Rune Hansen

A dosimetric comparison of real-time adaptive and non-adaptive radiotherapy: A multi-institutional study encompassing robotic, gimbaled, multileaf collimator and couch tracking.

Purpose A study of real-time adaptive radiotherapy systems was performed to test the hypothesis that, across delivery systems and institutions, the dosimetric accuracy is improved with adaptive treatments over non-adaptive radiotherapy in the presence of patient-measured tumor motion. Methods and materials Ten institutions with robotic(2), gimbaled(2), MLC(4) or couch tracking(2) used common materials including CT and structure sets, motion traces and planning protocols to create a lung and a prostate plan. For each motion trace, the plan was delivered twice to a moving dosimeter; with and without real-time adaptation. Each measurement was compared to a static measurement and the percentage of failed points for γ-tests recorded. Results For all lung traces all measurement sets show improved dose accuracy with a mean 2%/2 mm γ-fail rate of 1.6% with adaptation and 15.2% without adaptation (p < 0.001). For all prostate the mean 2%/2 mm γ-fail rate was 1.4% with adaptation and 17.3% without adaptation (p < 0.001). The difference between the four systems was small with an average 2%/2 mm γ-fail rate of <3% for all systems with adaptation for lung and prostate. Conclusions The investigated systems all accounted for realistic tumor motion accurately and performed to a similar high standard, with real-time adaptation significantly outperforming non-adaptive delivery methods.

Tolerance levels of EPID-based quality control for volumetric modulated arc therapy.

PURPOSE Volumetric modulated arc therapy (VMAT) includes features such as a variable dose rate and gantry speed in addition to the beam modulation achieved with multileaf collimator (MLC) motion patterns employed in intensity modulated radiotherapy. Three tests have previously been proposed for the evaluation of the performance of VMAT delivery. In order to enable a convenient and accurate routine machine quality control (QC) program, the present study proposes tolerance levels for these tests based on a department-wide implementation of an electronic portal imaging device (EPID)-based QC. METHODS Three different VMAT tests--a picket fence (PF) test, a dose rate versus gantry speed (DRGS) test, and a dose rate versus MLC leaf speed (DRMLC) test--were performed on nine accelerators using two different EPIDs (aS1000 and aS500, Varian Medical Systems). All tests were repeated six times for each accelerator. The images were analyzed using an in-house-developed software. For the PF test, the positions and widths of individual MLC leaf gaps were compared to the mean value. In the DRGS and DRMLC tests, different combinations of dose rate, gantry speed, and MLC leaf speed were used to deliver identical doses to separate parts of the EPID. The tests were evaluated by looking for deviations in the constancy of the measured dose for the preset combinations of dose rate, gantry speed, and MLC leaf speed. RESULTS For the PF test, a 0.3 mm tolerance level was suggested for the positioning of the MLC leaves. The tolerance level for the gap width was 0.5 mm. For the DRGS and DRMLC tests, a 3% tolerance level was proposed. CONCLUSIONS With the adapted levels of tolerance for an EPID-based approach, the PF, the DRGS, and the DRMLC tests offer a convenient and accurate machine QC program for linear accelerators used for VMAT.

Respiratory gating based on internal electromagnetic motion monitoring during stereotactic liver radiation therapy: First results

ABSTRACT Background. Intrafraction motion may compromise the target dose in stereotactic body radiation therapy (SBRT) of tumors in the liver. Respiratory gating can improve the treatment delivery, but gating based on an external surrogate signal may be inaccurate. This is the first paper reporting on respiratory gating based on internal electromagnetic monitoring during liver SBRT. Material and methods. Two patients with solitary liver metastases were treated with respiratory-gated SBRT guided by three implanted electromagnetic transponders. The treatment was delivered in end-exhale with beam-on when the centroid of the three transponders deviated less than 3 mm [left-right (LR) and anterior-posterior (AP) directions] and 4mm [cranio-caudal (CC)] from the planned position. For each treatment fraction, log files were used to determine the transponder motion during beam-on in the actual gated treatments and in simulated treatments without gating. The motion was used to reconstruct the dose to the clinical target volume (CTV) with and without gating. The reduction in D95 (minimum dose to 95% of the CTV) relative to the plan was calculated for both treatment courses. Results. With gating the maximum course mean (standard deviation) geometrical error in any direction was 1.2 mm (1.8 mm). Without gating the course mean error would mainly increase for Patient 1 [to -2.8 mm (1.6 mm) (LR), 7.1 mm (5.8 mm) (CC), −2.6 mm (2.8mm) (AP)] due to a large systematic cranial baseline drift at each fraction. The errors without gating increased only slightly for Patient 2. The reduction in CTV D95 was 0.5% (gating) and 12.1% (non-gating) for Patient 1 and 0.3% (gating) and 1.7% (non-gating) for Patient 2. The mean duty cycle was 55%. Conclusion. Respiratory gating based on internal electromagnetic motion monitoring was performed for two liver SBRT patients. The gating added robustness to the dose delivery and ensured a high CTV dose even in the presence of large intrafraction motion.

