Motion adaptive x-ray therapy: a feasibility study

Simultaneous object detection and segmentation for patient-specific markerless lung tumor tracking in simulated radiographs with deep learning

BACKGROUND

Real-time tumor tracking is one motion management method to address motion-induced uncertainty. To date, fiducial markers are often required to reliably track lung tumors with X-ray imaging, which carries risks of complications and leads to prolonged treatment time. A markerless tracking approach is thus desirable. Deep learning-based approaches have shown promise for markerless tracking, but systematic evaluation and procedures to investigate applicability in individual cases are missing. Moreover, few efforts have been made to provide bounding box prediction and mask segmentation simultaneously, which could allow either rigid or deformable multi-leaf collimator tracking.

PURPOSE

The purpose of this study was to implement a deep learning-based markerless lung tumor tracking model exploiting patient-specific training which outputs both a bounding box and a mask segmentation simultaneously. We also aimed to compare the two kinds of predictions and to implement a specific procedure to understand the feasibility of markerless tracking on individual cases.

METHODS

We first trained a Retina U-Net baseline model on digitally reconstructed radiographs (DRRs) generated from a public dataset containing 875 CT scans and corresponding lung nodule annotations. Afterwards, we used an independent cohort of 97 lung patients to develop a patient-specific refinement procedure. In order to determine the optimal hyperparameters for automatic patient-specific training, we selected 13 patients for validation where the baseline model predicted a bounding box on planning CT (PCT)-DRR with intersection over union (IoU) with the ground-truth higher than 0.7. The final test set contained the remaining 84 patients with varying PCT-DRR IoU. For each testing patient, the baseline model was refined on the PCT-DRR to generate a patient-specific model, which was then tested on a separate 10-phase 4DCT-DRR to mimic the intrafraction motion during treatment. A template matching algorithm served as benchmark model. The testing results were evaluated by four metrics: the center of mass (COM) error and the Dice similarity coefficient (DSC) for segmentation masks, and the center of box (COB) error and the DSC for bounding box detections. Performance was compared to the benchmark model including statistical testing for significance.

RESULTS

A PCT-DRR IoU value of 0.2 was shown to be the threshold dividing inconsistent (68%) and consistent (100%) success (defined as mean bounding box DSC > 0.6) of PS models on 4DCT-DRRs. Thirty-seven out of the eighty-four testing cases had a PCT-DRR IoU above 0.2. For these 37 cases, the mean COM error was 2.6 mm, the mean segmentation DSC was 0.78, the mean COB error was 2.7 mm, and the mean box DSC was 0.83. Including the validation cases, the model was applicable to 50 out of 97 patients when using the PCT-DRR IoU threshold of 0.2. The inference time per frame was 170 ms. The model outperformed the benchmark model on all metrics, and the comparison was significant (p < 0.001) over the 37 PCT-DRR IoU > 0.2 cases, but not over the undifferentiated 84 testing cases.

CONCLUSIONS

The implemented patient-specific refinement approach based on a pre-trained baseline model was shown to be applicable to markerless tumor tracking in simulated radiographs for lung cases.

Impact of respiratory gating and ECG gating on 18F-FDG PET/CT for cardiac sarcoidosis

BACKGROUND

The aim of this study was to estimate the impact of respiratory and electrocardiogram (ECG)-gated FDG positron emission tomography (PET)/computed tomography (CT) on the diagnosis of cardiac sarcoidosis (CS).

METHODS AND RESULTS

Imaging from thirty-one patients was acquired on a PET/CT scanner equipped with a respiratory- and ECG-gating system. Non-gated PET images and three kinds of gated PET/CT images were created from identical list-mode clinical PET data: respiratory-gated PET during expiration (EX), ECG-gated PET at end diastole (ED), and ECG-gated PET at end systole (ES). The maximum standardized uptake value (SUVmax) and cardiac metabolic volume (CMV) were measured, and the locations of FDG accumulation were analyzed using a polar map. The mean SUVmax of the subjects was significantly higher after application of either respiratory-gated or ECG-gated reconstruction. Conversely, the mean CMV was significantly lower following the application of respiratory-gated or ECG-gated reconstruction. The segment showing maximum accumulation was shifted to the adjacent segment in 25.8%, 38.7%, and 41.9% of cases in EX, ED, and ES images, respectively.

CONCLUSION

In FDG PET/CT scanning for the diagnosis of CS, gated scanning is likely to increase quantitative accuracy, but the effect depends on the location and synchronization method.

Evaluation of real-time tumor contour prediction using LSTM networks for MR-guided radiotherapy

BACKGROUND AND PURPOSE

Magnetic resonance imaging guided radiotherapy (MRgRT) with deformable multileaf collimator (MLC) tracking would allow to tackle both rigid displacement and tumor deformation without prolonging treatment. However, the system latency must be accounted for by predicting future tumor contours in real-time. We compared the performance of three artificial intelligence (AI) algorithms based on long short-term memory (LSTM) modules for the prediction of 2D-contours 500ms into the future.

MATERIALS AND METHODS

Models were trained (52 patients, 3.1h of motion), validated (18 patients, 0.6h) and tested (18 patients, 1.1h) with cine MRs from patients treated at one institution. Additionally, we used three patients (2.9h) treated at another institution as second testing set. We implemented 1) a classical LSTM network (LSTM-shift) predicting tumor centroid positions in superior-inferior and anterior-posterior direction which are used to shift the last observed tumor contour. The LSTM-shift model was optimized both in an offline and online fashion. We also implemented 2) a convolutional LSTM model (ConvLSTM) to directly predict future tumor contours and 3) a convolutional LSTM combined with spatial transformer layers (ConvLSTM-STL) to predict displacement fields used to warp the last tumor contour.

RESULTS

The online LSTM-shift model was found to perform slightly better than the offline LSTM-shift and significantly better than the ConvLSTM and ConvLSTM-STL. It achieved a 50% Hausdorff distance of 1.2mm and 1.0mm for the two testing sets, respectively. Larger motion ranges were found to lead to more substantial performance differences across the models.

CONCLUSION

LSTM networks predicting future centroids and shifting the last tumor contour are the most suitable for tumor contour prediction. The obtained accuracy would allow to reduce residual tracking errors during MRgRT with deformable MLC-tracking.

A phantom-based analysis for tracking intra-fraction pancreatic tumor motion by ultrasound imaging during radiation therapy

Purpose

In this study, we aim to further evaluate the accuracy of ultrasound tracking for intra-fraction pancreatic tumor motion during radiotherapy by a phantom-based study.

Methods

Twelve patients with pancreatic cancer who were treated with stereotactic body radiation therapy were enrolled in this study. The displacement points of the respiratory cycle were acquired from 4DCT and transferred to a motion platform to mimic realistic breathing movements in our phantom study. An ultrasound abdominal phantom was placed and fixed in the motion platform. The ground truth of phantom movement was recorded by tracking an optical tracker attached to this phantom. One tumor inside the phantom was the tracking target. In the evaluation of the results, the monitoring results from the ultrasound system were compared with the phantom motion results from the infrared camera. Differences between infrared monitoring motion and ultrasound tracking motion were analyzed by calculating the root-mean-square error.

Results

The 82.2% ultrasound tracking motion was within a 0.5 mm difference value between ultrasound tracking displacement and infrared monitoring motion. 0.7% ultrasound tracking failed to track accurately (a difference value &gt; 2.5 mm). These differences between ultrasound tracking motion and infrared monitored motion do not correlate with respiratory displacements, respiratory velocity, or respiratory acceleration by linear regression analysis.

Conclusions

The highly accurate monitoring results of this phantom study prove that the ultrasound tracking system may be a potential method for real-time monitoring targets, allowing more accurate delivery of radiation doses.

