Dosimetric impact of intra-fraction prostate motion under a tumour-tracking system in hypofractionated robotic radiosurgery

Abstract

For CyberKnife-mediated prostate cancer treatment, a tumour-tracking approach is applied to correct the target location by acquiring X-ray images of implanted fiducial markers intermittently. This study investigated the dosimetric impact of intra-fraction prostate motion during CyberKnife treatment. We retrospectively analyzed 16 patients treated using the CyberKnife (35 Gy delivered in five fractions). Using log files of recorded prostate motion, the intra-fraction prostate motion was simulated. We defined the worst-case intra-fraction prostate motion as the difference between pre- and post-deviation on log files and shifted structure sets according to the corresponding offsets for each beam. The dose–volume indices were calculated and compared with the original plan in terms of clinical target volume (CTV), planning target volume (CTV plus a 2-mm margin), rectum, bladder, and urethra. Prostate motions of >3, >5, and >10 mm were observed for 31.3, 9.1, and 0.5% of the 1929 timestamps, respectively. Relative differences between the simulated and original plans were mostly less than 1%. Although significant decreases were observed in D50% and D98% of the target, absolute dose differences were <0.1 Gy compared with the planned dose. The dosimetric impact of intra-fraction prostate motion may be small even with longer treatment durations, indicating that the tumour tracking using the CyberKnife could be a robust system for examining prostate motion.

Clinical Assessment of Prostate Displacement and Planning Target Volume Margins for Stereotactic Body Radiotherapy of Prostate Cancer

“…Arc 4 is not included, as there is no IGRT performed after completion of the fourth arc delivery. greatest in the SI and AP directions, and was of similar magnitude in these two directions though slightly more pronounced anteriorly-posteriorly, which is consistent with prior reports (8,16,23,25). Greater intra-fractional displacement has also been reported, up to 2.0 ± 1.8 mm (SI) and 1.9 ± 2.0 mm (AP) based on pre-and post-treatment orthogonal imaging (19).…”

“…Our measured intra-fractional displacements and calculated margins are similar to previously published data for SBRT and conventional fractionation (8,(28)(29)(30)(31)(32). Mean displacement was FIGURE 3 | Mean implemented shifts for prostate displacement sequentially over the phases of treatment: from cone beam computed tomography (CBCT) to planar imaging just prior to initiation of arc 1, and between each of the partial arcs incorporating the displacement detected on the image pair obtained subsequent to the corresponding arc.…”

“…Our large patient numbers provide weight to our conclusions regarding average target displacement and required margins. Indeed, our standard deviations of 0.5-0.9 mm for prostate displacement are smaller than those reported in many published datasets, including those which found smaller mean directional displacements, as standard deviations of up to 2 mm have been reported (8,9,16,19,25). Importantly, the calculated margins reflect a scenario in which intra-fractional shifts are not applied in real-time.…”

“…By definition, SBRT necessitates a high level of accuracy and precision during treatment delivery, as each fraction is responsible for delivering a high proportion of the total prescription dose (2,3). Thus, errors in the precision and accuracy of actual dose delivered can lead to incomplete target coverage and/or overdosing of adjacent organs-at-risk (8,9).…”

“…Multiple image-guided motion management strategies exist (22), and a wide range of required prostate radiotherapy margins have been proposed based on varied techniques. However, the numbers of patients included in studies to estimate required margins are relatively modest, and proposed margins based on data from the intra-fractional, interarc kilovoltage (kV) imaging method is lacking (8,9,16,19,20,(23)(24)(25)(26)(27).…”