Electromagnetic guided couch and multileaf collimator tracking on a TrueBeam accelerator

PURPOSE Couch and MLC tracking are two promising methods for real-time motion compensation during radiation therapy. So far, couch and MLC tracking experiments have mainly been performed by different research groups, and no direct comparison of couch and MLC tracking of volumetric modulated arc therapy (VMAT) plans has been published. The Varian TrueBeam 2.0 accelerator includes a prototype tracking system with selectable couch or MLC compensation. This study provides a direct comparison of the two tracking types with an otherwise identical setup. METHODS Several experiments were performed to characterize the geometric and dosimetric performance of electromagnetic guided couch and MLC tracking on a TrueBeam accelerator equipped with a Millennium MLC. The tracking system latency was determined without motion prediction as the time lag between sinusoidal target motion and the compensating motion of the couch or MLC as recorded by continuous MV portal imaging. The geometric and dosimetric tracking accuracies were measured in tracking experiments with motion phantoms that reproduced four prostate and four lung tumor trajectories. The geometric tracking error in beams eye view was determined as the distance between an embedded gold marker and a circular MLC aperture in continuous MV images. The dosimetric tracking error was quantified as the measured 2%/2 mm gamma failure rate of a low and a high modulation VMAT plan delivered with the eight motion trajectories using a static dose distribution as reference. RESULTS The MLC tracking latency was approximately 146 ms for all sinusoidal period lengths while the couch tracking latency increased from 187 to 246 ms with decreasing period length due to limitations in the couch acceleration. The mean root-mean-square geometric error was 0.80 mm (couch tracking), 0.52 mm (MLC tracking), and 2.75 mm (no tracking) parallel to the MLC leaves and 0.66 mm (couch), 1.14 mm (MLC), and 2.41 mm (no tracking) perpendicular to the leaves. The motion-induced gamma failure rate was in mean 0.1% (couch tracking), 8.1% (MLC tracking), and 30.4% (no tracking) for prostate motion and 2.9% (couch), 2.4% (MLC), and 41.2% (no tracking) for lung tumor motion. The residual tracking errors were mainly caused by inadequate adaptation to fast lung tumor motion for couch tracking and to prostate motion perpendicular to the MLC leaves for MLC tracking. CONCLUSIONS Couch and MLC tracking markedly improved the geometric and dosimetric accuracies of VMAT delivery. However, the two tracking types have different strengths and weaknesses. While couch tracking can correct perfectly for slowly moving targets such as the prostate, MLC tracking may have considerably larger dose errors for persistent target shift perpendicular to the MLC leaves. Advantages of MLC tracking include faster dynamics with better adaptation to fast moving targets, the avoidance of moving the patient, and the potential to track target rotations and deformations.

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

Purpose/objective: 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. Materials and methods: 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 beams 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. Results: 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 0.20 mm(0.23 mm) and 0.1 mm(0.23 mm) for MLC tracking parallel and perpendicular to the MLC leaves, and 0.40 mm(0.46 mm), 0.09 mm(0.25 mm), and 0.20 mm(0.46 mm) 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. Conclusion: 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.

Automatic online and real-time tumour motion monitoring during stereotactic liver treatments on a conventional linac by combined optical and sparse monoscopic imaging with kilovoltage x-rays (COSMIK)

