Impact Of Intrafraction Prostate Motion Research Articles

Dosimetric Impact of Intrafraction Prostate Motion and Interfraction Anatomical Changes in Dose-Escalated Linac-Based SBRT

The aim of this study was to investigate the impact of intrafraction prostate motion and interfraction anatomical changes on dose metrics and the effect of beam gating and motion correction in dose-escalated Linac-based SBRT. Thirteen patients (56 fractions) with organ-confined prostate cancer underwent dose-escalated FFF-VMAT SBRT. Accurate patient setup was ensured by CBCT, and real-time 3D prostate motion data were obtained using a novel electromagnetic tracking device. Treatment was interrupted when the signals exceeded a 2 mm threshold in any of the three spatial directions and couch position was corrected unless the offset was transient. Prostate trajectories with and without beam gating and motion correction events were reconstructed and analyzed. Rectum and bladder volumes on each daily CBCT were recorded and compared with volumes at simulation. The prostate motion observed for each fraction was incorporated into the patient original treatment plan with an isocenter shift method. Actually, delivered treatments were then obtained by recalculating the reconstructed motion-encoded plan on deformed CTs reflecting the patient CBCT-anatomy of the day. Non-gated treatments were also simulated using the prostate motion data assuming that no treatment interruptions have occurred. Treatment interruptions because of target motion trespassing the predefined threshold occurred in 25 fractions (45%). Rectum and bladder volume changes were relevant in most patients. The mean relative dose differences between actually delivered and planned treatments were -3.0% [-18.5 - 2.8] for CTV D99% and -2.6% [-17.8 - 1.0] for PTV D95% over all 56 fractions. However, the median cumulative CTV coverage with 93% of the prescribed dose turned out to be satisfactory. Urethra sparing was slightly degraded and a mean reduction of rectum and bladder dose was seen in all but two dose metrics: the maximum dose to rectum mucosa and the bladder D40%. Nevertheless, only 2 major deviations in rectum mucosa D0.035cc were observed at the end of the treatment. The greatest contribution to target missing and OARs doses came from the interfraction anatomical variations, while intrafraction prostate motion marginally contributed in gated treatments. In non-gated treatments, further deteriorations by 2.4 - 2.8% in minimum target coverage metrics and by 3.1 - 11.6% in bladder dose parameters would have occurred on average. The implemented motion management strategy and the strict patient preparation regimen, along with current PTV margins, robustness of original treatment plans, and fast FFF beam delivery, ensured no significant degradations of dose metrics due to both intrafraction motion and interfraction anatomical changes. Non-gated treatments would have more widely deteriorated the dosimetry of some individual fractions. Thus, continuous monitoring, beam gating and motion correction are recommended to safely deliver dose-escalated prostate SBRT.

Dosimetric Impact of Intrafraction Prostate Motion and Interfraction Anatomical Changes in Dose-Escalated Linac-Based SBRT

The dosimetric impact of intrafraction prostate motion and interfraction anatomical changes and the effect of beam gating and motion correction were investigated in dose-escalated linac-based SBRT. Fifty-six gated fractions were delivered using a novel electromagnetic tracking device with a 2 mm threshold. Real-time prostate motion data were incorporated into the patient's original plan with an isocenter shift method. Delivered dose distributions were obtained by recalculating these motion-encoded plans on deformed CTs reflecting the patient's CBCT daily anatomy. Non-gated treatments were simulated using the prostate motion data assuming that no treatment interruptions have occurred. The mean relative dose differences between delivered and planned treatments were -3.0% [-18.5-2.8] for CTV D99% and -2.6% [-17.8-1.0] for PTV D95%. The median cumulative CTV coverage with 93% of the prescribed dose was satisfactory. Urethra sparing was slightly degraded, with the maximum dose increased by only 1.0% on average, and a mean reduction in the rectum and bladder doses was seen in almost all dose metrics. Intrafraction prostate motion marginally contributed in gated treatments, while in non-gated treatments, further deteriorations in the minimum target coverage and bladder dose metrics would have occurred on average. The implemented motion management strategy and the strict patient preparation regimen, along with other treatment optimization strategies, ensured no significant degradations of dose metrics in delivered treatments.

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

<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.

PO-1715 Assessing the dosimetric impact of intrafraction prostate motion in dose-escalated linac-based SBRT

PO-1715 Assessing the dosimetric impact of intrafraction prostate motion in dose-escalated linac-based SBRT

Single-fraction prostate stereotactic body radiotherapy: Dose reconstruction with electromagnetic intrafraction motion tracking