Predicting respiratory motion using a novel patient specific dual deep recurrent neural networks

Real-time tracking of a target volume is a promising solution for reducing the planning margins and both dosimetric and geometric uncertainties in the treatment of thoracic and upper-abdomen cancers. Respiratory motion prediction is an integral part of real-time tracking to compensate for the latency of tracking systems. The purpose of this work was to develop a novel method for accurate respiratory motion prediction using dual deep recurrent neural networks (RNNs). The respiratory motion data of 111 patients were used to train and evaluate the method. For each patient, two models (Network1 and Network2) were trained on 80% of the respiratory wave, and the remaining 20% was used for evaluation. The first network (Network 1) is a "coarse resolution" prediction of future points and second network (Network 2) provides a "fine resolution" prediction to interpolate between the future predictions. The performance of the method was tested using two types of RNN algorithms : Long Short-Term Memory (LSTM) and Gated Recurrent Unit (GRU). The accuracy of each model was evaluated using the root mean square error (RMSE) and mean absolute error (MAE). Overall, the RNN model with GRU- function had better accuracy than the RNN model with LSTM-function (RMSE (mm): 0.4±0.2 vs. 0.6±0.3; MAE (mm): 0.4±0.2 vs. 0.6±0.2). The GRU was able to predict the respiratory motion accurately (<1 mm) up to the latency period of 440 ms, and LSTM's accuracy was acceptable only up to 240 ms. The proposed method using GRU function can be used for respiratory-motion prediction up to a latency period of 440 ms.

Dosimetric impact of phase shifts on Radixact Synchrony tracking system with patient-specific breathing patterns

Purpose

The Synchrony tracking system of Radixact is capable of real‐time tumor tracking by building a correlation model between external light‐emitting diodes on the patient's chest and an internal marker. A phase shift between the chest wall and a lung tumor has been reported. Hence, this study focused on evaluating the accuracy of the tracking system, especially under a patient‐specific breathing pattern with respiratory phase shifts.

Methods

A phantom containing fiducial markers was placed on a moving platform. The intrinsic delivery accuracy was verified with a patient‐specific breathing pattern. Three patient‐specific breathing patterns were then implemented, for which phase shifts, φ, were introduced. Phase shifts with +0.3 s and +1 s were tested for dosimetric aspects, whereas ±0.3, ±0.6, and ±0.8 s shifts were used for tracking accuracy. The resultant dose distributions were analyzed by γ comparison. Dose profiles in the superior‐inferior and lateral directions were compared. Logfiles of the tracking information were extracted from the system and compared with the input breathing pattern. The root mean square (RMS) difference was used to quantify the consistency.

Results

When the φ value was as large as 1 s, a severe inconsistency was observed. The target was significantly underdosed, down to 89% of the originally planned dose. γ analysis revealed that the failed portion was concentrated in the target region. The RMS of the tracking difference was close to 1 mm when φ was ±0.3 s and approximately 4 mm when φ was ±0.8 s. Tracking errors increased with an increase in the degree of phase shifts.

Conclusion

Phase shifts between the patient chest wall and the internal target may hamper treatment delivery and jeopardize treatment using Synchrony Tracking. Hence, a larger planning target volume (PTV) may be necessary if a large phase shift is observed in a patient, especially when an external surrogate shows a lag in motion when compared with the tumor.

MR SIGnature MAtching (MRSIGMA) with retrospective self-evaluation for real-time volumetric motion imaging

Objective. MR SIGnature MAtching (MRSIGMA) is a real-time volumetric MRI technique to image tumor and organs at risk motion in real-time for radiotherapy applications, where a dictionary of high-resolution 3D motion states and associated motion signatures are computed first during offline training and real-time 3D imaging is performed afterwards using fast signature-only acquisition and signature matching. However, the lack of a reference image with similar spatial resolution and temporal resolution introduces significant challenges forin vivovalidation.Approach. This work proposes a retrospective self-validation for MRSIGMA, where the same data used for real-time imaging are used to create a non-real-time reference for comparison. MRSIGMA with self-validation is tested in patients with liver tumors using quantitative metrics defined on the tumor and nearby organs-at-risk structures. The dice coefficient between contours defined on the real-time MRSIGMA and non-real-time reference was used to assess motion imaging performance.Main Results. Total latency (including signature acquisition and signature matching) was between 250 and 314 ms, which is sufficient for organs affected by respiratory motion. Mean ± standard deviation dice coefficient over time was 0.74 ± 0.03 for patients imaged without contrast agent and 0.87 ± 0.03 for patients imaged with contrast agent, which demonstrated high-performance real-time motion imaging.Signficance. MRSIGMA with self-evaluation provides a means to perform real-time volumetric MRI for organ motion tracking with quantitative performance measures.

Target motion management in breast cancer radiation therapy

Background

Over the last two decades, breast cancer remains the main cause of cancer deaths in women. To treat this type of cancer, radiation therapy (RT) has proved to be efficient. RT for breast cancer is, however, challenged by intrafractional motion caused by respiration. The problem is more severe for the left-sided breast cancer due to the proximity to the heart as an organ-at-risk. While particle therapy results in superior dose characteristics than conventional RT, due to the physics of particle interactions in the body, particle therapy is more sensitive to target motion.

Conclusions

This review highlights current and emerging strategies for the management of intrafractional target motion in breast cancer treatment with an emphasis on particle therapy, as a modern RT technique. There are major challenges associated with transferring real-time motion monitoring technologies from photon to particles beams. Surface imaging would be the dominant imaging modality for real-time intrafractional motion monitoring for breast cancer. The magnetic resonance imaging (MRI) guidance and ultra high dose rate (FLASH)-RT seem to be state-of-the-art approaches to deal with 4D RT for breast cancer.

First-in-human imaging using a MR-compatible e4D ultrasound probe for motion management of radiotherapy

PURPOSE

Respiration-induced tumor or organ positional changes can impact the accuracy of external beam radiotherapy. Motion management strategies are used to account for these changes during treatment. The authors report on the development, testing, and first-in-human evaluation of an electronic 4D (e4D) MR-compatible ultrasound probe that was designed for hands-free operation in a MR and linear accelerator (LINAC) environment.

METHODS

Ultrasound components were evaluated for MR compatibility. Electromagnetic interference (EMI) shielding was used to enclose the entire probe and a factory-fabricated cable shielded with copper braids was integrated into the probe. A series of simultaneous ultrasound and MR scans were acquired and analyzed in five healthy volunteers.

RESULTS

The ultrasound probe led to minor susceptibility artifacts in the MR images immediately proximal to the ultrasound probe at a depth of <10 mm. Ultrasound and MR-based motion traces that were derived by tracking the salient motion of endogenous target structures in the superior-inferior (SI) direction demonstrated good concordance (Pearson correlation coefficients of 0.95-0.98) between the ultrasound and MRI datasets.

CONCLUSION

We have demonstrated that our hands-free, e4D probe can acquire ultrasound images during a MR acquisition at frame rates of approximately 4 frames per second (fps) without impacting either the MR or ultrasound image quality. This use of this technology for interventional procedures (e.g. biopsies and drug delivery) and motion compensation during imaging are also being explored.

Gated Radiotherapy Development and its Expansion

One of the most important challenges in treatment of patients with cancerous tumors of chest and abdominal areas is organ movement. The delivery of treatment radiation doses to tumor tissue is a challenging matter while protecting healthy and radio sensitive tissues. Since the movement of organs due to respiration causes a discrepancy in the middle of planned and delivered dose distributions. The moderation in the fatalistic effect of intra-fractional target travel on the radiation therapy correctness is necessary for cutting-edge methods of motion remote monitoring and cancerous growth irradiancy. Tracking respiratory milling and implementation of breath-hold techniques by respiratory gating systems have been used for compensation of respiratory motion negative effects. Therefore, these systems help us to deliver precise treatments and also protect healthy and critical organs. It seems aspiration should be kept under observation all over treatment period employing tracking seed markers (e.g. fiducials), skin surface scanners (e.g. camera and laser monitoring systems) and aspiration detectors (e.g. spirometers). However, these systems are not readily available for most radiotherapy centers around the word. It is believed that providing and expanding the required equipment, gated radiotherapy will be a routine technique for treatment of chest and abdominal tumors in all clinical radiotherapy centers in the world by considering benefits of respiratory gating techniques in increasing efficiency of patient treatment in the near future. This review explains the different technologies and systems as well as some strategies available for motion management in radiotherapy centers.

