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Duration-dependent margins for prostate radiotherapy—a practical motion mitigation strategy


Background and objective

The magnitude of intra-fractional prostate displacement (change from initial position over time) is associated with the duration of the patient lying on the radiotherapy treatment couch. This study reports a minute-by-minute association and calculates the impact of this displacement on duration-dependent margins using real-time intra-fractional position data monitored by four-dimensional transperineal ultrasound (4D TPUS).

Materials and methods

A total of 55 patients were recruited prospectively. Intra-fractional position of the prostate was monitored in real-time using a 4D TPUS Clarity® system. A total of 1745 monitoring sessions were analysed. Van Herk’s margin recipe (2.5∑ + 1.64((σ2 + σp2)1/2 − σp)) was used to estimate the duration-dependant margins for every minute, up to the 15th minute. Linear regression analysis was then performed on the overall margins against time and direction.


The mean intra-fractional position was 0.76 mm Inferior (Inf), 0 mm Lateral (Lat) and 0.94 mm Posterior (Post) at the 15th minute. A minimum margin expansion of 2.42 mm (Superior/Inf), 1.02 mm (Left/Right) and 2.65 mm (Anterior/Post) was required for an 8‑minute treatment compared to 4.29 mm (Sup/Inf), 1.84 mm (Lt/Rt) and 4.63 mm (Ant/Post) for a 15-minute treatment. The required margin expansion increased linearly (R2 = 0.99) in all directions (p < 0.01). However, while there was no statistically significant difference (p = 0.10) in the required margin expansion in the Sup/Inf and Ant/Post directions respective of the time duration, the margins were much bigger compared to those in the Lt/Rt direction (p < 0.01).


We report our experience in deriving the minimum duration-dependant margin to generate the required planning target volume for prostate radiotherapy. The required margin increases linearly in all directions within the 15-min duration; thus, the margin will depend on the duration of the technique chosen (IMRT/VMAT/3DCRT/proton).

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The authors express their appreciation to Elekta Pte Ltd for providing the 4D TPUS Clarity equipment and in-house training. We also thank the clinical coordinators for facilitating the consenting process and the radiation therapists involved in the treatment process.


This study did not receive any financial support/funding.

Author information

Correspondence to Eric Pei Ping Pang PhD.

Ethics declarations

Conflict of interest

M.L.K. Chua reports personal fees from Astellas; personal fees from Janssen; grants and personal fees from Ferring; grants, personal fees and non-financial support from Varian; non-financial support from AstraZeneca; non-financial support from GenomeDx Biosciences; non-financial support from MedLever Inc; non-financial support from PVMed Inc.; outside the submitted work. E.P.P. Pang, K. Knight, S.Y. Park, W. Lian, Z. Master, M. Baird, J.W.X. Chan, M.L.C. Wang, T.W.K. Tan, E.T. Chua, W.S. Looi, W.L. Nei and J.K.L. Tuan declare that they have no competing interests.

Additional information

Part of the findings in this manuscript was presented at ASTRO 2019 conference in Chicago, USA, in a poster entitled: “Estimation of duration-dependent margins for prostate radiotherapy”.

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Pang, E.P.P., Knight, K., Park, S.Y. et al. Duration-dependent margins for prostate radiotherapy—a practical motion mitigation strategy. Strahlenther Onkol (2020). https://doi.org/10.1007/s00066-019-01558-y

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  • Intra-fractional prostate displacement
  • Prostate cancer
  • Real-time tracking
  • Transperineal ultrasound
  • Margins