PurposeTo reconstruct the dose delivered during single-fraction urethra-sparing prostate stereotactic body radiotherapy (SBRT) accounting for intrafraction motion monitored by intraprostatic electromagnetic transponders (EMT). MethodsWe analyzed data of 15 patients included in the phase I/II “ONE SHOT” trial and treated with a single fraction of 19 Gy to the planning target volume (PTV) and 17 Gy to the urethra planning risk volume. During delivery, prostate motion was tracked with implanted EMT. SBRT was interrupted when a 3-mm threshold was trespassed and corrected unless the offset was transient. Motion-encoded reconstructed (MER) plans were obtained by splitting the original plans into multiple sub-beams with isocenter shifts based on recorded EMT positions, mimicking prostate motion during treatment. We analyzed intrafraction motion and compared MER to planned doses. ResultsThe median EMT motion range (±SD) during delivery was 0.26 ± 0.09, 0.22 ± 0.14 and 0.18 ± 0.10 cm in the antero-posterior, supero-inferior, and left–right axes, respectively. Treatment interruptions were needed for 8 patients because of target motion beyond limits in the antero-posterior (n = 6) and/or supero-inferior directions (n = 4). Comparing MER vs. original plan there was a median relative dose difference of −1.9% (range, −7.9 to −1.0%) and of +0.5% (−0.3–1.7%) for PTV D98% and D2%, respectively. The clinical target volume remained sufficiently covered with a median D98% difference of −0.3% (−1.6–0.5%). Bladder and rectum dosimetric parameters showed significant differences between original and MER plans, but mostly remained within acceptable limits. ConclusionsThe dosimetric impact of intrafraction prostate motion was minimal for target coverage for single-fraction prostate SBRT with real-time electromagnetic tracking combined with beam gating.

Impact of intrafraction prostate motion on clinical target coverage in proton therapy: A simulation study of dosimetric differences in two delivery techniques.

PurposeTo investigate the dosimetric impact of prostate intrafraction motion on proton double‐scattering (DS) and uniform scanning (US) treatments using electromagnetic transponder‐based prostate tracking data in simulated treatment deliveries.MethodsIn proton DS delivery, the spread‐out Bragg peak (SOBP) is created almost instantaneously by the constant rotation of the range modulator. US, however, delivers each entire energy layer of the SOBP sequentially from distal to proximal direction in time, which can interplay with prostate intrafraction motion. This spatiotemporal interplay during proton treatment was simulated to evaluate its dosimetric impact. Prostate clinical target volume (CTV) dose was obtained by moving CTV through dose matrices of the energy layers according to prostate‐motion traces. Fourteen prostate intrafraction motion traces of each of 17 prostate patients were used in the simulated treatment deliveries. Both single fraction dose‐volume histograms (DVHs) and fraction‐cumulative DVHs were obtained for both 2 Gy per fraction and 7.25 Gy per fraction stereotactic body radiotherapy (SBRT).ResultsThe simulation results indicated that CTV dose degradation depends on the magnitude and direction of prostate intrafraction motion and is patient specific. For some individual fractions, prescription dose coverage decreased in both US and DS treatments, and hot and cold spots inside the CTV were observed in the US results. However, fraction‐cumulative CTV dose coverage showed much reduced dose degradation for both DS and US treatments for both 2 Gy per fraction and SBRT simulations.ConclusionsThis study indicated that CTV dose inhomogeneity may exist for some patients with severe prostate intrafraction motion during US treatments. However, there are no statistically significant dose differences between DS and US treatment simulations. Cumulative dose of multiple‐fractions significantly reduced dose uncertainties.

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

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.

Intrafraction Prostate Translations and Rotations During Hypofractionated Robotic Radiation Surgery: Dosimetric Impact of Correction Strategies and Margins

To investigate the dosimetric impact of intrafraction prostate motion and the effect of robot correction strategies for hypofractionated CyberKnife treatments with a simultaneously integrated boost. A total of 548 real-time prostate motion tracks from 17 patients were available for dosimetric simulations of CyberKnife treatments, in which various correction strategies were included. Fixed time intervals between imaging/correction (15, 60, 180, and 360 seconds) were simulated, as well as adaptive timing (ie, the time interval reduced from 60 to 15 seconds in case prostate motion exceeded 3 mm or 2° in consecutive images). The simulated extent of robot corrections was also varied: no corrections, translational corrections only, and translational corrections combined with rotational corrections up to 5°, 10°, and perfect rotational correction. The correction strategies were evaluated for treatment plans with a 0-mm or 3-mm margin around the clinical target volume (CTV). We recorded CTV coverage (V100%) and dose-volume parameters of the peripheral zone (boost), rectum, bladder, and urethra. Planned dose parameters were increasingly preserved with larger extents of robot corrections. A time interval between corrections of 60 to 180 seconds provided optimal preservation of CTV coverage. To achieve 98% CTV coverage in 98% of the treatments, translational and rotational corrections up to 10° were required for the 0-mm margin plans, whereas translational and rotational corrections up to 5° were required for the 3-mm margin plans. Rectum and bladder were spared considerably better in the 0-mm margin plans. Adaptive timing did not improve delivered dose. Intrafraction prostate motion substantially affected the delivered dose but was compensated for effectively by robot corrections using a time interval of 60 to 180 seconds. A 0-mm margin required larger extents of additional rotational corrections than a 3-mm margin but resulted in lower doses to rectum and bladder.