AAPM Task Group 264: The safe clinical implementation of MLC tracking in radiotherapy

The era of real-time radiotherapy is upon us. Robotic and gimbaled linac tracking are clinically established technologies with the clinical realization of couch tracking in development. Multileaf collimators (MLCs) are a standard equipment for most cancer radiotherapy systems, and therefore MLC tracking is a potentially widely available technology. MLC tracking has been the subject of theoretical and experimental research for decades and was first implemented for patient treatments in 2013. The AAPM Task Group 264 Safe Clinical Implementation of MLC Tracking in Radiotherapy Report was charged to proactively provide the broader radiation oncology community with (a) clinical implementation guidelines including hardware, software, and clinical indications for use, (b) commissioning and quality assurance recommendations based on early user experience, as well as guidelines on Failure Mode and Effects Analysis, and (c) a discussion of potential future developments. The deliverables from this report include: an explanation of MLC tracking and its historical development; terms and definitions relevant to MLC tracking; the clinical benefit of, clinical experience with and clinical implementation guidelines for MLC tracking; quality assurance guidelines, including example quality assurance worksheets; a clinical decision pathway, future outlook and overall recommendations.

Variational multi-task MRI reconstruction: Joint reconstruction, registration and super-resolution

Motion degradation is a central problem in Magnetic Resonance Imaging (MRI). This work addresses the problem of how to obtain higher quality, super-resolved motion-free reconstructions from highly undersampled MRI data. In this work, we present for the first time a variational multi-task framework that allows joining three relevant tasks in MRI: reconstruction, registration and super-resolution. Our framework takes a set of multiple undersampled MR acquisitions corrupted by motion into a novel multi-task optimisation model, which is composed of an L² fidelity term that allows sharing representation between tasks, super-resolution foundations and hyperelastic deformations to model biological tissue behaviors. We demonstrate that this combination yields significant improvements over sequential models and other bi-task methods. Our results exhibit fine details and compensate for motion producing sharp and highly textured images compared to state of the art methods while keeping low CPU time. Our improvements are appraised on both clinical assessment and statistical analysis.

Couch and multileaf collimator tracking: A clinical feasibility study for pancreas and liver treatment

PURPOSE

Real-time tumor tracking through active correction by the multileaf collimator or treatment couch offers a promising strategy to mitigate delivery uncertainty due to intrafractional tumor motion. This study evaluated the performance of MLC and couch tracking using the prototype iTools Tracking system in TrueBeam Developer Mode and the application for abdominal cancer treatments.

METHODS

Experiments were carried out using a phantom with embedded Calypso transponders and a motion simulation platform. Geometric evaluations were performed using a circular conformal field with sinusoidal traces and pancreatic tumor motion traces. Geometric tracking accuracy was retrospectively calculated by comparing the compensational MLC or couch motion extracted from machine log files to the target motion reconstructed from real-time MV and kV images. Dosimetric tracking accuracy was measured with radiochromic films using clinical abdominal VMAT plans and pancreatic tumor traces.

RESULTS

Geometrically, the root-mean-square errors for MLC tracking were 0.5 and 1.8 mm parallel and perpendicular to leaf travel direction, respectively. Couch tracking, in contrast, showed an average of 0.8 mm or less geometric error in all directions. Dosimetrically, both MLC and couch tracking reduced motion-induced local dose errors compared to no tracking. Evaluated with five pancreatic tumor motion traces, the average 2%/2 mm global gamma pass rate of eight clinical abdominal VMAT plans was 67.4% (range: 26.4%-92.7%) without tracking, which was improved to 86.0% (range: 67.9%-95.6%) with MLC tracking, and 98.1% (range: 94.9%-100.0%) with couch tracking. In 16 out of 40 deliveries with different plans and motion traces, MLC tracking did not achieve clinically acceptable dosimetric accuracy with 3%/3mm gamma pass rate below 95%.

CONCLUSIONS

This study demonstrated the capability of MLC and couch tracking to reduce motion-induced dose errors in abdominal cases using a prototype tracking system. Clinically significant dose errors were observed with MLC tracking for certain plans which could be attributed to the inferior MLC tracking accuracy in the direction perpendicular to leaf travel, as well as the interplay between motion tracking and plan delivery for highly modulated plans. Couch tracking outperformed MLC tracking with consistently high dosimetric accuracy in all plans evaluated, indicating its clinical potential in the treatment of abdominal cancers.

The potential of Gantry beamline large momentum acceptance for real time tumour tracking in pencil beam scanning proton therapy

Tumour tracking is an advanced radiotherapy technique for precise treatment of tumours subject to organ motion. In this work, we addressed crucial aspects of dose delivery for its realisation in pencil beam scanning proton therapy, exploring the momentum acceptance and global achromaticity of a Gantry beamline to perform continuous energy regulation with a standard upstream degrader. This novel approach is validated on simulation data from three geometric phantoms of increasing complexity and one liver cancer patient using 4D dose calculations. Results from a standard high-to-low beamline ramping scheme were compared to alternative energy meandering schemes including combinations with rescanning. Target coverage and dose conformity were generally well recovered with tumour tracking even though for particularly small targets, large variations are reported for the different approaches. Meandering in energy while rescanning has a positive impact on target homogeneity and similarly, hot spots outside the targets are mitigated with a relatively fast convergence rate for most tracking scenarios, halving the volume of hot spots after as little as 3 rescans. This work investigates the yet unexplored potential of having a large momentum acceptance in medical beam line, and provides an alternative to take tumour tracking with particle therapy closer to clinical translation.

Automated MV markerless tumor tracking for VMAT

Tumor tracking during radiotherapy treatment can improve dose accuracy, conformity and sparing of healthy tissue. Many methods have been introduced to tackle this challenge utilizing multiple imaging modalities, including a template matching based approach using the megavoltage (MV) on-board portal imager demonstrated on 3D conformal treatments. However, the complexity of treatments is evolving with the introduction of VMAT and IMRT, and successful motion management is becoming more important due to a trend towards hypofractionation. We have developed a markerless lung tumor tracking algorithm, utilizing the electronic portal imager (EPID) of the treatment machine. The algorithm has been specifically adapted to track during complex treatment deliveries with gantry and MLC motion. The core of the algorithm is an adaptive template matching method that relies on template stability metrics and local relative orientations to perform multiple feature tracking simultaneously. Only a single image is required to initialize the algorithm and features are automatically added, modified or removed in response to the input images. This algorithm was evaluated against images collected during VMAT arcs of a dynamic thorax phantom. Dynamic phantom images were collected during radiation delivery for multiple lung SBRT breathing traces and an example patient data set. The tracking error was 1.34 mm for the phantom data and 0.68 mm for the patient data. A multi-region, markerless tracking algorithm has been developed, capable of tracking multiple features simultaneously without requiring any other a priori information. This novel approach delivers robust target localization during complex treatment delivery. The reported tracking error is similar to previous reports for 3D conformal treatments.

4D-MRI driven MR-guided online adaptive radiotherapy for abdominal stereotactic body radiation therapy on a high field MR-Linac: Implementation and initial clinical experience

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This is the first clinical use of 4D-MRI in an online radiation therapy workflow.

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Eleven patients received SBRT on an Elekta MR-Linac using ATP and ATS workflows.

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A parallel contour editing approach was successfully utilized in the ATS workflow.

