Feasibility of using ultrasound for real-time tracking during radiotherapy

Enhancing Image-Guided Radiation Therapy for Pancreatic Cancer: Utilizing Aligned Peak Response Beamforming in Flexible Array Transducers

To develop ultrasound-guided radiotherapy, we proposed an assistant structure with embedded markers along with a novel alternative method, the Aligned Peak Response (APR) method, to alter the conventional delay-and-sum (DAS) beamformer for reconstructing ultrasound images obtained from a flexible array. We simulated imaging targets in Field-II using point target phantoms with point targets at different locations. In the experimental phantom ultrasound images, image RF data were acquired with a flexible transducer with in-house assistant structures embedded with needle targets for testing the accuracy of the APR method. The lateral full width at half maximum (FWHM) values of the objective point target (OPT) in ground truth ultrasound images, APR-delayed ultrasound images with a flat shape, and images acquired with curved transducer radii of 500 mm and 700 mm were 3.96 mm, 4.95 mm, 4.96 mm, and 4.95 mm. The corresponding axial FWHM values were 1.52 mm, 4.08 mm, 5.84 mm, and 5.92 mm, respectively. These results demonstrate that the proposed assistant structure and the APR method have the potential to construct accurate delay curves without external shape sensing, thereby enabling a flexible ultrasound array for tracking pancreatic tumor targets in real time for radiotherapy.

A Multimodal Point Cloud-Based Method for Tumor Localization in Robotic Ultrasound-Guided Radiotherapy

Objectives: Part of the tumor localization methods in radiotherapy have poor real-time performance and may generate additional radiation. We propose a multimodal point cloud-based method for tumor localization in robotic ultrasound-guided radiotherapy, which only irradiates computed tomography (CT) during radiotherapy planning to avoid additional radiation. Methods: The tumor position was determined using the CT point cloud, and the red green blue depth (RGBD) point cloud was used to determine body surface scanning location corresponding to the tumor location. The relationship between the CT point cloud and RGBD point cloud was established through multi-modal point cloud registration. The point cloud was then used for robot tumor localization through coordinate transformation between camera and robot. Results: The maximum mean absolute error of the tumor location in the X, Y, and Z directions of the robot coordinate system were 0.781, 1.334, and 1.490 mm, respectively. The average point-to-point translation mean absolute error between the actual and predicted positions of the localization points was 1.847 mm. The maximum error in the random positioning experiment was 1.77 mm. Conclusion: The proposed method is radiation free and has real-time performance, with tumor localization accuracy that meets the requirements of radiotherapy. The proposed method, which potentially reduces the risks associated with radiation exposure while ensuring efficient and accurate tumor localization, represents a promising advancement in the field of radiotherapy.

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 > 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.

An integrated ultrasound imaging and abdominal compression device for respiratory motion management in radiation therapy

BACKGROUND

Radiotherapy to tumors in the abdomen is challenging because of the significant organ movement and tissue deformation caused by respiration.

PURPOSE

A motion management strategy that integrated ultrasound (US) imaging with abdominal compression was developed and evaluated, where US was used to real-time monitor organ motion after abdominal compression.

METHODS

A device that combined a US imaging system and an abdominal compression plate (ACP) was developed. Twenty-one healthy volunteers were involved to evaluate the motion management efficacy. Each volunteer was immobilized on a flat bench by the device. Abdominal US data were successively collected with and without ACP compression and experiments were repeated three times to verify the imaging reproducibility. A template matching algorithm based on normalized cross correlation (NCC) was implemented to track the targets (vessels in the liver, pancreas and stomach) automatically. The matching algorithm was validated by comparing with the manual references. Automatic tracking was judged as failed if the center of mass difference from manual tracking was beyond a failure threshold. Based on the locations obtained through the template matching algorithm, the motion correlation between liver and pancreas/stomach was investigated using Pearson correlation test. Paired Student's t-test was used to analyze the difference between the results without and with ACP compression.

