Session Item

The aim of this study was to investigate the impact of intrafraction prostate motion on dose metrics and the effect of beam gating and motion correction in dose-escalated linac-based SBRT. 

A total of 56 fractions from 13 patients treated with dose-escalated SBRT using VMAT technique with FFF arcs, were examined. Real-time 3D prostate motion data were acquired using a novel electromagnetic tracking device. Beam delivery was interrupted whenever the prostate trespassed a 2-mm safety tolerance in any of the three spatial directions and table couch position corrected unless the offset was transient. Prostate trajectories with and without beam gating and motion correction events were reconstructed and analyzed with in-house C++ code. Both actually delivered treatments (case A) and non-gated treatments (case B) were simulated by incorporating the observed prostate motion for each fraction into the patient original treatment plan with an isocenter shift method. The total dose of each patient was then estimated by accumulating the motion inclusive dose distributions recalculated with Monaco Monte Carlo TPS from all fractions. Target and organs at risk (OARs) parameters were derived from reconstructed DVHs and compared to planned values. In addition, all dosimetric parameters were compared with protocol dose constraints.

Average values of mean prostate displacements in case A were -0.2 mm [-1.5 – 0.8], 0.1 mm [-1.4 – 1.5], and -0.3 mm [-1.7 – 1.4] in lateral, longitudinal, and vertical directions, respectively. The same values in case B were -0.3 mm [-3.1 – 0.8], 0.0 mm [-4.2 – 3.7], and -0.6 mm [-3.5 – 1.9]. Degradations in CTV and PTV coverage relative to planned dose were generally limited. Mean relative dose differences were -0.1% [-1.8 – 1.0] for CTVD99% and -0.2% [-1.6 – 0.7] for PTVD95% in case A, and -1.2% [-8.8 – 0.8] and -1.2% [-5.9 – 0.7] in case B. Urethra planning organ at risk volume (uPRV) was slightly degraded after taking motion into account, with larger than 1% differences in uPRVD10% observed only for 1 patient in case B. Rectum and bladder dosimetric parameters showed major variations between reconstructed and original plans, with a favourable underexposition of rectum and an undesirable overdose to bladder. Nevertheless, no protocol dose constraints violations were observed for bladder due to posterior displacement of the prostate.

Current CTV to PTV margins, robustness of original treatment plans, and fast FFF beams delivery do not result in significant degradation of dose metrics for target and OARs due to intrafraction prostate motion in both cases. Anyway, beam gating and motion correction ensured superior results and are recommended to use in dose-escalated prostate SBRT. The dosimetric impact of daily anatomy will be also explored in future studies.