Prostate Motion Research Articles

Intrafraction monitoring of prostate motion during radiotherapy using the Clarity® Autoscan Transperineal Ultrasound (TPUS) system

IntroductionImplementation of the Clarity® Autoscan (Elekta) Transperineal Ultrasound (TPUS) system in Bristol is the first of its kind in the UK and we have already shown its utility in interfractional Image Guided Radiotherapy (IGRT).14 This study establishes the extent of intrafraction prostate motion as measured by Clarity and explores the potential benefits of TPUS for intrafraction monitoring. MethodsMonitoring data was analysed for 526 fractions from 20 localised prostate cancer patients. Intrafraction prostate displacements exceeding thresholds of 3 mm, 7 mm and 10 mm along patient axes were assessed for frequency and duration of motion. ResultsProstate motion exceeds the above displacement thresholds during 52%, 8%, and 2% of fractions analysed. Displacement at the 3 mm threshold occurred for 100% of patients, 60% at 7 mm and 35% at 10 mm. The mean frequency and duration of displacements is low for the overall population. In contrast specific patients exhibit much higher displacement values. Posterior motion is most common, averaging at 24% of the treatment time at 3 mm, 3% at 7 mm and 1% at 10 mm, ranging up to 92%, 35% and 10% for individual patients. ConclusionsIntrafraction monitoring with Clarity has the potential to improve accuracy through application of in-treatment motion correction. This is most beneficial for specific patients who exhibit a higher frequency and/or duration of prostate motion. Consideration must be given to the added time implications and radiographer workload in clinical practice to correct for prostate motion. Clarity could help facilitate future protocols using tighter treatment margins, although further research is required.

Impact of robotic ultrasound image guidance on plan quality in SBRT of the prostate.

Ultrasound provides good image quality, fast volumetric imaging and is established for abdominal image guidance. Robotic transducer placement may facilitate intrafractional motion compensation in radiation therapy. We consider integration with the CyberKnife and study whether the kinematic redundancy of a seven-degrees-of-freedom robot allows for acceptable plan quality for prostate treatments. Reference treatment plans were generated for 10 prostate cancer cases previously treated with the CyberKnife. Considering transducer and prostate motion by different safety margins, 10 different robot poses, and 3 different elbow configurations, we removed all beams colliding with robot or transducer. For each combination, plans were generated using the same strict dose constraints and the objective to maximize the target coverage. Additionally, plans for the union of all unblocked beams were generated. In 9 cases the planning target coverage with the ultrasound robot was within 1.1 percentage points of the reference coverage. It was 1.7 percentage points for one large prostate. For one preferable robot position, kinematic redundancy decreased the average number of blocked beam directions from 23.1 to 14.5. The impact of beam blocking can largely be offset by treatment planning and using a kinematically redundant robot. Plan quality can be maintained by carefully choosing the ultrasound robot position and pose. For smaller planning target volumes the difference in coverage is negligible for safety margins of up to 35 mm. Advances in knowledge: Integrating a robot for online intrafractional image guidance based on ultrasound can be realized while maintaining acceptable plan quality for prostate cancer treatments with the CyberKnife.

Performance assessment of a programmable five degrees-of-freedom motion platform for quality assurance of motion management techniques in radiotherapy.

Inter-fraction and intra-fraction motion management methods are increasingly applied clinically and require the development of advanced motion platforms to facilitate testing and quality assurance program development. The aim of this study was to assess the performance of a 5 degrees-of-freedom (DoF) programmable motion platform HexaMotion (ScandiDos, Uppsala, Sweden) towards clinically observed tumor motion range, velocity, acceleration and the accuracy requirements of SABR prescribed in AAPM Task Group 142. Performance specifications for the motion platform were derived from literature regarding the motion characteristics of prostate and lung tumor targets required for real time motion management. The performance of the programmable motion platform was evaluated against (1) maximum range, velocity and acceleration (5 DoF), (2) static position accuracy (5 DoF) and (3) dynamic position accuracy using patient-derived prostate and lung tumor motion traces (3 DoF). Translational motion accuracy was compared against electromagnetic transponder measurements. Rotation was benchmarked with a digital inclinometer. The static accuracy and reproducibility for translation and rotation was <0.1mm or <0.1°, respectively. The accuracy of reproducing dynamic patient motion was <0.3mm. The motion platform's range met the need to reproduce clinically relevant translation and rotation ranges and its accuracy met the TG 142 requirements for SABR. The range, velocity and acceleration of the motion platform are sufficient to reproduce lung and prostate tumor motion for motion management. Programmable motion platforms are valuable tools in the investigation, quality assurance and commissioning of motion management systems in radiation oncology.