RESULTS

The liver motion amplitude over all 21 volunteers was significantly (p<0.001) reduced from 14.9 ± 5.5/3.4 ± 1.8 mm in superior-inferior (SI)/anterior-posterior (AP) direction before ACP compression to 7.3 ± 1.5/1.6 ± 0.7 mm after ACP compression. The mean liver motion standard deviation before compression was on average 2.8/1.4 mm in SI/AP direction, and was significantly (p<0.001) reduced to 0.9/0.4 mm after compression. The failure rates of automatic tracking for liver, pancreas and stomach were reduced for failure thresholds of 1-5 mm after applying ACP. The Pearson correlation coefficients between liver and pancreas/stomach were 0.98/0.97 without ACP and 0.96/0.94 with ACP in SI direction, and were 0.68/0.68 and 0.43/0.42 in AP direction. The motion prediction errors for pancreas/stomach with ACP have significantly (p<0.001) reduced to 0.45 ± 0.36/0.52 ± 0.43 mm from 0.69 ± 0.56/0.71 ± 0.66 mm without ACP in SI direction, and to 0.38 ± 0.33/0.39 ± 0.27 mm from 0.44 ± 0.35/0.61 ± 0.59 mm in AP direction.

CONCLUSIONS

The proposed strategy that combines real-time US imaging and abdominal compression has the potential to reduce the abdominal organ motion while improving both target tracking reliability and motion reproducibility. Furthermore, the observed correlation between liver and pancreas/stomach motion indicates the possibility of indirect pancreas/stomach tracking using liver markers as tracking surrogates. The strategy is expected to provide an alternative for respiratory motion management in the radiation treatment of abdominal tumors. This article is protected by copyright. All rights reserved.

Deep Learning for Ultrasound Beamforming in Flexible Array Transducer

Ultrasound imaging has been developed for image-guided radiotherapy for tumor tracking, and the flexible array transducer is a promising tool for this task. It can reduce the user dependence and anatomical changes caused by the traditional ultrasound transducer. However, due to its flexible geometry, the conventional delay-and-sum (DAS) beamformer may apply incorrect time delay to the radio-frequency (RF) data and produce B-mode images with considerable defocusing and distortion. To address this problem, we propose a novel end-to-end deep learning approach that may alternate the conventional DAS beamformer when the transducer geometry is unknown. Different deep neural networks (DNNs) were designed to learn the proper time delays for each channel, and they were expected to reconstruct the undistorted high-quality B-mode images directly from RF channel data. We compared the DNN results to the standard DAS beamformed results using simulation and flexible array transducer scan data. With the proposed DNN approach, the averaged full-width-at-half-maximum (FWHM) of point scatters is 1.80 mm and 1.31 mm lower in simulation and scan results, respectively; the contrast-to-noise ratio (CNR) of the anechoic cyst in simulation and phantom scan is improved by 0.79 dB and 1.69 dB, respectively; and the aspect ratios of all the cysts are closer to 1. The evaluation results show that the proposed approach can effectively reduce the distortion and improve the lateral resolution and contrast of the reconstructed B-mode images.

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.

3D dosimetric validation of ultrasound-guided radiotherapy with a dynamically deformable abdominal phantom

OBJECTIVES

The purpose of this study was to dosimetrically benchmark gel dosimetry measurements in a dynamically deformable abdominal phantom for intrafraction image guidance through a multi-dosimeter comparison. Once benchmarked, the study aimed to perform a proof-of-principle study for validation measurements of an ultrasound image-guided radiotherapy delivery system.

METHODS

The phantom was dosimetrically benchmarked by delivering a liver VMAT plan and measuring the 3D dose distribution with DEFGEL dosimeters. Measured doses were compared to the treatment planning system and measurements acquired with radiochromic film and an ion chamber. The ultrasound image guidance validation was performed for a hands-free ultrasound transducer for the tracking of liver motion during treatment.

RESULTS

Gel dosimeters were compared to the TPS and film measurements, showing good qualitative dose distribution matches, low γ values through most of the high dose region, and average 3%/5 mm γ-analysis pass rates of 99.2%(0.8%) and 90.1%(0.8%), respectively. Gel dosimeter measurements matched ion chamber measurements within 3%. The image guidance validation study showed the measurement of the treatment delivery improvements due to the inclusion of the ultrasound image guidance system. Good qualitative matching of dose distributions and improvements of the γ-analysis results were observed for the ultrasound-gated dosimeter compared to the ungated dosimeter.

CONCLUSIONS

DEFGEL dosimeters in phantom showed good agreement with the planned dose and other dosimeters for dosimetric benchmarking. Ultrasound image guidance validation measurements showed good proof-of-principle of the utility of the phantom system as a method of validating ultrasound-based image guidance systems and potentially other image guidance methods.