Background and purpose

In this report, we describe our implementation and initial clinical experience using 4D-MRI driven MR-guided online adaptive radiotherapy (MRgOART) for abdominal stereotactic body radiotherapy (SBRT) on the Elekta Unity MR-Linac.

Materials and methods

Eleven patients with abdominal malignancies were treated with free-breathing SBRT in three to five fractions on a 1.5 T MR-Linac. Online adaptive plans were generated using Adapt-To-Position (ATP) or Adapt-To-Shape (ATS) workflows based on motion averaged or mid-position images derived from a pre-beam 4D-MRI. A high performance server positioned on the local MR-Linac machine network was utilized for 4D-MR image reconstruction. A parallel contour editing approach was employed in the ATS workflow. Intravoxel incoherent motion (IVIM) and T2 mapping sequences were acquired during adaptive planning in both ATP and ATS workflows for treatment response monitoring. Adaptive plans were delivered under real-time cine image motion monitoring.

Results

The shortest 4D-MRI time-to-image was the motion averaged image, followed by mid position and respiratory binned images. In this cohert of patients, 50% of treatments utilized the ATS workflow; the remaining treatments utilized the ATP workflow. Mid-position images were utilized as daily planning images for two of the eleven patients. The mean daily adaptive plan secondary dose calculation and ArcCheck 3D Gamma passing rates were 97.5% (92.1–100.0%) and 99.3% (96.2–100.0%), respectively. The median overall treatment times for abdominal SBRT was 46 and 62 min for ATP and ATS workflows, respectively.

Conclusion

We have successfully implemented and utilized a 4D-MRI driven MRgOART process with ATP and ATS workflows for free-breathing abdominal SBRT on a 1.5 T Elekta Unity MR-Linac.

Technical Note: Comprehensive performance tests of the first clinical real-time motion tracking and compensation system using MLC and jaws

Purpose

To evaluate the performance of the first clinical real‐time motion tracking and compensation system using multileaf collimator (MLC) and jaws during helical tomotherapy delivery.

Methods

Appropriate mechanical and dosimetry tests were performed on the first clinical real‐time motion tracking system (Synchrony on Radixact, Accuray Inc) recently installed in our institution. kV radiography dose was measured by CTDIw using a pencil chamber. Changes of beam characteristics with jaw offset and MLC leaf shift were evaluated. Various dosimeters and phantoms including A1SL ion chamber (Standard Imaging), Gafchromic EBT3 films (Ashland), TomoPhantom (Med Cal), ArcCheck (Sun Nuclear), Delta4 (ScandiDos), with fiducial or high contrast inserts, placed on two dynamical motion platforms (CIRS dynamic motion‐CIRS, Hexamotion‐ScandiDos), were used to assess the dosimetric accuracy of the available Synchrony modalities: fiducial tracking with nonrespiratory motion (FNR), fiducial tracking with respiratory modeling (FR), and fiducial free (e.g., lung tumor tracking) with respiratory modeling (FFR). Motion detection accuracy of a tracking target, defined as the difference between the predicted and instructed target positions, was evaluated with the root mean square (RMS). The dose accuracy of motion compensation was evaluated by verifying the dose output constancy and by comparing measured and planned (predicted) three‐dimensional (3D) dose distributions based on gamma analysis.

Results

The measured CTDIw for a single radiograph with a 120 kVp and 1.6 mAs protocol was 0.084 mGy, implying a low imaging dose of 8.4 mGy for a typical Synchrony motion tracking fraction with 100 radiographs. The dosimetric effect of the jaw swing or MLC leaf shift was minimal on depth dose (<0.5%) and was <2% on both beam profile width and output for typical motions. The motion detection accuracies, that is, RMS, were 0.84, 1.13, and 0.48 mm for FNR, FR, and FFR, respectively, well within the 1.5 mm recommended tolerance. Dose constancy with Synchrony was found to be within 2%. The gamma passing rates of 3D dose measurements for a variety of Synchrony plans were well within the acceptable level.

Conclusions

The motion tracking and compensation using kV radiography, MLC shifting, and jaw swing during helical tomotherapy delivery was tested to be mechanically and dosimetrically accurate for clinical use.

Investigating neutron activated contrast agent imaging for tumor localization in proton therapy: a feasibility study for proton neutron gamma-x detection (PNGXD)

Proton neutron gamma-x detection (PNGXD) is a novel imaging concept being investigated for tumor localization during proton therapy that uses secondary neutron interactions with a gadolinium contrast agent (GDCA) to produce characteristic photons within the 40-200 keV energy region. The purpose of this study is to experimentally investigate the feasibility of implementing this procedure by performing experimental measurements on a passive double scattering proton treatment unit. Five experimental measurements were performed with varying concentrations and irradiation conditions. Photon spectra were measured with a 25 mm², 1 mm thick uncollimated X-123 CdTe spectrometer. For a 10.4 Gy administration on a 100 ml volume phantom with 10 mg g^-1 Gd solution placed in a water phantom, 1129  ±  184 K-shell Gd counts were detected. For an administered dose of 21 Gy and the same Gd solution measured in air, resulted in 3296  ±  256 counts. A total of 1094  ±  171, 421  ±  150 and 23  ±  141 K-shell Gd counts were measured for Gd concentrations of 10 mg g^-1, 1 mg g^-1 and 0 mg g^-1 for 7 Gy dose in air. The signal to noise ratio for these five measurements were: 7, 15, 6, 3, and 0.2, respectively. The spectrum contained 43 keV K _α and 49 keV K _β peaks, however a small amount of 79.5 and 181.9 keV prompt gamma rays were detected from gadolinium neutron capture. This discrepancy is due to a drop in the intrinsic detection efficiency of the CdTe spectrometer over this energy range. The measurements were compared with Monte-Carlo simulation to determine the contributions of Gd neutron capture from internal and external neutrons on a passive scattering proton therapy unit and to investigate the discrepancy in detected characteristic x-rays versus prompt gamma rays.

Prediction of lung tumor motion using nonlinear autoregressive model with exogenous input

The present note addresses the development of a lung tumor position predictor to be used in dynamic tumor tracking radiotherapy, abbreviated as DTT-RT. As there exists 50-500 ms positioning lag in the control of the multi-leaf collimator (MLC) of commercial medical linear accelerators, prediction of future lung tumor position with sufficiently long prediction horizon is inevitable for the successful implementation of DTT-RT. The present article proposes a lung tumor position predictor, which is classified as a nonlinear autoregressive model with exogenous input (NARX). The proposed predictor was trained using seven lung tumor motion trajectories of patients who underwent respiratory gated radiotherapy at Yamaguchi University Hospital. We considered three different prediction horizons, 600 ms, 800 ms and 1 s, which were sufficiently long to compensate for the possible positioning control lag of the MLC. A patient-specific model corresponding to an intended prediction horizon was obtained by training it using the selected tumor motion trajectory with the specified horizon. Accordingly, we obtained three NARX predictors for a single patient. We calculated two performance metrics: the RMS prediction errors and the rate of coverage of the entire tumor trajectory defined by the number of samples of the measured tumor position which was inside the 4 mm cube centered at the corresponding predicted tumor position. The latter quantifies the feasibility of the predictors to generate future gating cubes in the implementation of DTT-RT. The [Formula: see text] (mean [Formula: see text] standard deviation) values of the rates of 600 ms, 800 ms and 1 s prediction horizon calculated using the proposed NARX predictors were [Formula: see text]%, [Formula: see text]% and [Formula: see text]%, respectively.