A new method to reconstruct intra-fractional prostate motion in volumetric modulated arc therapy

Intra-fractional motion is a concern during prostate radiation therapy, as it may cause deviations between planned and delivered radiation doses. Because accurate motion information during treatment delivery is critical to address dose deviation, we developed the projection marker matching method (PM3), a novel method for prostate motion reconstruction in volumetric modulated arc therapy. The purpose of this method is to reconstruct in-treatment prostate motion trajectory using projected positions of implanted fiducial markers measured in kV x-ray projection images acquired during treatment delivery. We formulated this task as a quadratic optimization problem. The objective function penalized the distance from the reconstructed 3D position of each fiducial marker to the corresponding straight line, defined by the x-ray projection of the marker. Rigid translational motion of the prostate and motion smoothness along the temporal dimension were assumed and incorporated into the optimization model. We tested the motion reconstruction method in both simulation and phantom experimental studies. We quantified the accuracy using 3D normalized root-mean-square (RMS) error defined as the norm of a vector containing ratios between the absolute RMS errors and corresponding motion ranges in three dimensions. In the simulation study with realistic prostate motion trajectories, the 3D normalized RMS error was on average (range from to ). In an experimental study, a prostate phantom was driven to move along a realistic prostate motion trajectory. The 3D normalized RMS error was . We also examined the impact of the model parameters on reconstruction accuracy, and found that a single set of parameters can be used for all the tested cases to accurately reconstruct the motion trajectories. The motion trajectory derived by PM3 may be incorporated into novel strategies, including 4D dose reconstruction and adaptive treatment replanning to address motion-induced dose deviation.

Robust 2-D-3-D Registration Optimization for Motion Compensation During 3-D TRUS-Guided Biopsy Using Learned Prostate Motion Data.

In magnetic resonance (MR)-targeted, 3-D transrectal ultrasound (TRUS)-guided biopsy, prostate motion during the procedure increases the needle targeting error and limits the ability to accurately sample MR-suspicious tumor volumes. The robustness of the 2-D-3-D registration methods for prostate motion compensation is impacted by local optima in the search space. In this paper, we analyzed the prostate motion characteristics and investigated methods to incorporate such knowledge into the registration optimization framework to improve robustness against local optima. Rigid motion of the prostate was analyzed adopting a mixture-of-Gaussian (MoG) model using 3-D TRUS images acquired at bilateral sextant probe positions with a mechanically assisted biopsy system. The learned motion characteristics were incorporated into Powell's direction set method by devising multiple initial search positions and initial search directions. Experiments were performed on data sets acquired during clinical biopsy procedures, and registration error was evaluated using target registration error (TRE) and converged image similarity metric values after optimization. After incorporating the learned initialization positions and directions in Powell's method, 2-D-3-D registration to compensate for motion during prostate biopsy was performed with rms ± std TRE of 2.33 ± 1.09 mm with ~3 s mean execution time per registration. This was an improvement over 3.12 ± 1.70 mm observed in Powell's standard approach. For the data acquired under clinical protocols, the converged image similarity metric value improved in ≥8% of the registrations whereas it degraded only ≤1% of the registrations. The reported improvements in optimization indicate useful advancements in robustness to ensure smooth clinical integration of a registration solution for motion compensation that facilitates accurate sampling of the smallest clinically significant tumors.

Patient specific methods for room-mounted x-ray imagers for monoscopic/stereoscopic prostate motion monitoring.

PurposeTo investigate the improvement of combined monoscopic/stereoscopic prostate motion monitoring with room‐mounted dual x‐ray systems by adopting patient specific methods.MethodsThe linac couch was used as a motion stage to simulate 40 highly dynamic real patient prostate trajectories. For each trajectory, 40 s pretreatment and 120 s treatment periods were extracted to represent a typical treatment fraction. Motion was monitored via continuous stereoscopic x‐ray imaging of a single gold fiducial and images were retrospectively divided into periods of stereoscopic and monoscopic imaging to simulate periodic blocking of the room‐mounted system by the gantry during arc‐based therapy. The accuracy of the combined motion monitoring was assessed by comparison with the linac couch log files. To estimate 3‐D marker position during monoscopic imaging, the use of population statistics was compared to both maximum likelihood estimation and stereoscopic localization based estimation of individualized prostate probability density functions (PDFs) from the pretreatment period. The inclusion of intrafraction updating was compared to pretreatment initialization alone.ResultsCombined mono/stereoscopic localization was successfully implemented. During the transitions from stereoscopic to monoscopic imaging, fiducial localization exhibits sharp discontinuities when population PDFs were employed. Patient specific PDFs successfully reduced the localization error when estimated from stereoscopic localizations, whereas maximum likelihood estimation (MLE) was too unstable in the room‐mounted geometry. Intrafraction stereoscopic updating provided further increases in accuracy. Residual error tended to decrease throughout the treatment fraction, as the patient‐specific PDFs became more refined.ConclusionsThis is the first demonstration of toggled monoscopic/stereoscopic localization using room‐mounted dual x‐ray imagers, enabling continuous intrafraction motion monitoring for these systems. We showed that both pretreatment individualization and intrafraction updating should be used to provide the most accurate motion monitoring.