Systematic review of the role of high intensity focused ultrasound (HIFU) in treating malignant lesions of the hepatobiliary system

BACKGROUND

High Intensity Focused Ultrasound (HIFU) is an emerging non-invasive, targeted treatment of malignancy. The aim of this review was to assess the efficacy, safety and optimal technical parameters of HIFU to treat malignant lesions of the hepatobiliary system.

METHODS

A systematic search of the English literature was performed until March 2020, interrogating Pubmed, Embase and Cochrane Library databases. The following key-words were input in various combinations: 'HIFU', 'High intensity focussed ultrasound', 'Hepatobiliary', 'Liver', 'Cancer' and 'Carcinoma'. Extracted content included: Application type, Exposure parameters, Patient demographics, and Treatment outcomes.

RESULTS

Twenty-four articles reported on the clinical use of HIFU in 940 individuals to treat malignant liver lesions. Twenty-one studies detailed the use of HIFU to treat hepatocellular carcinoma only. Mean tumour size was 5.1 cm. Across all studies, HIFU resulted in complete tumour ablation in 55% of patients. Data on technical parameters and the procedural structure was very heterogeneous. Ten studies (n = 537 (57%) patients) described the use of HIFU alongside other modalities including TACE, RFA and PEI; 66% of which resulted in complete tumour ablation. Most common complications were skin burns (15%), local pain (5%) and fever (2%).

CONCLUSION

HIFU has demonstrated benefit as a treatment modality for malignant lesions of the hepatobiliary system. Combining HIFU with other ablative therapies, particularly TACE, increases the efficacy without increasing complications. Future human clinical studies are required to determine the optimal treatment parameters, better define outcomes and explore the risks and benefits of combination therapies.

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.

An improved abdominal phantom for intrafraction image guidance validation

A dynamically compressible phantom of the human abdomen that simulates organ motion with breathing is being developed for possible testing of image-gated beam delivery in radiotherapy. The polyvinyl chloride plastisol (PVCP) phantom features a cavity that can contain a deformable normoxic polyacrylamide gel (nPAG) dosimeter that is intended for use with MRI to provide dosimetric data. The phantom has been improved by the inclusion of new components that are more realistic anatomically and exhibit CT values similar to those of the tissues they mimic. Component organs were made from 3D-printed molds developed from CT contours of a real patient and their radiodensities adjusted by varying the mass ratios of the PVCP hardener and softener during manufacture. To make the phantom more compatible with ultrasound imaging a graphite scatterer was mixed into some of the phantom components to produce a background speckle pattern. This provided contrast between the body and a moving anatomical target intended for motion tracking. Phantom insert motion magnitude and repeatibility was assessed using CT by imaging two phantom inserts, one containing fiducial markers and the other containing iodinated gelatin, at the same position after repeated cycles of deformation. The maximum motion of a phantom fiducial at the position of the phantom treatment target was found to be 12.2 mm. The phantom design resulted in dosimeter motion with a point-to-point repatability within 0.3 mm on average and contour repeatability resulting in Dice coefficients exceeding 0.98 on average.

Dosimetric assessment of patient-specific breath-hold reproducibility on liver motion for SBRT planning

PURPOSE

To investigate the impact of breath-hold reproducibility on liver motion using a respiratory motion management device.

METHODS

Forty-four patients with hepatic tumors, treated with SBRT with breath-hold, were randomly selected for this study. All patients underwent three consecutive computed tomography (CT) scans using active breath-hold coordinator (ABC) with three repeated single breath-hold during simulation. The three CT scans were labeled as ABC1-CT, ABC2-CT, and ABC3-CT. Displacements of centroids of the entire livers among the three ABC-CTs were measured as a surrogate for intrafractional motion. For each patient, two different treatment plans were prepared: (a) a clinical plan using a 5-mm expansion of an ITV that encompassed all three GTVs from each of the three ABC-CTs, and (b) a research plan using a 5-mm expansion of the GTV from only ABC1-CT to create PTV. The clinical plan acceptance criteria were that 95% of the PTV and 99% of the GTV received 100% of the prescription dose. Dosimetric endpoints were analyzed and compared for the two plans.

RESULTS

All shifts in the medial-lateral direction (range: -3.9 to 2.0 mm) were within 5 mm while 7% of shifts in the anterior-posterior direction (range: -10.5 to 16.7 mm) and 11% of shifts in the superior-inferior direction (range: -17.0 to 8.7 mm) exceeded 5 mm. Six patients (14%) had an intrafraction motion greater than 5 mm in any direction. For these six patients, if a plan was created based on a PTV from a single CT (ex. ABC1-CT), 5 of 12 GTVs captured from other ABC-CTs would fail to meet the clinical acceptance criteria due to poor breath-hold reproducibility.