The purpose of this study was to develop, validate and clinically demonstrate fully automatic tumour motion monitoring on a conventional linear accelerator by combined optical and sparse monoscopic imaging with kilovoltage x-rays (COSMIK). COSMIK combines auto-segmentation of implanted fiducial markers in cone-beam computed tomography (CBCT) projections and intra-treatment kV images with simultaneous streaming of an external motion signal. A pre-treatment CBCT is acquired with simultaneous recording of the motion of an external marker block on the abdomen. The 3-dimensional (3D) marker motion during the CBCT is estimated from the auto-segmented positions in the projections and used to optimize an external correlation model (ECM) of internal motion as a function of external motion. During treatment, the ECM estimates the internal motion from the external motion at 20 Hz. KV images are acquired every 3 s, auto-segmented, and used to update the ECM for baseline shifts between internal and external motion. The COSMIK method was validated using Calypso-recorded internal tumour motion with simultaneous camera-recorded external motion for 15 liver stereotactic body radiotherapy (SBRT) patients. The validation included phantom experiments and simulations hereof for 12 fractions and further simulations for 42 fractions. The simulations compared the accuracy of COSMIK with ECM-based monitoring without model updates and with model updates based on stereoscopic imaging as well as continuous kilovoltage intrafraction monitoring (KIM) at 10 Hz without an external signal. Clinical real-time tumour motion monitoring with COSMIK was performed offline for 14 liver SBRT patients (41 fractions) and online for one patient (two fractions). The mean 3D root-mean-square error for the four monitoring methods was 1.61 mm (COSMIK), 2.31 mm (ECM without updates), 1.49 mm (ECM with stereoscopic updates) and 0.75 mm (KIM). COSMIK is the first combined kV/optical real-time motion monitoring method used clinically online on a conventional accelerator. COSMIK gives less imaging dose than KIM and is in addition applicable when the kV imager cannot be deployed such as during non-coplanar fields.

TH‐AB‐303‐01: Benchmarking Real‐Time Adaptive Radiotherapy Systems: A Multi‐ Platform Multi‐Institutional Study

Purpose: The era of real-time adaptive radiotherapy is here: patients are being treated by CyberKnife (since 2004), Vero (2011) and MLC tracking (2013) technology, with couch tracking planned to be clinical in 2015. We have developed a common set of tools for benchmarking real-time adaptive radiotherapy systems and to test the hypothesis that, across delivery systems and institutions, real-time adaptive radiotherapy improves the dosimetric accuracy over non-adaptive radiotherapy in the presence of realistic tumor motion. Methods: Ten institutions with CyberKnife, Vero, MLC or couch tracking technology were involved in the study. Common materials were anonymized lung and prostate CT and structure sets, patient-measured motion traces (four lung, four prostate) and SBRT planning protocols (lung: RTOG1021, prostate: RTOG0938). The institutions delivered lung and prostate plans to a moving dosimeter programmed with tumor motion. For each trace the plan was delivered twice; with and without motion adaptation, each measurement was compared to the static dosimeter dose and the percentage of failed points for γ-tests recorded. Results: Eleven measurement sets were obtained for this study; two CyberKnife, two Vero, five MLC and two couch tracking sets. For all lung traces all sets show improved dose accuracy from a mean 2%/2mm γ-failrate of 1.6% with adaptation and 14.7% with no motion correction(p<0.001). For all prostate traces the mean 2%/2mm γ-failrate was 1.6% with adaptation and 17.4% with no motion correction (p<0.001). The difference between the four adaptive systems was small with an average 2%/2mm γ-failrate of <3% for all systems with adaptation for lung and prostate. Conclusion: A common set of tools has been developed for benchmarking real-time adaptive radiotherapy systems and a multi-platform multi-institutional study performed. The results show the systems all account for realistic tumor motion accurately and performed to a similar high standard, with real-time adaptation significantly outperforming non-adaptive methods.

Validation of fast motion-including dose reconstruction for proton scanning therapy in the liver

This study validates a method of fast motion-including dose reconstruction for proton pencil beam scanning in the liver. The method utilizes a commercial treatment planning system (TPS) and calculates the delivered dose for any translational 3D target motion. Data from ten liver patients previously treated with photon radiotherapy with intrafraction tumour motion monitoring were used. The dose reconstruction method utilises an in-house developed program to incorporate beams-eye-view tumour motion by shifting each spot in the opposite direction of the tumour and in-depth motion as beam energy changes for each spot. The doses are then calculated on a single CT phase in the TPS. Two aspects of the dose reconstruction were assessed: (1) The accuracy of reconstruction, by comparing dose reconstructions created using 4DCT motion with ground truth doses obtained by calculating phase specific doses in all 4DCT phases and summing up these partial doses. (2) The error caused by assuming 4DCT motion, by comparing reconstructions with 4DCT motion and actual tumour motion. The CTV homogeneity index (HI) and the root-mean-square (rms) dose error for all dose points receiving  >70%, >80% and  >90% of the prescribed dose were calculated. The dose reconstruction resulted in mean (range) absolute CTV HI errors of 1.0% (0.0-3.0)% and rms dose errors of 2.5% (1.0%-5.3%), 2.1% (0.9%-4.5%), and 1.8% (0.7%-3.7%) for  >70%, >80% and  >90% doses, respectively, when compared with the ground truth. The assumption of 4DCT motion resulted in mean (range) absolute CTV HI errors of 5.9% (0.0-15.0)% and rms dose errors of 6.3% (3.9%-12.6%), 5.9% (3.4%-12.5%), and 5.4% (2.6%-12.1%) for  >70%, >80% and  >90% doses, respectively. The investigated method allows tumour dose reconstruction with the actual tumour motion and results in significantly smaller dose errors than those caused by assuming that motion at treatment is identical to the 4DCT motion.