Validation of dynamic treatment-couch tracking for prostate SBRT

In stereotactic body radiation therapy (SBRT) of prostatic cancer, a high dose per fraction is applied to the target with steep dose gradients. Intrafractional prostate motion can occur unpredictably during the treatment and lead to target miss. This work investigated the dosimetric benefit of motion compensation with dynamic treatment-couch tracking for prostate SBRT treatments in the presence of prostatic motion. Ten SBRT treatment plans for prostate cancer patients with integrated boosts to their index lesion were prepared. The treatment plans were applied with a TrueBeam linear accelerator to a phantom in (a) static reference position, (b) moved with five prostate motion trajectories without any motion compensation, and (c) with real-time compensation using transponder-guided couch tracking. The geometrical position of the electromagnetic transponder was evaluated in the tracked and untracked situation. The dosimetric performance of couch tracking was evaluated, using Gamma agreement indices (GAI) and other dose parameters. These were evaluated within the phantoms biplanar diode array, as well as target- and organ-specific. The root-mean-square error of the motion traces (range: 0.8-4.4 mm) was drastically reduced with couch tracking (0.2-0.4 mm). Residual motion was mainly observed at abrupt direction changes with steep motion gradients. The phantom measurements showed significantly better GAI1%/1mm with tracked (range: 83.4%-100.0%) than with untracked motion (28.9%-99.7%). Also GAI2%/2mm was significantly superior for the tracked (98.4%-100.0%) than the untracked motion (52.3%-100.0%). The organ-specific evaluation showed significantly better target coverage with tracking. The dose to the rectum and bladder showed a dependency on the anterior-posterior motion direction. Couch tracking clearly improved the dosimetric accuracy of prostate SBRT treatments. The treatment couch was able to compensate the prostatic motion with only some minor residual motion. Therefore, couch tracking combined with electromagnetic position monitoring for prostate SBRT is feasible and improves the accuracy in treatment delivery when prostate motion is present.

The first clinical implementation of a real-time six degree of freedom target tracking system during radiation therapy based on Kilovoltage Intrafraction Monitoring (KIM)

PurposeWe present the first clinical implementation of a real-time six-degree of freedom (6DoF) Kilovoltage Intrafraction Monitoring (KIM) system which tracks the cancer target translational and rotational motions during treatment. The method was applied to measure and correct for target motion during stereotactic body radiotherapy (SBRT) for prostate cancer. MethodsPatient: A patient with prostate adenocarcinoma undergoing SBRT with 36.25Gy, delivered in 5 fractions was enrolled in the study. 6DoF KIM technology: 2D positions of three implanted gold markers in each of the kV images (125kV, 10mA at 11Hz) were acquired continuously during treatment. The 2D→3D target position estimation was based on a probability distribution function. The 3D→6DoF target rotation was calculated using an iterative closest point algorithm. The accuracy and precision of the KIM method was measured by comparing the real-time results with kV-MV triangulation. ResultsOf the five treatment fractions, KIM was utilised successfully in four fractions. The intrafraction prostate motion resulted in three couch shifts in two fractions when the prostate motion exceeded the pre-set action threshold of 2mm for more than 5s. KIM translational accuracy and precision were 0.3±0.6mm, −0.2±0.3mm and 0.2±0.7mm in the Left-Right (LR), Superior-Inferior (SI) and Anterior-Posterior (AP) directions, respectively. The KIM rotational accuracy and precision were 0.8°±2.0°, −0.5°±3.3° and 0.3°±1.6° in the roll, pitch and yaw directions, respectively. ConclusionThis treatment represents, to the best of our knowledge, the first time a cancer patient’s tumour position and rotation have been monitored in real-time during treatment. The 6 DoF KIM system has sub-millimetre accuracy and precision in all three translational axes, and less than 1° accuracy and 4° precision in all three rotational axes.

Current modulated volume-of-interest imaging for kilovoltage intrafaction monitoring of the prostate.