CONCLUSIONS

Non-negligible intrafractional motion occurs in patients with poor breath-hold reproducibility. To identify this subgroup of patients, acquiring three CTs with active breath-hold during simulation is a feasible practical method.

The first prospective implementation of markerless lung target tracking in an experimental quality assurance procedure on a standard linear accelerator

The ability to track tumour motion without implanted markers on a standard linear accelerator (linac) could enable wide access to real-time adaptive radiotherapy for cancer patients. We previously have retrospectively validated a method for 3D markerless target tracking using intra-fractional kilovoltage (kV) projections acquired on a standard linac. This paper presents the first prospective implementation of markerless lung target tracking on a standard linac and its quality assurance (QA) procedure. The workflow and the algorithm developed to track the 3D target position during volumetric modulated arc therapy treatment delivery were optimised. The linac was operated in clinical QA mode, while kV projections were streamed to a dedicated computer using a frame-grabber software. The markerless target tracking accuracy and precision were measured in a lung phantom experiment under the following conditions: static localisation of seven distinct positions, dynamic localisation of five patient-measured motion traces, and dynamic localisation with treatment interruption. The QA guidelines were developed following the AAPM Task Group 147 report with the requirement that the tracking margin components, the margins required to account for tracking errors, did not exceed 5 mm in any direction. The mean tracking error ranged from 0.0 to 0.9 mm (left-right), -0.6 to  -0.1 mm (superior-inferior) and  -0.7 to 0.1 mm (anterior-posterior) over the three tests. Larger errors were found in cases with large left-right or anterior-posterior and small superior-inferior motion. The tracking margin components did not exceed 5 mm in any direction and ranged from 0.4 to 3.2 mm (left-right), 0.7 to 1.6 mm (superior-inferior) and 0.8 to 1.5 mm (anterior-posterior). This study presents the first prospective implementation of markerless lung target tracking on a standard linac and provides a QA procedure for its safe clinical implementation, potentially enabling real-time adaptive radiotherapy for a large population of lung cancer patients.

Long-Duration Tracking of Cervical-Spine Kinematics With Ultrasound

Cervical-spine (C-spine) pathoanatomy is commonly evaluated by plane radiographs, computed tomography (CT), or magnetic resonance imaging (MRI); however, these modalities are unable to directly measure the dynamic mechanical properties of the functional spinal units (FSU) comprising the C-spine that account for its functional performance. We have developed an ultrasound-based technique that provides a non-invasive, real-time, quantitative, in vivo assessment of C-spine kinematics and FSU viscoelastic properties. The fidelity of the derived measurements is predicated on accurate tracking of vertebral motion over a prolonged time duration. The purpose of this work was to present a bundle adjustment method that enables accurate tracking of the relative motion of contiguous cervical vertebrae from ultrasound radio-frequency data. The tracking method was validated using both a plastic anatomical model of a cervical vertebra undergoing prescribed displacements and also human cadaveric C-spine specimens subjected to physiologically relevant loading configurations. While the velocity of motion and thickness of the surrounding soft tissue envelope affected accuracy, using the bundle adjustment method, B-mode ultrasound was capable of accurately tracking vertebral motion under clinically relevant physiologic conditions. Therefore, B-mode ultrasound can be used to evaluate in vivo real-time C-spine kinematics and FSU mechanical properties in environments where radiographs, CT, or MRI cannot be used.

Correlation between intrafractional motion and dosimetric changes for prostate IMRT: Comparison of different adaptive strategies

Purpose

To retrospectively analyze and estimate the dosimetric benefit of online and offline motion mitigation strategies for prostate IMRT.

Methods

Intrafractional motion data of 21 prostate patients receiving intensity‐modulated radiotherapy was acquired with an electromagnetic tracking system. Target trajectories of 734 fractions were analyzed per delivered multileaf‐collimator segment in five motion metrics: three‐dimensional displacement, distance from beam axis (DistToBeam), and three orthogonal components. Time‐resolved dose calculations have been performed by shifting the target according to the sampled motion for the following scenarios: without adaptation, online‐repositioning with a minimum threshold of 3 mm, and an offline approach using a modified field order applying horizontal before vertical beams. Change of D95 (targets) or V65 (organs at risk) relative to the static case, that is, ΔD95 or ΔV65, was extracted per fraction in percent. Correlation coefficients (CC) between the motion metrics and the dose metrics were extracted. Mean of patient‐wise CC was used to evaluate the correlation of motion metric and dosimetric changes. Mean and standard deviation of the patient‐wise correlation slopes (in %/mm) were extracted.