2D ultrasound imaging based intra-fraction respiratory motion tracking for abdominal radiation therapy using machine learning

We have previously developed a robotic ultrasound imaging system for motion monitoring in abdominal radiation therapy. Owing to the slow speed of ultrasound image processing, our previous system could only track abdominal motions under breath-hold. To overcome this limitation, a novel 2D-based image processing method for tracking intra-fraction respiratory motion is proposed. Fifty-seven different anatomical features acquired from 27 sets of 2D ultrasound sequences were used in this study. Three 2D ultrasound sequences were acquired with the robotic ultrasound system from three healthy volunteers. The remaining datasets were provided by the 2015 MICCAI Challenge on Liver Ultrasound Tracking. All datasets were preprocessed to extract the feature point, and a patient-specific motion pattern was extracted by principal component analysis and slow feature analysis (SFA). The tracking finds the most similar frame (or indexed frame) by a k-dimensional-tree-based nearest neighbor search for estimating the tracked object location. A template image was updated dynamically through the indexed frame to perform a fast template matching (TM) within a learned smaller search region on the incoming frame. The mean tracking error between manually annotated landmarks and the location extracted from the indexed training frame is 1.80  ±  1.42 mm. Adding a fast TM procedure within a small search region reduces the mean tracking error to 1.14  ±  1.16 mm. The tracking time per frame is 15 ms, which is well below the frame acquisition time. Furthermore, the anatomical reproducibility was measured by analyzing the location's anatomical landmark relative to the probe; the position-controlled probe has better reproducibility and yields a smaller mean error across all three volunteer cases, compared to the force-controlled probe (2.69 versus 11.20 mm in the superior-inferior direction and 1.19 versus 8.21 mm in the anterior-posterior direction). Our method reduces the processing time for tracking respiratory motion significantly, which can reduce the delivery uncertainty.

Dynamic MLC Tracking Using 4D Lung Tumor Motion Modelling and EPID Feedback

BACKGROUND

Respiratory motion causes thoracic movement and reduces targeting accuracy in radiotherapy.

OBJECTIVE

This study proposes an approach to generate a model to track lung tumor motion by controlling dynamic multi-leaf collimators.

MATERIAL AND METHODS

All slices which contained tumor were contoured in the 4D-CT images for 10 patients. For modelling of respiratory motion, the end-exhale phase of these images has been considered as the reference and they were analyzed using neuro-fuzzy method to predict the magnitude of displacement of the lung tumor. Then, the predicted data were used to determine the leaf motion in MLC. Finally, the trained algorithm was figured out using Shaper software to show how MLCs could track the moving tumor and then imported on the Varian Linac equipped with EPID.

RESULTS

The root mean square error (RMSE) was used as a statistical criterion in order to investigate the accuracy of neuro-fuzzy performance in lung tumor prediction. The results showed that RMSE did not have a considerable variation. Also, there was a good agreement between the images obtained by EPID and Shaper for a respiratory cycle.

CONCLUSION

The approach used in this study can track the moving tumor with MLC based on the 4D modelling, so it can improve treatment accuracy, dose conformity and sparing of healthy tissues because of low error in margins that can be ignored. Therefore, this method can work more accurately as compared with the gating and invasive approaches using markers.

Technical Note: In silico and experimental evaluation of two leaf-fitting algorithms for MLC tracking based on exposure error and plan complexity

PURPOSE

Multileaf collimator (MLC) tracking is being clinically pioneered to continuously compensate for thoracic and pelvic motion during radiotherapy. The purpose of this work was to characterize the performance of two MLC leaf-fitting algorithms, direct optimization and piecewise optimization, for real-time motion compensation with different plan complexity and tumor trajectories.

METHODS

To test the algorithms, both in silico and phantom experiments were performed. The phantom experiments were performed on a Trilogy Varian linac and a HexaMotion programmable motion platform. High and low modulation VMAT plans for lung and prostate cancer cases were used along with eight patient-measured organ-specific trajectories. For both MLC leaf-fitting algorithms, the plans were run with their corresponding patient trajectories. To compare algorithms, the average exposure errors, i.e., the difference in shape between ideal and fitted MLC leaves by the algorithm, plan complexity and system latency of each experiment were calculated.

RESULTS

Comparison of exposure errors for the in silico and phantom experiments showed minor differences between the two algorithms. The average exposure errors for in silico experiments with low/high plan complexity were 0.66/0.88 cm² for direct optimization and 0.66/0.88 cm² for piecewise optimization, respectively. The average exposure errors for the phantom experiments with low/high plan complexity were 0.73/1.02 cm² for direct and 0.73/1.02 cm² for piecewise optimization, respectively. The measured latency for the direct optimization was 226 ± 10 ms and for the piecewise algorithm was 228 ± 10 ms. In silico and phantom exposure errors quantified for each treatment plan demonstrated that the exposure errors from the high plan complexity (0.96 cm² mean, 2.88 cm² 95% percentile) were all significantly different from the low plan complexity (0.70 cm² mean, 2.18 cm² 95% percentile) (P < 0.001, two-tailed, Mann-Whitney statistical test).

CONCLUSIONS

The comparison between the two leaf-fitting algorithms demonstrated no significant differences in exposure errors, neither in silico nor with phantom experiments. This study revealed that plan complexity impacts the overall exposure errors significantly more than the difference between the algorithms.

ELPHA: Dynamically deformable liver phantom for real-time motion-adaptive radiotherapy treatments

PURPOSE

Real-time motion-adaptive radiotherapy of intrahepatic tumors needs to account for motion and deformations of the liver and the target location within. Phantoms representative of anatomical deformations are required to investigate and improve dynamic treatments. A deformable phantom capable of testing motion detection and motion mitigation techniques is presented here.

METHODS

The dynamically dEformable Liver PHAntom (ELPHA) was designed to fulfill three main constraints: First, a reproducibly deformable anatomy is required. Second, the phantom should provide multimodality imaging contrast for motion detection. Third, a time-resolved dosimetry system to measure temporal effects should be provided. An artificial liver with vasculature was casted from soft silicone mixtures. The silicones allow for deformation and radiographic image contrast, while added cellulose provides ultrasonic contrast. An actuator was used for compressing the liver in the inferior direction according to a prescribed respiratory motion trace. Electromagnetic (EM) transponders integrated in ELPHA help provide ground truth motion traces. They were used to quantify the motion reproducibility of the phantom and to validate motion detection based on ultrasound imaging. A two-dimensional ultrasound probe was used to follow the position of the vessels with a template-matching algorithm. This detected vessel motion was compared to the EM transponder signal by calculating the root-mean-square error (RMSE). ELPHA was then used to investigate the dose deposition of dynamic treatment deliveries. Two dosimetry systems, radio-chromic film and plastic scintillation dosimeters (PSD), were integrated in ELPHA. The PSD allow for time-resolved measurement of the delivered dose, which was compared to a time-resolved dose of the treatment planning system. Film and PSD were used to investigate dose delivery to the deforming phantom without motion compensation and with treatment-couch tracking for motion compensation.

RESULTS

ELPHA showed densities of 66 and 45 HU in the liver and the surrounding tissues. A high motion reproducibility with a submillimeter RMSE (<0.32 mm) was measured. The motion of the vasculature detected with ultrasound agreed well with the EM transponder position (RMSE < 1 mm). A time-resolved dosimetry system with a 1 Hz time resolution was achieved with the PSD. The agreement of the planned and measured dose to the PSD decreased with increasing motion amplitude: A dosimetric RMSE of 1.2, 2.1, and 2.7 cGy/s was measured for motion amplitudes of 8, 16, and 24 mm, respectively. With couch tracking as motion compensation, these values decreased to 1.1, 1.4, and 1.4 cGy/s. This is closer to the static situation with 0.7 cGy/s. Film measurements showed that couch tracking was able to compensate for motion with a mean target dose within 5% of the static situation (-5% to +1%), which was higher than in the uncompensated cases (-41% to -1%).

CONCLUSIONS

ELPHA is a deformable liver phantom with high motion reproducibility. It was demonstrated to be suitable for the verification of motion detection and motion mitigation modalities. Based on the multimodality image contrast, a high accuracy of ultrasound based motion detection was shown. With the time-resolved dosimetry system, ELPHA is suitable for performance assessment of real-time motion-adaptive radiotherapy, as was shown exemplary with couch tracking.