The focus of this work was to improve the available kV image quality for continuous intrafraction monitoring of the prostate during volumetric modulated arc therapy. This is investigated using a novel blade collimation system enabling tube current modulated (TCM) volume-of-interest (VOI) imaging of prostate fiducial markers during radiotherapy, and Monte Carlo simulation of MV scatter. A four-blade dynamic kV collimator was used to track a VOI containing gold fiducial markers embedded in a dynamic pelvis phantom during gantry rotation. For each fiducial, a VOI margin around each marker was set to be 2σ of the population covariance matrix characterizing prostate motion. This was used to conform to a single or several fiducials and compared to a static field. DRRs were used to calculate the kV attenuation for each VOI as a function of angle and used to optimize x-ray tube current during acquisition. Image quality was assessed with regard to contrast-to-noise ratio (CNR), fiducial detectability, and imaging dose. Monte Carlo simulations in EGSnrc were used to calculate the imaging dose to the phantom and MV scatter fluence to the imaging panel. Fiducials can be accurately located using a VOI containing a single or several fiducials using a relatively high constant kV output. However, when using a 6 × 6 cm2 field the dose can be upwards of 1.5 Gy in bone for constant kV output and 3.1 Gy when applying TCM at 1 Hz imaging over the course of 40 fractions. This can be mitigated through tailoring the imaging field to a single or several fiducials, in which the integral dose is reduced by a factor of 15.6 and 3.7, respectively. For a constant MV treatment field size, the scattered fluence reaching the kV panel varies by less than a factor of two for a completely rotation of the gantry. However, the MV scatter spectrum overlaps with the detector response for a deleterious effect, with a peak MV scatter energy of approximately 100 keV. TCM can be used to overcome the variability in image quality throughout the rotation and therefore improve fiducial CNR and detectability during periods of high kV attenuation. The combination of VOI and TCM introduces an advantageous approach in intrafraction monitoring of the prostate during radiotherapy by both reducing and localizing the imaging dose, while improving image quality and fiducial detectability during periods of high kV attenuation. In addition, the influence of MV scatter has been shown to be most important in low attenuation regions, with a variation by a factor of two.

Motion corrected DWI with integrated T2-mapping for simultaneous estimation of ADC, T2-relaxation and perfusion in prostate cancer

ObjectiveMultiparametric magnetic resonance imaging (MRI) and PI-RADS (Prostate Imaging – Reporting and Data System) has become the standard to determine a probability score for a lesion being a clinically significant prostate cancer. T2-weighted and diffusion-weighted imaging (DWI) are essential in PI-RADS, depending partly on visual assessment of signal intensity, while dynamic-contrast enhanced imaging is less important. To decrease inter-rater variability and further standardize image evaluation, complementary objective measures are in need. MethodsWe here demonstrate a sequence enabling simultaneous quantification of apparent diffusion coefficient (ADC) and T2-relaxation, as well as calculation of the perfusion fraction f from low b-value intravoxel incoherent motion data. Expandable wait pulses were added to a FOCUS DW SE-EPI sequence, allowing the effective echo time to change at run time. To calculate both ADC and f, b-values 200s/mm2 and 600s/mm2 were chosen, and for T2-estimation 6 echo times between 64.9ms and 114.9ms were used. ResultsThree patients with prostate cancer were examined and all had significantly decreased ADC and T2-values, while f was significantly increased in 2 of 3 tumors. T2 maps obtained in phantom measurements and in a healthy volunteer were compared to T2 maps from a SE sequence with consecutive scans, showing good agreement. In addition, a motion correction procedure was implemented to reduce the effects of prostate motion, which improved T2-estimation. ConclusionsThis sequence could potentially enable more objective tumor grading, and decrease the inter-rater variability in the PI-RADS classification.

Uncertainties of cumulative dose assessment for prostate IMRT

Intensity-modulated radiation therapy (IMRT) is capable of providing conformal doses to the target volume while minimizing doses to nearby normal tissue for radiotherapy of localized prostate cancer (1). However, the clinical gains of prostate IMRT may be limited by geometric uncertainties of a patient. The prostate movement between fractions, i.e., inter-fractional motion, is associated with patient setup uncertainty as well as internal organ displacement between fractions. The inter-fractional motion contributes to the deviation in the delivered doses to the target volume and organs at risk (OARs) (2). Depending on the filling state of the rectum and bladder, the prostate can move by more than 1 cm (3,4). Image guidance before each fraction can reduce these errors and deliver a precise prescription dose to the target volume while sparing doses to OARs. On the other hand, intra-fractional motion refers to the external patient movement as well as internal organ motion during the actual treatment. The major intra-fractional motion of the prostate gland is generally due to changes of gas in the rectum and surrounding digestive track (5). This temporal behavior of prostate motion can vary rapidly and substantially, which could result in large discrepancies between planned doses and actually delivered doses to a patient.

