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

Pei Ping Eric Pang, Kellie Knight, Sung Yong Park, Weixiang Lian, Zubin Master, Marilyn Baird, Jason Wei Xiang Chan, Michael Lian Chek Wang, Terence Wee Kiat Tan, Melvin L.K. Chua, Eu Tiong Chua, Wen Shen Looi, Wen Long Nei, Jeffrey Kit Loong Tuan

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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 (2.5P+ 1.64((σ2+ σp 2)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. Results The mean intra-fractional position was 0.76mm Inferior (Inf), 0mm Lateral (Lat) and 0.94mm Posterior (Post) at the 15th minute. A minimum margin expansion of 2.42mm (Superior/Inf), 1.02mm (Left/Right) and 2.65mm (Anterior/ Post) was required for an 8-minute treatment compared to 4.29mm (Sup/Inf), 1.84mm (Lt/Rt) and 4.63mm (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). Conclusion 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).
Original languageEnglish
Pages (from-to)657-663
Number of pages7
JournalStrahlentherapie Und Onkologie
Issue number7
Publication statusPublished - Jul 2020


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

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