Tracking, gating, free-breathing, which technique to use for lung stereotactic treatments? A dosimetric comparison

BACKGROUND

The management of breath-induced tumor motion is a major challenge for lung stereotactic body radiation therapy (SBRT). Three techniques are currently available for these treatments: tracking (T), gating (G) and free-breathing (FB).

AIM

To evaluate the dosimetric differences between these three treatment techniques for lung SBRT.

MATERIALS AND METHODS

Pretreatment 4DCT data were acquired for 10 patients and sorted into 10 phases of a breathing cycle, such as 0% and 50% phases defined respectively as the inhalation and exhalation maximum. GTVph, PTVph (=GTVph + 3 mm) and the ipsilateral lung were contoured on each phase.For the tracking technique, 9 fixed fields were adjusted to each PTVph for the 10 phases. The gating technique was studied with 3 exhalation phases (40%, 50% and 60%). For the free-breathing technique, ITVFB was created from a sum of all GTVph and a 3 mm margin was added to define a PTVFB. Fields were adjusted to PTVFB and dose distributions were calculated on the average intensity projection (AIP) CT. Then, the beam arrangement with the same monitor units was planned on each CT phase.The 3 modalities were evaluated using DVHs of each GTVph, the homogeneity index and the volume of the ipsilateral lung receiving 20 Gy (V 20Gy).

RESULTS

The FB system improved the target coverage by increasing D mean (75.87(T)-76.08(G)-77.49(FB)Gy). Target coverage was slightly more homogeneous, too (HI: 0.17(T and G)-0.15(FB)). But the lung was better protected with the tracking system (V 20Gy: 3.82(T)-4.96(G)-6.34(FB)%).

CONCLUSIONS

Every technique provides plans with a good target coverage and lung protection. While irradiation with free-breathing increases doses to GTV, irradiation with the tracking technique spares better the lung but can dramatically increase the treatment complexity.

The impact of 2D cine MR imaging parameters on automated tumor and organ localization for MR-guided real-time adaptive radiotherapy

2D cine MR imaging may be utilized to monitor rapidly moving tumors and organs-at-risk for real-time adaptive radiotherapy. This study systematically investigates the impact of geometric imaging parameters on the ability of 2D cine MR imaging to guide template-matching-driven autocontouring of lung tumors and abdominal organs. Abdominal 4D MR images were acquired of six healthy volunteers and thoracic 4D MR images were obtained of eight lung cancer patients. At each breathing phase of the images, the left kidney and gallbladder or lung tumor, respectively, were outlined as volumes of interest. These images and contours were used to create artificial 2D cine MR images, while simultaneously serving as 3D ground truth. We explored the impact of five different imaging parameters (pixel size, slice thickness, imaging plane orientation, number and relative alignment of images as well as strategies to create training images). For each possible combination of imaging parameters, we generated artificial 2D cine MR images as training and test images. A template-matching algorithm used the training images to determine the tumor or organ position in the test images. Subsequently, a 3D base contour was shifted to the determined position and compared to the ground truth via centroid distance and Dice similarity coefficient. The median centroid distance between adapted and ground truth contours was 1.56 mm for the kidney, 3.81 mm for the gallbladder and 1.03 mm for the lung tumor (median Dice similarity coefficient: 0.95, 0.72 and 0.93). We observed that a decrease in image resolution led to a modest decrease in localization accuracy, especially for the small gallbladder. However, for all volumes of interest localization accuracy varied substantially more between subjects than due to the different imaging parameters. Automated tumor and organ localization using 2D cine MR imaging and template-matching-based autocontouring is robust against variation of geometric imaging parameters. Future work and optimization efforts of 2D cine MR imaging for real-time adaptive radiotherapy is needed to characterize the influence of sequence- and anatomical site-specific imaging contrast.

SBRT targets that move with respiration

The technology of treating SBRT targets that move with respiration has undergone profound changes over the last 20 years. This review article summarizes modern image guidance to localize the target in real-time to account for intra-fraction motion. The state-of-the art respiratory motion compensation techniques will be discussed, including the determination and application of appropriate margins. This includes compression, gating and breath-hold, including the use of audiovisual feedback to manage motion. Approaches to real-time tracking include the use of hybrid external-internal imaging to build a skin-to-tumor correlation, which can then be tracked with a mobile robot (CyberKnife Synchrony, clinical since 2003) as well as the use of non-ionizing electromagnetic tumor surrogate localization followed by real-time tracking with a moving MLC (in clinical trials in Europe and Australia). Lastly, the clinical application of real-time MRI soft-tissue imaging to deliver adaptive, iso-toxic treatments will be presented.

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.

Electromagnetic-Guided MLC Tracking Radiation Therapy for Prostate Cancer Patients: Prospective Clinical Trial Results

PURPOSE

To report on the primary and secondary outcomes of a prospective clinical trial of electromagnetic-guided multileaf collimator (MLC) tracking radiation therapy for prostate cancer.

METHODS AND MATERIALS

Twenty-eight men with prostate cancer were treated with electromagnetic-guided MLC tracking with volumetric modulated arc therapy. A total of 858 fractions were delivered, with the dose per fraction ranging from 2 to 13.75 Gy. The primary outcome was feasibility, with success determined if >95% of fractions were successfully delivered. The secondary outcomes were (1) the improvement in beam-target geometric alignment, (2) the improvement in dosimetric coverage of the prostate and avoidance of critical structures, and (3) no acute grade ≥3 genitourinary or gastrointestinal toxicity.

RESULTS

All 858 planned fractions were successfully delivered with MLC tracking, demonstrating the primary outcome of feasibility (P < .001). MLC tracking improved the beam-target geometric alignment from 1.4 to 0.90 mm (root-mean-square error). MLC tracking improved the dosimetric coverage of the prostate and reduced the daily variation in dose to critical structures. No acute grade ≥3 genitourinary or gastrointestinal toxicity was observed.

CONCLUSIONS

Electromagnetic-guided MLC tracking radiation therapy for prostate cancer is feasible. The patients received improved geometric targeting and delivered dose distributions that were closer to those planned than they would have received without electromagnetic-guided MLC tracking. No significant acute toxicity was observed.

Initial clinical observations of intra- and interfractional motion variation in MR-guided lung SBRT

OBJECTIVE

To evaluate variations in intra- and interfractional tumour motion, and the effect on internal target volume (ITV) contour accuracy, using deformable image registration of real-time two-dimensional-sagittal cine-mode MRI acquired during lung stereotactic body radiation therapy (SBRT) treatments.

METHODS

Five lung tumour patients underwent free-breathing SBRT treatments on the ViewRay system, with dose prescribed to a planning target volume (defined as a 3-6 mm expansion of the 4DCT-ITV). Sagittal slice cine-MR images (3.5 × 3.5 mm² pixels) were acquired through the centre of the tumour at 4 frames per second throughout the treatments (3-4 fractions of 21-32 min). Tumour gross tumour volumes (GTVs) were contoured on the first frame of the MR cine and tracked for the first 20 min of each treatment using offline optical-flow based deformable registration implemented on a GPU cluster. A ground truth ITV (MR-ITV_20 min) was formed by taking the union of tracked GTV contours. Pseudo-ITVs were generated from unions of the GTV contours tracked over 10 s segments of image data (MR-ITV_10 s).

RESULTS

Differences were observed in the magnitude of median tumour displacement between days of treatments. MR-ITV_10 s areas were as small as 46% of the MR-ITV_20 min.

CONCLUSION

An ITV offers a "snapshot" of breathing motion for the brief period of time the tumour is imaged on a specific day. Real-time MRI over prolonged periods of time and over multiple treatment fractions shows that ITV size varies. Further work is required to investigate the dosimetric effect of these results. Advances in knowledge: Five lung tumour patients underwent free-breathing MRI-guided SBRT treatments, and their tumours tracked using deformable registration of cine-mode MRI. The results indicate that variability of both intra- and interfractional breathing amplitude should be taken into account during planning of lung radiotherapy.