Results

For ΔD95 of the prostate, mean DistToBeam per fraction showed the highest correlation for all scenarios with a relative change of −0.6 ± 0.7%/mm without adaptation and −0.4 ± 0.5%/mm for the repositioning and field order strategies. For ΔV65 of the bladder and the rectum, superior–inferior and posterior–anterior motion components per fraction showed the highest correlation, respectively. The slope of bladder (rectum) was 14.6 ± 5.8 (15.1 ± 6.9) %/mm without adaptation, 14.0 ± 4.9 (14.5 ± 7.4) %/mm for repositioning with 3 mm, and 10.6 ± 2.5 (8.1 ± 4.6) %/mm for the field order approach.

Conclusions

The correlation slope is a valuable concept to estimate dosimetric deviations from static plan quality directly based on the observed motion. For the prostate, both mitigation strategies showed comparable benefit. For organs at risk, the field order approach showed less sensitive response regarding motion and reduced interpatient variation.

A block matching based approach with multiple simultaneous templates for the real-time 2D ultrasound tracking of liver vessels

PURPOSE

The implementation of motion management techniques in radiation therapy can aid in mitigating uncertainties and reducing margins. For motion management to be effective, it is necessary to track key structures both accurately and at a real-time speed. Therefore, the focus of this work was to develop a 2D algorithm for the real-time tracking of ultrasound features to aid in radiation therapy motion management.

MATERIALS AND METHODS

The developed algorithm utilized a similarity measure-based block matching algorithm incorporating training methods and multiple simultaneous templates. The algorithm is broken down into three primary components, all of which use normalized cross-correlation (NCC) as a similarity metric. First, a global feature shift to account for gross displacements from the previous frame is determined using large block sizes which encompass the entirety of the feature. Second, the most similar reference frame is chosen from a series of training images that are accumulated during the first K frames of tracking to aid in contour consistency and provide a starting point for the localized template initialization. Finally, localized block matching is performed through the simultaneous use of both a training frame and the previous frame. The localized block matching utilizes a series of templates positioned at the boundary points of the training and previous contours. The weighted final boundary points from both the previous and the training frame are ultimately combined and used to determine an affine transformation from the previous frame to the current frame.

RESULTS

A mean tracking error of 0.72 ± 1.25 mm was observed for 85 point-landmarks across 39 ultrasound sequences relative to manual ground truth annotations. The image processing speed per landmark with the GPU implementation was between 41 and 165 frames per second (fps) during the training set accumulation, and between 73 and 234 fps after training set accumulation. Relative to a comparable multithreaded CPU approach using OpenMP, the GPU implementation resulted in speedups between -30% and 355% during training set accumulation, and between -37% and 639% postaccumulation.

CONCLUSIONS

Initial implementations indicated an accuracy that was comparable to or exceeding those achieved by alternative 2D tracking methods, with a computational speed that is more than sufficient for real-time applications in a radiation therapy environment. While the overall performance reached levels suitable for implementation in radiation therapy, the observed increase in failures for smaller features, as well as the algorithm's inability to be applied to nonconvex features warrants additional investigation to address the shortcomings observed.

A Monte Carlo study of the effect of an ultrasound transducer on surface dose during intrafraction motion imaging for external beam radiation therapy

PURPOSE

The aim of this study was to estimate changes in surface dose due to the presence of the Clarity Autoscan™ ultrasound (US) probe during prostate radiotherapy using Monte Carlo (MC) methods.

METHODS

MC models of the Autoscan US probe were developed using the BEAMnrc/DOSXYZnrc code based on kV and MV CT images. CT datasets were converted to voxelized mass density phantoms using a CT number-to-mass density calibration. The dosimetric effect of the probe, in the contact region (an 8 mm × 12 mm single layer of voxels), was investigated using a phantom set-up mimicking two scenarios (a) a transperineal imaging configuration (radiation beam perpendicular to the central US axial direction), and (b) a transabdominal imaging configuration (radiation beam parallel to the central US axial direction). For scenario (a), the dosimetric effect was evaluated as a function of the probe to inferior radiation field edge distance. Clinically applicable distances from 5 mm separation to 2 mm overlap were determined from the radiotherapy plans of 27 patients receiving Clarity imaging. Overlaps of 3 to 14 (1 to 3 SD) mm were also considered to include the effect of interfraction motion correction. The influence of voxel size on surface dose estimation was investigated. Approved clinical plans from two prostate patients were used to simulate worst-case dosimetric impact of the probe when large couch translations were applied to correct for interfraction prostate motion.