An experimentally validated couch and MLC tracking simulator used to investigate hybrid couch-MLC tracking.

Couch and MLC tracking are two novel techniques to mitigate intrafractional tumor motion on a conventional linear accelerator, but both techniques still have residual dosimetric errors. Here, we first propose and experimentally validate a software tool to simulate couch and MLC tracking, and then use the simulator to study hybrid couch-MLC tracking for improved tracking performance. The tracking simulator requires a treatment plan and a motion trajectory as input and simulates the delivered monitor units and motion of all accelerator parts as function of time. The simulator outputs accelerator log files synchronized with the target motion as well as the MLC exposure error, which is a simple dose error surrogate. A series of couch and MLC tracking experiments were used to determine appropriate parameters for the simulator dynamics and to validate the simulator by its ability to reproduce the experimental tracking accuracy. Three hybrid couch-MLC tracking strategies were investigated. All strategies divided the target motion in beam's eye view into motion perpendicular and parallel to the MLC leaves. In the hybrid strategies, couch tracking compensated for the following target motion components (in order of decreasing couch tracking contribution): (a) all perpendicular motion, (b) residual perpendicular motion less than half a leaf width, and (c) persistent residual perpendicular motion that was stable at a time scale of 1s. MLC tracking compensated for the remaining target motion. All tracking strategies were simulated with two prostate and two lung cancer single-arc VMAT plans using 695 prostate trajectories and 160 lung tumor trajectories. The tracking error was quantified as the MLC exposure error. The couch motion was quantified as the mean speed, acceleration, and jerk of the couch. The simulator reproduced the experimental gantry position with a mean (maximum) root-mean-square (rms) error of 0.07°(0.2°). The geometrical rms tracking error was reproduced with mean (maximum) absolute errors of 0.20mm(0.23mm) and 0.1mm(0.23mm) for MLC tracking parallel and perpendicular to the MLC leaves, and 0.40mm(0.46mm), 0.09mm(0.25mm), and 0.20mm(0.46mm) for couch tracking in the left-right, anterior-posterior, and cranio-caudal directions. The MLC exposure error of VMAT MLC tracking was reproduced with a mean absolute error of 5.6%. All hybrid tracking strategies reduced the couch motion relative to pure couch tracking and improved the tracking accuracy compared with pure MLC tracking. The mean MLC exposure error reduction relative to no tracking was 66.6% (couch tracking), 72.9% (hybrid (1)), 70.2% (2), 59.1% (3), and 55.6% (MLC tracking) for lung tumor motion and 76.5% (couch tracking), 76.1% (1), 74.3% (2), 72.3% (3), and 35.9% (MLC tracking) for prostate motion. For prostate motion, pure MLC tracking resulted in rather large MLC exposure errors that were more than halved with all hybrid tracking strategies. A couch and MLC tracking simulator was developed and experimentally validated against a series of tracking experiments. All hybrid couch-MLC tracking strategies improved MLC tracking. Two strategies also improved couch tracking of lung tumors. In particular, MLC tracking of prostate may be greatly improved by a modest degree of couch motion.

An accurate method to quantify breathing-induced prostate motion for patients implanted with electromagnetic transponders.

To validate and apply a method for the quantification of breathing-induced prostate motion (BIPM) for patients treated with radiotherapy and implanted with electromagnetic transponders for prostate localization and tracking. For the analysis of electromagnetic transponder signal, dedicated software was developed and validated with a programmable breathing simulator phantom. The software was then applied to 1,132 radiotherapy fractions of 30 patients treated in supine position, and to a further 61 fractions of 2 patients treated in prone position. Application of the software in phantom demonstrated reliability of the developed method in determining simulated breathing frequencies and amplitudes. For supine patients, the in vivo analysis of BIPM resulted in median (maximum) amplitudes of 0.10 mm (0.35 mm), 0.24 mm (0.66 mm), and 0.17 mm (0.61 mm) in the left-right (LR), cranio-caudal (CC), and anterior-posterior (AP) directions, respectively. Breathing frequency ranged between 7.73 and 29.43 breaths per minute. For prone patients, the ranges of the BIPM amplitudes were 0.1-0.5 mm, 0.5-1.3 mm, and 0.7-1.7 mm in the LR, CC, and AP directions, respectively. The developed method was able to detect the BIPM with sub-millimeter accuracy. While for patients treated in supine position the BIPM represents a reduced source of treatment uncertainty, for patients treated in prone position, it can be higher than 3 mm.

