margins are utilised in prostate radiotherapy. Smaller PTV margins and longer treatment time require real-time monitoring to avoid geographical miss.

Keywords: PTV margins; external beam radiotherapy; intra‐fractional monitoring; prostate motion; transperineal ultrasound.