RESULTS

The dosimetric impact of both the MV and kV probe models agreed within ±2% for both beam configurations. For scenario (a) and 1 mm voxel model, the probe gave mean dose increases of 1.2% to 4.6% (of the dose at isocenter) for 5 mm separation to 0 mm overlap in the probe-phantom contact region, respectively. This increased to 27.5% for the largest interfraction motion correction considered (14 mm overlap). For separations of ≥ 2 mm dose differences were < 2%. Simulated dose perturbations were found to be superficial; for the 14 mm overlap the dose increase reduced to < 3% at 5.0 mm within the phantom. For scenario (b), dose increases due to the probe were < 5% in all cases. The dose increase was underestimated by up to ~13% when the voxel size was increased from 1 mm to 3 mm. MC simulated dose to the PTV and OARs for the two clinical plans considered showed good agreement with commercial treatment planning system results (within 2%). Mean dose increases due to the presence of the probe, after the maximum interfraction motion correction, were ~16.3% and ~8.0%, in the contact region, for plan 1 and plan 2, respectively.

CONCLUSIONS

The presence of the probe results in superficial dose perturbations for patients with an overlap between the probe and the radiation field present in either the original treatment plan or due to translation of the radiation field to simulate correction of interfraction internal prostate motion.

Investigation of the optimum location of external markers for patient setup accuracy enhancement at external beam radiotherapy

In external beam radiotherapy, one of the most common and reliable methods for patient geometrical setup and/or predicting the tumor location is use of external markers. In this study, the main challenging issue is increasing the accuracy of patient setup by investigating external markers location. Since the location of each external marker may yield different patient setup accuracy, it is important to assess different locations of external markers using appropriate selective algorithms. To do this, two commercially available algorithms entitled a) canonical correlation analysis (CCA) and b) principal component analysis (PCA) were proposed as input selection algorithms. They work on the basis of maximum correlation coefficient and minimum variance between given datasets. The proposed input selection algorithms work in combination with an adaptive neuro-fuzzy inference system (ANFIS) as a correlation model to give patient positioning information as output. Our proposed algorithms provide input file of ANFIS correlation model accurately. The required dataset for this study was prepared by means of a NURBS-based 4D XCAT anthropomorphic phantom that can model the shape and structure of complex organs in human body along with motion information of dynamic organs. Moreover, a database of four real patients undergoing radiation therapy for lung cancers was utilized in this study for validation of proposed strategy. Final analyzed results demonstrate that input selection algorithms can reasonably select specific external markers from those areas of the thorax region where root mean square error (RMSE) of ANFIS model has minimum values at that given area. It is also found that the selected marker locations lie closely in those areas where surface point motion has a large amplitude and a high correlation.

The feasibility of using Microsoft Kinect v2 sensors during radiotherapy delivery

Consumer‐grade distance sensors, such as the Microsoft Kinect devices (v1 and v2), have been investigated for use as marker‐free motion monitoring systems for radiotherapy. The radiotherapy delivery environment is challenging for such sensors because of the proximity to electromagnetic interference (EMI) from the pulse forming network which fires the magnetron and electron gun of a linear accelerator (linac) during radiation delivery, as well as the requirement to operate them from the control area. This work investigated whether using Kinect v2 sensors as motion monitors was feasible during radiation delivery. Three sensors were used each with a 12 m USB 3.0 active cable which replaced the supplied 3 m USB 3.0 cable. Distance output data from the Kinect v2 sensors was recorded under four conditions of linac operation: (i) powered up only, (ii) pulse forming network operating with no radiation, (iii) pulse repetition frequency varied between 6 Hz and 400 Hz, (iv) dose rate varied between 50 and 1450 monitor units (MU) per minute. A solid water block was used as an object and imaged when static, moved in a set of steps from 0.6 m to 2.0 m from the sensor and moving dynamically in two sinusoidal‐like trajectories. Few additional image artifacts were observed and there was no impact on the tracking of the motion patterns (root mean squared accuracy of 1.4 and 1.1 mm, respectively). The sensors’ distance accuracy varied by 2.0 to 3.8 mm (1.2 to 1.4 mm post distance calibration) across the range measured; the precision was 1 mm. There was minimal effect from the EMI on the distance calibration data: 0 mm or 1 mm reported distance change (2 mm maximum change at one position). Kinect v2 sensors operated with 12 m USB 3.0 active cables appear robust to the radiotherapy treatment environment.

