Dosimetric Impact of Intrafraction Prostate Motion in Dose-Escalated Linac-Based SBRT

Abstract

<h3>Purpose/Objective(s)</h3> The aim of this study was to investigate the impact of observed intrafraction prostate motion on dose metrics and the effect of beam gating and motion correction using a novel electromagnetic (EM) tracking device in dose-escalated Linac-based SBRT. <h3>Materials/Methods</h3> Thirteen patients (56 fractions) with organ-confined prostate cancer received 40 Gy in 5 fractions or 38 Gy in 4 fractions using VMAT technique with flattening filter-free (FFF) arcs. The EM tracking device consisted of an integrated Foley catheter with a transmitter and provided real-time 3D prostate motion data. Treatment was interrupted when the signals exceeded a 2 mm threshold in any of the three spatial directions and, unless the offset was transient, target position was re-defined by repeating a ConeBeam-CT (CBCT). Prostate trajectories with and without beam gating and motion correction events were reconstructed and analyzed. 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. Target and organs at risk (OARs) dosimetric parameters of the two motion-inclusive plans were compared to planned values and protocol dose constraints. <h3>Results</h3> Treatment interruptions were needed in 25 fractions (45%) because of target motion beyond the predefined threshold. The prostate was found within 2 mm from its initial position in 96% of the treatment time, i.e., in 94% of the time during the setup, and in 98% during the delivery. In non-gated treatments, the prostate would have been found outside the tolerance in 8% of the total session time, in 4% of the time during the setup, and in 14% during beam-on. The predominant motion pattern was posterior and its probability increased with time, with a mean motion ≤ 2 mm occurring within 10 minutes. Mean relative dose differences respect to original plans were -0.1% [-1.8 – 1.0] for CTV D99% and -0.2% [-1.6 – 0.7] for PTV D95% in case A. The same values were -1.2% [-8.8 – 0.8] and -1.2% [-5.9 – 0.7] in case B. In both cases, urethra planning organ at risk volume was slightly degraded after taking motion into account, and the rectum and bladder dose metrics showed a favorable underexposition of rectum and an undesirable overdose to bladder. Nevertheless, no protocol dose constraints violations were observed. <h3>Conclusion</h3> Our findings showed that the majority of the observed prostate motion was modest; however, there were a non-insignificant number of fractions with motion exceeding the predetermined threshold. The current CTV to PTV margins, the robustness of original treatment plans, and the fast FFF beams delivery led to minimal degradations of dose metrics for the target and OARs in both cases. Anyway, real-time monitoring with beam gating and motion correction ensured superior results and are recommended in dose-escalated prostate SBRT.