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).
- Intra-fractional prostate displacement
- Prostate cancer
- Real-time tracking
- Transperineal ultrasound