Quantification of intrafraction prostate motion and its dosimetric effect on VMAT.

Intrafraction prostate motion degrades the accuracy of radiation therapy (RT) delivery. Whilst a number of metrics in the literature have been used to quantify intrafraction prostate motion, it has not been established whether these metrics reflect the effect of motion on the RT dose delivered to the patients. In this study, prostate motion during volumetric modulated arc therapy (VMAT) treatment of 18 patients and a total of 294 fractions was quantified through novel metrics as well as those available in the literature. The impact of the motion on VMAT dosimetry was evaluated using these metrics and dose reconstructions based on a previously validated and published method. The dosimetric impact of the motion on planning target volume (PTV) and clinical target volume (CTV) coverage and organs at risk (OARs) was correlated with the motion metrics, using the coefficient of determination (R 2 ), to evaluate their utility. Action level threshold for the prostate motion metric that best described the dosimetric impact on the PTV D95% was investigated through iterative regression analysis. The average (range) of the mean motion for the patient cohort was 1.5mm (0.3-9.9mm). A number of motion metrics were found to be strongly correlated with PTV D95%, the range of R 2 was 0.43-0.81. For all the motion measures, correlations with CTV D99% (range of R 2 was 0.12-0.62), rectum V65% (range of R 2 was 0.33-0.58) and bladder V65% (range of R 2 was 0.51-0.69) were not as strong as for PTV D95%. The mean of the highest 50% of motion metric was one of the best indicator of dosimetric impact on PTV D95%. Action level threshold value for this metric was found to be 3.0mm. For an individual fraction, when the metric value was greater than 3.0mm then the PTV D95% was reduced on average by 6.2%. This study demonstrated that several motion metrics are well correlated with the dosimetric impact (PTV D95%) of individual fraction prostate motion on VMAT delivery and could be used for treatment course adaptation.

Analysis of electromagnetic transponders tracking data to quantify intrafraction prostate motion during radiotherapy treatments

The Calypso tracking system (Varian, Palo Alto, CA, USA) is used to track the prostate isocenter on patients undergoing prostate radiotherapy after implantation of electromagnetic transponders. Aim of this study was to assign 226 recorded prostate tracks to different patterns of prostate intrafraction motion (i.e. stable target (ST), continuous target drift (CTD) and irregular wave motion (IWM)) and excursion (i.e. transient excursion (TE), persistent excursion (PE) and high-frequency excursion (HFE)). Relative frequencies of STs, CTDs and IWMs were 51.8%, 44.6% and 3.6%, respectively. TEs, PEs and HFEs were revealed in 9.4%, 5.4% and 14.3% cases, respectively, with maximum values of 8.0 mm, 8.7 mm and 15.5 mm, respectively. The equation D(t) = 8.0*10-3 mm/s * t + 0.93 mm was established to calculate the average prostate drift D with time t. Intrafraction prostate motion and excursions can be significant and should be in particular taken into account with treatment deliveries that require a prolonged treatment time, as for instance stereotactic body radiotherapy (SBRT) or hadrontherapy.

P31. Commissioning, quality assurance and initial experience of a Calypso R system: Geneva experience