Simultaneous tumor and surrogate motion tracking with dynamic MRI for radiation therapy planning

Respiration-induced tumor motion is a major obstacle for achieving high-precision radiotherapy of cancers in the thoracic and abdominal regions. Surrogate-based estimation and tracking methods are commonly used in radiotherapy, but with limited understanding of quantified correlation to tumor motion. In this study, we propose a method to simultaneously track the lung tumor and external surrogates to evaluate their spatial correlation in a quantitative way using dynamic MRI, which allows real-time acquisition without ionizing radiation exposure. To capture the lung and whole tumor, four MRI-compatible fiducials are placed on the patient's chest and upper abdomen. Two different types of acquisitions are performed in the sagittal orientation including multi-slice 2D cine MRIs to reconstruct 4D-MRI and two-slice 2D cine MRIs to simultaneously track the tumor and fiducials. A phase-binned 4D-MRI is first reconstructed from multi-slice MR images using body area as a respiratory surrogate and groupwise registration. The 4D-MRI provides 3D template volumes for different breathing phases. 3D tumor position is calculated by 3D-2D template matching in which 3D tumor templates in the 4D-MRI reconstruction and the 2D cine MRIs from the two-slice tracking dataset are registered. 3D trajectories of the external surrogates are derived via matching a 3D geometrical model of the fiducials to their segmentations on the 2D cine MRIs. We tested our method on ten lung cancer patients. Using a correlation analysis, the 3D tumor trajectory demonstrates a noticeable phase mismatch and significant cycle-to-cycle motion variation, while the external surrogate was not sensitive enough to capture such variations. Additionally, there was significant phase mismatch between surrogate signals obtained from the fiducials at different locations.

MRI-guided lung SBRT: Present and future developments

Stereotactic body radiotherapy (SBRT) is rapidly becoming an alternative to surgery for the treatment of early-stage non-small cell lung cancer patients. Lung SBRT is administered in a hypo-fractionated, conformal manner, delivering high doses to the target. To avoid normal-tissue toxicity, it is crucial to limit the exposure of nearby healthy organs-at-risk (OAR). Current image-guided radiotherapy strategies for lung SBRT are mostly based on X-ray imaging modalities. Although still in its infancy, magnetic resonance imaging (MRI) guidance for lung SBRT is not exposure-limited and MRI promises to improve crucial soft-tissue contrast. Looking beyond anatomical imaging, functional MRI is expected to inform treatment decisions and adaptations in the future. This review summarises and discusses how MRI could be advantageous to the different links of the radiotherapy treatment chain for lung SBRT: diagnosis and staging, tumour and OAR delineation, treatment planning, and inter- or intrafractional motion management. Special emphasis is placed on a new generation of hybrid MRI treatment devices and their potential for real-time adaptive radiotherapy.

Sub-10 nm Water-Dispersible β-NaGdF4:X% Eu3+ Nanoparticles with Enhanced Biocompatibility for in Vivo X-ray Luminescence Computed Tomography

As a novel molecular and functional imaging modality, X-ray luminescence computed tomography (XLCT) has shown its potentials in biomedical and preclinic applications. However, there are still some limitations of X-ray-excited luminescent materials, such as low luminescence efficiency, poor biocompatibility, and cytotoxicity, making in vivo XLCT imaging quite challenging. In this study, for the very first time, we present on using sub-10 nm β-NaGdF₄:X% Eu³⁺ nanoparticles with poly(acrylic acid) (PAA) surface modification, which demonstrate outstanding luminescence efficiency, uniform size distribution, water dispersity, and biosafety, as the luminescent probes for in vivo XLCT application. The pure hexagonal phase (β-) NaGdF₄ has been successfully synthesized and characterized by X-ray powder diffraction (XRD) and transmission electron microscopy (TEM), and then the results of X-ray photoelectron spectroscopy (XPS), energy-dispersive X-ray spectrometry  (EDX), and elemental mapping further confirm Eu³⁺ ions doped into NaGdF₄ host. Under X-ray excitation, the β-NaGdF₄ nanoparticles with a doping level of 15% Eu³⁺ exhibited the most efficient luminescence intensity. Notably, the doping level of Eu³⁺ has no effect on the crystal phase and morphology of the NaGdF₄-based host. Afterward, β-NaGdF₄:15% Eu³⁺ nanoparticles were modified with PAA to enhance the water dispersity and biocompatibility. The compatibility of in vivo XLCT imaging using such nanoparticles was systematically studied via in vitro cytotoxicity, physical phantom, and in vivo imaging experiments. The ultralow cytotoxicity of PAA-modified nanoparticles, which is confirmed by over 80% cell viability of SH-SY5Y cells when treated by high nanoparticle concentration of 200 μg/mL, overcome the major obstacle for in vivo application. In addition, the high luminescence intensity of PAA-modified nanoparticles enables the location error of in vivo XLCT imaging less than 2 mm, which is comparable to that using commercially available bulk material Y₂O₃:15% Eu³⁺. The proposed nanoparticles promote XLCT research into an in vivo stage. Further modification of these nanoparticles with biofunctional molecules could enable the potential of targeting XLCT imaging.

Magnitude, Impact, and Management of Respiration-induced Target Motion in Radiotherapy Treatment: A Comprehensive Review

Tumors in thoracic and upper abdomen regions such as lungs, liver, pancreas, esophagus, and breast move due to respiration. Respiration-induced motion introduces uncertainties in radiotherapy treatments of these sites and is regarded as a significant bottleneck in achieving highly conformal dose distributions. Recent developments in radiation therapy have resulted in (i) motion-encompassing, (ii) respiratory gating, and (iii) tracking methods for adapting the radiation beam aperture to account for the respiration-induced target motion. The purpose of this review is to discuss the magnitude, impact, and management of respiration-induced tumor motion.

Examination of a deformable motion model for respiratory movements and 4D dose calculations using different driving surrogates

PURPOSE

The aim of this study was to evaluate a surrogate-driven motion model based on four-dimensional computed tomography that is able to predict CT volumes corresponding to arbitrary respiratory phases. Furthermore, the comparison of three different driving surrogates is examined and the feasibility of using the model for 4D dose re-calculation will be discussed.

METHODS

The study is based on repeated 4DCTs of twenty patients treated for bronchial carcinoma and metastasis. The motion model was estimated from the planning 4DCT through deformable image registration. To predict a certain phase of a follow-up 4DCT, the model considers inter-fractional variations (baseline correction) and intra-fractional respiratory parameters (amplitude and phase) derived from surrogates. The estimated volumes resulting from the model were compared to ground-truth clinical 4DCTs using absolute HU differences in the lung region and landmarks localized using the Scale Invariant Feature Transform. Finally, the γ-index was used to evaluate the dosimetric effects of the intensity differences measured between the estimated and the ground-truth CT volumes.

RESULTS

The results show absolute HU differences between estimated and ground-truth images with median value (± standard deviation) of (61.3 ± 16.7) HU. Median 3D distances, measured on about 400 matching landmarks in each volume, were (2.9 ± 3.0) mm. 3D errors up to 28.2 mm were found for CT images with artifacts or reduced quality. Pass rates for all surrogate approaches were above 98.9% with a γ-criterion of 2%/2 mm.

CONCLUSION

The results depend mainly on the image quality of the initial 4DCT and the deformable image registration. All investigated surrogates can be used to estimate follow-up 4DCT phases, however, uncertainties decrease for volumetric approaches. Application of the model for 4D dose calculations is feasible.

Validation of dynamic treatment-couch tracking for prostate SBRT

PURPOSE

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.

METHODS

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.