The Dosimetric Effect of Intrafraction Prostate Motion on Step-and-Shoot Intensity-Modulated Radiation Therapy Plans: Magnitude, Correlation With Motion Parameters, and Comparison With Helical Tomotherapy Plans

To determine the daily and cumulative dosimetric effects of intrafraction prostate motion on step-and-shoot (SNS) intensity-modulated radiation therapy (IMRT) plans, to evaluate the correlation of dosimetric effect with motion-based metrics, and to compare on a fraction-by-fraction basis the dosimetric effect induced in SNS and helical tomotherapy plans. Intrafraction prostate motion data from 486 fractions and 15 patients were available. A motion-encoded dose calculation technique was used to determine the variation of the clinical target volume (CTV) D(95%) values with respect to the static plan for SNS plans. The motion data were analyzed separately, and the correlation coefficients between various motion-based metrics and the dosimetric effect were determined. The dosimetric impact was compared with that incurred during another IMRT technique to assess correlation across different delivery techniques. The mean (±1 standard deviation [SD]) change in D(95%) in the CTV over all 486 fractions was 0.2 ± 0.5%. After the delivery of five and 12 fractions, the mean (±1 SD) changes over the 15 patients in CTV D(95%) were 0.0 ± 0.2% and 0.1 ± 0.2%, respectively. The correlation coefficients between the CTV D(95%) changes and the evaluated motion metrics were, in general, poor and ranged from r = -0.2 to r = -0.39. Dosimetric effects introduced by identical motion in SNS and helical tomotherapy IMRT techniques were poorly correlated with a correlation coefficient of r = 0.32 for the CTV. The dosimetric impact of intrafraction prostate motion on the CTV is, in general, small. In only 4% of all fractions did the dosimetric consequence exceed 1% in the CTV. As expected, the cumulative effect was further reduced with fractionation. The poor correlations between the calculated motion parameters and the subsequent dosimetric effect implies that motion-based thresholds are of limited value in predicting the dosimetric impact of intrafraction motion. The dosimetric effects between the two evaluated delivery techniques were poorly correlated.

Correlation between dosimetric effect and intrafraction motion during prostate treatments delivered with helical tomotherapy

The dosimetric impact of intrafraction prostate motion was investigated for helical tomotherapy treatments. Measured motion tracks were used to calculate the dosimetric impact on delivered target dose distributions. A dynamic dose calculation engine was developed to facilitate this evaluation. It was found that the D95% (minimum dose to 95% of the volume) changes in the prostate were well correlated with D95% changes in the PTV. This means that the dosimetric impact of intrafraction motion is not restricted to the periphery of the target. The amount of motion was not well correlated with the dosimetric impact (measured in target D95% changes) of motion. The relationship between motion and its dosimetric impact is complex and depends on the timing and direction of the movement. These findings have implications for motion management techniques. It appears that the use of target margins is not an effective strategy to protect the prostate from the effects of observed intrafraction motion. The complex relationship between motion and its dosimetric effect renders simple threshold-based intervention schemes inefficient. Monitoring of actual prostate motion would allow the documentation of the dosimetric impact and implementation of corrective action if needed. However, when motion management techniques are evaluated, it should be kept in mind that the dosimetric impact of observed prostate motion is small for the majority of fractions.

SU‐GG‐J‐53: Comparison of Dosimetric Consequences of Intrafraction Prostate Motion Between IMRT and IMPT

Purpose: To compare the dosimetric impact of intrafraction prostate motion in IMRT and intensity modulated proton therapy (IMPT), with an emphasis on determining the necessary CTV‐to‐PTV margins to compensate for the motion in these two modalities. Method and Materials: Intrafraction prostate motion of 35 patients was tracked in real‐time using the Calypso System. Data were categorized into three groups according to the treatment intervention performed in response to the motion: (1) if motion larger than 3 mm was observed for 30 seconds during a fraction, dose delivery was interrupted and the patient was re‐aligned; (2) as above, but with motion threshold of 5 mm for 30 seconds; (3) no treatment intervention performed regardless of the motion amplitude. Dose delivered in presence of motion was evaluated by convolving the static dose distribution matrix with the motion probability density matrix. IMRT and IMPT plans for the same patient data were constructed with Pinnacle and UFORT planning systems respectively, using CTV‐PTV margins from zero to 11 mm. Results: For single fractions with the largest average motion in groups (1) to (3), CTV‐PTV margins of 2, 5, and 11 mm for IMRT, and 3, 7, 11 mm for IMPT, are necessary to maintain minimum CTV doses above 95% of nominal dose respectively. In the individual patient with the largest average motion over an entire treatment course (37 fractions), the necessary CTV‐PTV margin was 2 and 3 mm for IMRT and IMPT respectively. Using these margins, any compromise in CTV DVH and the generalized equivalent uniform dose (gEUD) under the corresponding motion was insignificant. Conclusion: The required CTV‐PTV margin for IMPT is slightly larger than that for IMRT, due largely to the sharper penumbra of proton beam relative to photons. However, IMPT resulted in superior sparing of the urinary bladder and rectum compared with IMRT.