Introduction The Calypso R system (Varian medical systems), that uses radiofrequency signals for wireless tumor localization (absolute or relative to a zero set on the day) and tracking during radiotherapy, was installed in Geneva in 2015. Since January 2016 it is used to treat prostate patients (pts) with dynamic edge gating (allows to set motion thresholds which disable radiation delivery if the beacons, representing the target, move outside these thresholds). Here we present our early experience with its use with prostate implanted beacons as well as commissioning results and daily quality assurance (QA). Materials/methods For commissioning, specific tools were made available by the vendor. Furthermore our Rando R phantom was implanted with prostatic transponders to verify Calypso R versus cone beam computed tomography (CBCT) beacon’s position and isocenter correspondence as well as to realize an end-to end QA test (to export beacons info from CT scan, import etc). Commissioning QA involved checking: the system calibration and accuracy (isocenter Calypso R versus linac), the optical system monitoring the electro-magnetic array, the localization and tracking functionality, the dynamic edge gating, the correct creation of the reports and beam on radiation detection. Daily QA is performed with the QA fixture phantom and system calibration uses the Isocenter calibration fixture, both delivered with the system. Five patients with different targets are actually under treatment with volumetric modulated arc therapy plans (2 pts for whole prostate gland, 2 pts for pelvic lymph nodes + prostate and 1 pt for prostatic bed). Beacons are implanted with a trans-perineal procedure (not trans-rectal, as suggested by vendor). All patients are positioned supine with a feet immobilization and a knee cousin element on the dedicated Kevlar table top. Calypso R is first used to position the patient and to monitor CBCT image acquisition, for patient overall positioning, and after for patient tracking. Results Commissioning was made in half a day, with values within vendor’s specifications (Calypso R-Linac isocenter 0.1 cm). Daily QA shows a stability of the system (over more than 50 days, Calypso R-Linac isocenter 0.13 cm). Concerning patients, no beacons implant complications was registered, nor transponder migration seen on CBCT images or detected by Calypso R, even after several weeks of treatment. All patients could be treated using ”absolute” isocenter localization and tracking. Care was needed to position the array to avoid possible knee collisions if readjusting the patient or moving the table. When patient’s bladder and rectal filling reproduce simulation conditions, only 5 min extra are necessary to the session for array positioning, tracking and imaging. Clearly, one patient is reproducible on a daily basis (within 0.3 cm) and one patients shows a continuous drift of the tracking curve. Other patients do not present a distinct tracking curve behavior. Conclusions Overall Calypso R usage is easy and fast. A lower knee cousin will be used in the future to increase clearance between patient and array. Calypso R, detecting intra-fraction prostate movements, is introducing in our department a new way to perceive organ motion.

Volumetric modulated arc therapy with dynamic collimator rotation for improved multileaf collimator tracking of the prostate

PurposeTo improve MLC tracking of prostate VMAT plans by dynamic rotation of the collimator to align the MLC leaves with the dominant prostate motion direction. MethodsFor 22 prostate cancer patients, two dual arc VMAT plans were made with (1) fixed collimators (45° and 315°) and (2) a rotating collimator that aligned the MLC leaves with the dominant prostate motion direction (population-based first principal component). The fixed and rotating collimator plan quality was compared using selected dose–volume indices. Next, MLC tracking treatments were simulated with 695 patient-measured prostate traces. The MLC exposure error (under- and overexposed MLC area in beam’s eye view) was calculated as a surrogate for the MLC tracking error. Finally, motion including dose reconstruction was performed for 35 motion traces for one patient, and the root-mean-square dose error was compared with the MLC exposure error. ResultsRotating collimator VMAT plans were of similar quality as the fixed collimator plans, but significantly improved MLC tracking with 33% lower MLC exposure errors (p≪0.0001). The reductions in MLC exposure error correlated significantly with dose error reductions. ConclusionProstate VMAT plans with rotating collimator were of similar quality as fixed collimator plans, but more suitable for MLC tracking with significantly better agreement between planned and delivered dose distributions. MLC tracking for prostate cancer patients can therefore be improved without the requirement of additional efforts or hardware changes.

An analysis of interfractional and intrafractional prostate motion in hypofractionated precise radiotherapy for prostate cancer

Objective To investigate the interfractional and intrafractional prostate motion during hypofractionated precise radiotherapy for prostate cancer. Methods From 2013 to 2016, twenty-eight patients who received 5 Gy radiotherapy in 9 fractions for prostate cancer were enrolled as subjects. Every patient had three gold fiducials implanted into the prostate by transrectal ultrasound guidance two weeks before computed tomography (CT) simulation. All patients underwent CT scans in the supine position with full bladders and rectal balloons filled with 60 ml air. The Pinnacle planning system was used to design the treatment plans. Twenty-three patients were treated with a Synergy accelerator. Those patients underwent cone-beam CT (CBCT) scans prior to treatment. The set-up error was recorded by bone alignment between CBCT images and planning CT images. The prostate displacement was then recorded by gold fiducial alignment. The interfraction prostate displacement was defined by the difference between the above two parameters. The other 5 patients were treated with a Novalis accelerator. Based on gold fiducial alignment, the real-time tracking of gold fiducials was carried out by the ExacTrac system to evaluate the intrafractional prostate displacement. Results A total of 207 measurements of interfractional prostate displacement were made in the 23 patients before treatment. The mean interfractional prostate displacements in the left-right (LR), superior-inferior (SI), and anterior-posterior (AP) directions were (0.05±0.10), (0.20±0.22), and (0.19±0.18) cm, respectively. The displacements larger than 0.3 cm in the above three directions were observed in 1, 52, and 49 measurements, respectively, while the displacements larger than 0.5 cm in the three directions were observed in 1, 29, and 16 measurements, respectively. A total of 225 measurements of gold fiducial displacement were made in the 5 patients during treatment. The mean intrafractional prostate displacements in LR, SI, and AP directions were (0.61±0.50), (0.68±0.69), and (0.70±0.67) mm, respectively. The displacements larger than 3 mm in the three directions were observed in 0, 1, and 1 measurement, respectively. Conclusions In hypofractionated precise radiotherapy for prostate cancer, the interfractional prostate displacement is significantly larger than the intrafractional prostate displacement. The interfractional prostate displacement must be corrected before radiotherapy. In order to avoid off-target irradiation due to postural changes in patients, the tracking of the intrafractional prostate displacement is still necessary although the displacement is relatively small. The rectal balloon can help immobilize the prostate. Key words: Prostate motion; Interfraction; Intrafraction; Prostate neoplasms/hypofraction-ated radiotherapy