Geometric and dosimetric comparison of four intrafraction motion adaptation strategies for stereotactic liver radiotherapy

The accuracy of stereotactic body radiotherapy (SBRT) in the liver is limited by tumor motion. Selection of the most suitable motion mitigation strategy requires good understanding of the geometric and dosimetric consequences. This study compares the geometric and dosimetric accuracy of actually delivered respiratory gated SBRT treatments for 15 patients with liver tumors with three simulated alternative motion adaptation strategies. The simulated alternatives are MLC tracking, baseline shift adaptation by inter-field couch corrections and no intrafraction motion adaptation. The patients received electromagnetic transponder-guided respiratory gated IMRT or conformal treatments in three fractions with a 3-4 mm gating window around the full exhale position. The CTV-PTV margin was 5 mm axially and 7-10 mm cranio-caudally. The CTV and PTV were covered with 95% and 67% of the prescribed mean CTV dose, respectively. The time-resolved target position error during treatments with the four investigated motion adaptation strategies was used to calculate motion margins and the motion-induced reduction in CTV D 95 relative to the planned dose (ΔD 95). The mean (range) number of couch corrections per treatment session to compensate for tumor drift was 2.8 (0-7) with gating, 1.4 (0-5) with baseline shift adaptation, and zero for the other strategies. The motion margins were 3.5 mm (left-right), 9.4 mm (cranio-caudal) and 3.9 mm (anterior-posterior) without intrafraction motion adaptation, approximately half of that with baseline shift adaptation, and 1-2 mm with MLC tracking and gating. With 7 mm CC margins the mean (range) of ΔD 95 for the CTV was 8.1 (0.6-29.4)%-points (no intrafraction motion adaptation), 4.0 (0.4-13.3)%-points (baseline shift adaptation), 1.0 (0.3-2.2)%-points (MLC tracking) and 0.8 (0.1-1.8)%-points (gating). With 10 mm CC margins ΔD 95 was instead 4.8 (0.3-14.8)%-points (no intrafraction motion adaptation) and 2.9 (0.2-9.8)%-points (baseline shift adaptation). In conclusion, baseline shift adaptation can mitigate gross dose deficits without the requirement of real-time motion adaptation. MLC tracking and gating, however, more effectively ensure high similarity between planned and delivered doses.

Simultaneous acquisition of 4D ultrasound and wireless electromagnetic tracking for in-vivo accuracy validation

Abstract Ultrasound is being increasingly investigated for real-time target localization in image-guided interventions. Yet, in-vivo validation remains challenging due to the difficulty to obtain a reliable ground truth. For this purpose, real-time volumetric (4D) ultrasound imaging was performed simultaneously with electromagnetic localization of three wireless transponders implanted in the liver of a radiotherapy patient. 4D ultrasound and electromagnetic tracking were acquired at framerates of 12Hz and 8Hz, respectively, during free breathing over 8 min following treatment. The electromagnetic antenna was placed directly above and the ultrasound probe on the right side of the patient to visualize the liver transponders. It was possible to record 25.7 s of overlapping ultrasound and electromagnetic position data of one transponder. Good spatial alignment with 0.6 mm 3D root-mean-square error between both traces was achieved using a rigid landmark transform. However, data acquisition was impaired since the electromagnetic tracking highly influenced the ultrasound equipment and vice versa. High intensity noise streaks appeared in the ultrasound scan lines irrespective of the chosen frequency (1.7-3.3 MHz, 2/4 MHz harmonic). To allow for target visualization and tracking in the ultrasound volumes despite the artefacts, an online filter was designed where corrupted pixels in the newest ultrasound frame were replaced with non-corrupted pixels from preceding frames. Aside from these artefacts, the recorded electromagnetic tracking data was fragmented and only the transponder closest to the antenna could be detected over a limited period of six consecutive breathing cycles. This problem was most likely caused by interference from the metal holder of the ultrasound probe and was solved in a subsequent experiment using a 3D-printed non-metal probe fixation. Real-time wireless electromagnetic tracking was compared with 4D ultrasound imaging in-vivo for the first time. For stable tracking, large metal components need to be avoided during data acquisition and ultrasound filtering is required.