Dosimetric impact of real-time motion and daily anatomical changes during ultra-hypofractionated prostate cancer radiotherapy.

During ultra-hypofractionated prostate cancer radiotherapy treatments, prostate motion and daily anatomical variations degrade the delivered dose to the clinical target volume (CTV) and to the organs at risk. Hence, patients would benefit from real-time prostate tracking and from a strict protocol for bladder and rectal filling. Our purpose was to study the synchronization of prostate's displacements with the beam's geometry and to evaluate the dosimetric impact of real-time motion and daily anatomical changes. A trans-perineal ultrasound probe was used to track prostate translations for 15 patients treated by volumetric-modulated arc therapy. The prescription dose was $36.25 {Gy}$ to $100%$ of the CTV-prostate in 5 fractions. The daily dose distribution was calculated generating synthetic CTs for dose accumulation. Afterwards, original RT-plans were first divided into sub-arcs following the plan's control points and then calculated in the treatment planning system. Finally, the voxel shifting method was used to synchronize prostate's displacements with beam's geometry. The shifted dose maps were then summed and the dosimetric parameters were evaluated in the presence of motion.