PACS number(s): 87.53 JW, 87.55 N‐, 87.63 L‐

A Feasibility Study on Ribs as Anatomical Landmarks for Motion Tracking of Lung and Liver Tumors at External Beam Radiotherapy

At external beam radiotherapy for some tumors located at thorax region due to lack of information in gray scale fluoroscopic images tumor position determination is problematic. One of the clinical strategies is to implant clip as internal fiducial marker inside or near tumor to represent tumor position while the contrast of implanted clip is highly observable rather than tumor. As alternative, using natural anatomical landmarks located at thorax region of patient body is proposed to extract tumor position information without implanting clips that is invasive method with possible side effect. Among natural landmarks, ribs of rib-cage structure that result proper visualization at X-ray images may be optimal as representative for tumor motion. In this study, we investigated the existence of possible correlation between ribs as natural anatomical landmarks and various lung and liver tumors located at different sites as challenging issue. A simulation study was performed using data extracted from 4-dimensional extended cardiac-torso anthropomorphic phantom that is able to simulate motion effect of dynamic organs, as well. Several tumor sites with predefined distances originated from chosen ribs at anterior-posterior direction were simulated at 3 upper, middle, and lower parts of chest. Correlation coefficient between ribs and tumors was calculated to investigate the robustness of ribs as anatomical landmarks for tumor motion tracking. Moreover, a consistent correlation model was taken into account to track tumor motion with a rib as best candidate among selected ribs. Final results represent availability of using rib cage as anatomical landmark to track lung and liver tumors in a noninvasive way. Observations of our calculations showed a proper correlation between tumors and ribs while the degree of this correlation is changing depends on tumor site while lung tumors are more varied and complex with less correlation with ribs motion against liver tumors.

Review of ultrasound image guidance in external beam radiotherapy part II: intra-fraction motion management and novel applications

Imaging has become an essential tool in modern radiotherapy (RT), being used to plan dose delivery prior to treatment and verify target position before and during treatment. Ultrasound (US) imaging is cost-effective in providing excellent contrast at high resolution for depicting soft tissue targets apart from those shielded by the lungs or cranium. As a result, it is increasingly used in RT setup verification for the measurement of inter-fraction motion, the subject of Part I of this review (Fontanarosa et al 2015 Phys. Med. Biol. 60 R77-114). The combination of rapid imaging and zero ionising radiation dose makes US highly suitable for estimating intra-fraction motion. The current paper (Part II of the review) covers this topic. The basic technology for US motion estimation, and its current clinical application to the prostate, is described here, along with recent developments in robust motion-estimation algorithms, and three dimensional (3D) imaging. Together, these are likely to drive an increase in the number of future clinical studies and the range of cancer sites in which US motion management is applied. Also reviewed are selections of existing and proposed novel applications of US imaging to RT. These are driven by exciting developments in structural, functional and molecular US imaging and analytical techniques such as backscatter tissue analysis, elastography, photoacoustography, contrast-specific imaging, dynamic contrast analysis, microvascular and super-resolution imaging, and targeted microbubbles. Such techniques show promise for predicting and measuring the outcome of RT, quantifying normal tissue toxicity, improving tumour definition and defining a biological target volume that describes radiation sensitive regions of the tumour. US offers easy, low cost and efficient integration of these techniques into the RT workflow. US contrast technology also has potential to be used actively to assist RT by manipulating the tumour cell environment and by improving the delivery of radiosensitising agents. Finally, US imaging offers various ways to measure dose in 3D. If technical problems can be overcome, these hold potential for wide-dissemination of cost-effective pre-treatment dose verification and in vivo dose monitoring methods. It is concluded that US imaging could eventually contribute to all aspects of the RT workflow.