Fiduciels intraprostatiques dans le cadre de la radiothérapie stéréotaxique du cancer de la prostate

PurposeImage-guided radiotherapy for prostate cancer is widely used in radiotherapy departments. Intraprostatic gold fiducial markers are used to visualize prostate position and motion before and during treatment. The aim of this report is to describe our experience of implanting intraprostatic fiducial markers under local anesthesia before hypofractionated stereotactic radiotherapy for prostate cancer and to assess its tolerance and reproducibility. Patients and methodsOver a 6 and a half year period, 226 patients with prostate cancer received a stereotactic radiotherapy using the CyberKnife® system (Accuray) in our institution. Eighteen patients were treated for recurrence after prostatectomy; these patients were excluded from the study. Among the 208 remaining patients, 94 patients (45.2%) received stereotactic radiotherapy as a boost after external beam radiotherapy (three fractions of 6Gy); 36 patients (17.3%) were had a re-irradiation (six fractions of 6Gy) and 78 patients (37.5%) had a exclusive stereotactic radiotherapy (68 patients received five fractions of 7.25Gy and 11 patients five fractions of 6.25Gy). Four markers were implanted in all patients using transrectal ultrasound; the procedure was performed under local anesthesia, using transperineal access. The four fiducial markers were implanted in two strands with two fiducial each one, 1cm apart. In order to follow the recommendations of the image-guided radiotherapy system, the two strands of the two markers were located on the same plane in the middle of the prostate, at least 2cm apart from the midline. After insertion, correct positioning of fiducials markers was verified by X-ray. Dosimetry scanning was performed after the implantation procedure; prostate position tracking was possible before and during treatment through the kilovoltage incorporated system of the robotic accelerator. Clinical data, X-ray verification and dosimetry scanner have been retrospectively reviewed for all patients. ResultsThe tolerance to procedure was excellent; only four patients (1.8%) described pain related to implant. No urinary side effects were reported. Median time from fiducial implantation to dosimetry scanner was 16 days (4–113 days). Four fiducials were found within the prostate at dosimetry scanner in 181 patients and three in 27 remaining patients. All intraprostatic fiducials were used to track the prostate gland before and during treatment. ConclusionsIntraprostatic fiducial markers implantation is a safe and reproducible procedure that allows us to have reliable prostate information before and during stereotactic radiotherapy.

Real-time Image-guided Adaptive-predictive Prostate Radiotherapy using Rectal Diameter as a Predictor of Motion

AimsTo investigate a relationship between maximum rectal diameter (MRD) on pre-treatment cone beam computed tomography (CBCT) and intra-fraction prostate motion, in the context of an adaptive image-guided radiotherapy (IGRT) method. Materials and methodsThe MRD was measured on 2125 CBCTs from 55 retrospective patient datasets and related to prostate displacement from intra-fraction imaging. A linear regression model was developed to determine a threshold MRD associated with a high probability of small prostate displacement. Standard and reduced adaptive margin plans were created to compare rectum and bladder normal tissue complication probability (NTCP) with each method. ResultsA per-protocol analysis carried out on 1910 fractions from 51 patients showed with 90% confidence that for a MRD≤3 cm, prostate displacement will be ≤5 mm and that for a MRD≤3.5 cm, prostate displacement will be ≤5.5 mm. In the first scenario, if adaptive therapy was used instead of standard therapy, median reductions in NTCP for rectum and bladder were 0.5% (from 9.5% to 9%) and 1.3% (from 6.6% to 5.3%), respectively. In the second scenario, the NTCP for rectum and bladder would have median reductions of 1.1% and 2.6%, respectively. ConclusionsWe have identified a potential method for adaptive prostate IGRT based upon predicting small prostate intra-fraction motion by measuring MRD on pre-treatment CBCT.