RESULTS

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 GAI_1%/1mm with tracked (range: 83.4%-100.0%) than with untracked motion (28.9%-99.7%). Also GAI_2%/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.

CONCLUSIONS

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.

Advances in radiotherapy techniques and delivery for non-small cell lung cancer: benefits of intensity-modulated radiation therapy, proton therapy, and stereotactic body radiation therapy

The 21st century has seen several paradigm shifts in the treatment of non-small cell lung cancer (NSCLC) in early-stage inoperable disease, definitive locally advanced disease, and the postoperative setting. A key driver in improvement of local disease control has been the significant evolution of radiation therapy techniques in the last three decades, allowing for delivery of definitive radiation doses while limiting exposure of normal tissues. For patients with locally-advanced NSCLC, the advent of volumetric imaging techniques has allowed a shift from 2-dimensional approaches to 3-dimensional conformal radiation therapy (3DCRT). The next generation of 3DCRT, intensity-modulated radiation therapy and volumetric-modulated arc therapy (VMAT), have enabled even more conformal radiation delivery. Clinical evidence has shown that this can improve the quality of life for patients undergoing definitive management of lung cancer. In the early-stage setting, conventional fractionation led to poor outcomes. Evaluation of altered dose fractionation with the previously noted technology advances led to advent of stereotactic body radiation therapy (SBRT). This technique has dramatically improved local control and expanded treatment options for inoperable, early-stage patients. The recent development of proton therapy has opened new avenues for improving conformity and the therapeutic ratio. Evolution of newer proton therapy techniques, such as pencil-beam scanning (PBS), could improve tolerability and possibly allow reexamination of dose escalation. These new progresses, along with significant advances in systemic therapies, have improved survival for lung cancer patients across the spectrum of non-metastatic disease. They have also brought to light new challenges and avenues for further research and improvement.

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 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.

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.

Online 4D ultrasound guidance for real-time motion compensation by MLC tracking

PURPOSE

With the trend in radiotherapy moving toward dose escalation and hypofractionation, the need for highly accurate targeting increases. While MLC tracking is already being successfully used for motion compensation of moving targets in the prostate, current real-time target localization methods rely on repeated x-ray imaging and implanted fiducial markers or electromagnetic transponders rather than direct target visualization. In contrast, ultrasound imaging can yield volumetric data in real-time (3D + time = 4D) without ionizing radiation. The authors report the first results of combining these promising techniques-online 4D ultrasound guidance and MLC tracking-in a phantom.

METHODS

A software framework for real-time target localization was installed directly on a 4D ultrasound station and used to detect a 2 mm spherical lead marker inside a water tank. The lead marker was rigidly attached to a motion stage programmed to reproduce nine characteristic tumor trajectories chosen from large databases (five prostate, four lung). The 3D marker position detected by ultrasound was transferred to a computer program for MLC tracking at a rate of 21.3 Hz and used for real-time MLC aperture adaption on a conventional linear accelerator. The tracking system latency was measured using sinusoidal trajectories and compensated for by applying a kernel density prediction algorithm for the lung traces. To measure geometric accuracy, static anterior and lateral conformal fields as well as a 358° arc with a 10 cm circular aperture were delivered for each trajectory. The two-dimensional (2D) geometric tracking error was measured as the difference between marker position and MLC aperture center in continuously acquired portal images. For dosimetric evaluation, VMAT treatment plans with high and low modulation were delivered to a biplanar diode array dosimeter using the same trajectories. Dose measurements with and without MLC tracking were compared to a static reference dose using 3%/3 mm and 2%/2 mm γ-tests.

RESULTS

The overall tracking system latency was 172 ms. The mean 2D root-mean-square tracking error was 1.03 mm (0.80 mm prostate, 1.31 mm lung). MLC tracking improved the dose delivery in all cases with an overall reduction in the γ-failure rate of 91.2% (3%/3 mm) and 89.9% (2%/2 mm) compared to no motion compensation. Low modulation VMAT plans had no (3%/3 mm) or minimal (2%/2 mm) residual γ-failures while tracking reduced the γ-failure rate from 17.4% to 2.8% (3%/3 mm) and from 33.9% to 6.5% (2%/2 mm) for plans with high modulation.

CONCLUSIONS

Real-time 4D ultrasound tracking was successfully integrated with online MLC tracking for the first time. The developed framework showed an accuracy and latency comparable with other MLC tracking methods while holding the potential to measure and adapt to target motion, including rotation and deformation, noninvasively.

Does Motion Assessment With 4-Dimensional Computed Tomographic Imaging for Non-Small Cell Lung Cancer Radiotherapy Improve Target Volume Coverage?

Introduction:

Modern radiotherapy with 4-dimensional computed tomographic (4D-CT) image acquisition for non–small cell lung cancer (NSCLC) captures respiratory-mediated tumor motion to provide more accurate target delineation. This study compares conventional 3-dimensional (3D) conformal radiotherapy (3DCRT) plans generated with standard helical free-breathing CT (FBCT) with plans generated on 4D-CT contoured volumes to determine whether target volume coverage is affected.

Materials and methods:

Fifteen patients with stage I to IV NSCLC were enrolled in the study. Free-breathing CT and 4D-CT data sets were acquired at the same simulation session and with the same immobilization. Gross tumor volume (GTV) for primary and/or nodal disease was contoured on FBCT (GTV_3D). The 3DCRT plans were obtained, and the patients were treated according to our institution’s standard protocol using FBCT imaging. Gross tumor volume was contoured on 4D-CT for primary and/or nodal disease on all 10 respiratory phases and merged to create internal gross tumor volume (IGTV)_4D. Clinical target volume margin was 5 mm in both plans, whereas planning tumor volume (PTV) expansion was 1 cm axially and 1.5 cm superior/inferior for FBCT-based plans to incorporate setup errors and an estimate of respiratory-mediated tumor motion vs 8 mm isotropic margin for setup error only in all 4D-CT plans. The 3DCRT plans generated from the FBCT scan were copied on the 4D-CT data set with the same beam parameters. GTV_3D, IGTV_4D, PTV, and dose volume histogram from both data sets were analyzed and compared. Dice coefficient evaluated PTV similarity between FBCT and 4D-CT data sets.

Results:

In total, 14 of the 15 patients were analyzed. One patient was excluded as there was no measurable GTV. Mean GTV_3D was 115.3 cm³ and mean IGTV_4D was 152.5 cm³ (P = .001). Mean PTV_3D was 530.0 cm³ and PTV_4D was 499.8 cm³ (P = .40). Both gross primary and nodal disease analyzed separately were larger on 4D compared with FBCT. D95 (95% isodose line) covered 98% of PTV_3D and 88% of PTV_4D (P = .003). Mean dice coefficient of PTV_3D and PTV_4D was 84%. Mean lung V20 was 24.0% for the 3D-based plans and 22.7% for the 4D-based plans (P = .057). Mean heart V40 was 12.1% for the 3D-based plans and 12.7% for the 4D-based plans (P = .53). Mean spinal cord D_max was 2517 and 2435 cGy for 3D-based and 4D-based plans, respectively (P = .019). Mean esophageal dose was 1580 and 1435 cGy for 3D and 4D plans, respectively (P = .13).

Conclusions:

IGTV_4D was significantly larger than GTV_3D for both primary and nodal disease combined or separately. Mean PTV_3D was larger than PTV_4D, but the difference was not statistically significant. The PTV_4D coverage with 95% isodose line was inferior, indicating the importance of incorporating the true size and shape of the target volume. Relatively less dose was delivered to spinal cord and esophagus with plans based on 4D data set. Dice coefficient analysis for degree of similarity revealed that 16% of PTVs from both data sets did not overlap, indicating different anatomical positions of the PTV due to tumor/nodal motion during a respiratory cycle. All patients with lung cancer planned for radical radiotherapy should have 4D-CT simulation to ensure accurate coverage of the target volumes.