Optimum location of external markers using feature selection algorithms for real-time tumor tracking in external-beam radiotherapy: a virtual phantom study

In external-beam radiotherapy, using external markers is one of the most reliable tools to predict tumor position, in clinical applications. The main challenge in this approach is tumor motion tracking with highest accuracy that depends heavily on external markers location, and this issue is the objective of this study. Four commercially available feature selection algorithms entitled 1) Correlation-based Feature Selection, 2) Classifier, 3) Principal Components, and 4) Relief were proposed to find optimum location of external markers in combination with two "Genetic" and "Ranker" searching procedures. The performance of these algorithms has been evaluated using four-dimensional extended cardiac-torso anthropomorphic phantom. Six tumors in lung, three tumors in liver, and 49 points on the thorax surface were taken into account to simulate internal and external motions, respectively. The root mean square error of an adaptive neuro-fuzzy inference system (ANFIS) as prediction model was considered as metric for quantitatively evaluating the performance of proposed feature selection algorithms. To do this, the thorax surface region was divided into nine smaller segments and predefined tumors motion was predicted by ANFIS using external motion data of given markers at each small segment, separately. Our comparative results showed that all feature selection algorithms can reasonably select specific external markers from those segments where the root mean square error of the ANFIS model is minimum. Moreover, the performance accuracy of proposed feature selection algorithms was compared, separately. For this, each tumor motion was predicted using motion data of those external markers selected by each feature selection algorithm. Duncan statistical test, followed by F-test, on final results reflected that all proposed feature selection algorithms have the same performance accuracy for lung tumors. But for liver tumors, a correlation-based feature selection algorithm, in combination with a genetic search algorithm, proved to yield best performance accuracy for selecting optimum markers.

[Ultrasound motion tracking for radiation therapy]

BACKGROUND

In modern radiotherapy the radiation dose can be applied with an accuracy in the range of 1-2 mm provided that the exact position of the target is known. If, however, the target (the tumor) is located in the lungs or the abdomen, respiration or peristalsis can cause substantial movement of the target.

METHODS

Various methods for intrafractional motion detection and compensation are currently under consideration or are already applied in clinical practice. Sonography is one promising option, which is now on the brink of clinical implementation. Ultrasound is particularly suited for this purpose due to the high soft tissue contrast, real-time capability, the absence of ionizing radiation and low acquisition costs. Ultrasound motion tracking is an image-based approach, i.e. the target volume or an adjacent structure is directly monitored and the motion is tracked automatically on the ultrasound image. Diverse algorithms are presently available that provide the real-time target coordinates from 2D as well as 3D images. Definition of a suitable sonographic window is not, however, trivial and a gold standard for positioning and mounting of the transducer has not yet been developed. Furthermore, processing of the coordinate information in the therapy unit and the dynamic adaptation of the radiation field are challenging tasks.

CONCLUSION

It is not clear whether ultrasound motion tracking will become established in the clinical routine although all technical prerequisites can be considered as fulfilled, such that exciting progress in this field of research is still to be expected.

Monte Carlo modeling of ultrasound probes for image guided radiotherapy

PURPOSE

To build Monte Carlo (MC) models of two ultrasound (US) probes and to quantify the effect of beam attenuation due to the US probes for radiation therapy delivered under real-time US image guidance.

METHODS

MC models of two Philips US probes, an X6-1 matrix-array transducer and a C5-2 curved-array transducer, were built based on their megavoltage (MV) CT images acquired in a Tomotherapy machine with a 3.5 MV beam in the EGSnrc, BEAMnrc, and DOSXYZnrc codes. Mass densities in the probes were assigned based on an electron density calibration phantom consisting of cylinders with mass densities between 0.2 and 8.0 g/cm(3). Beam attenuation due to the US probes in horizontal (for both probes) and vertical (for the X6-1 probe) orientation was measured in a solid water phantom for 6 and 15 MV (15 × 15) cm(2) beams with a 2D ionization chamber array and radiographic films at 5 cm depth. The MC models of the US probes were validated by comparison of the measured dose distributions and dose distributions predicted by MC. Attenuation of depth dose in the (15 × 15) cm(2) beams and small circular beams due to the presence of the probes was assessed by means of MC simulations.

RESULTS

The 3.5 MV CT number to mass density calibration curve was found to be linear with R(2) > 0.99. The maximum mass densities in the X6-1 and C5-2 probes were found to be 4.8 and 5.2 g/cm(3), respectively. Dose profile differences between MC simulations and measurements of less than 3% for US probes in horizontal orientation were found, with the exception of the penumbra region. The largest 6% dose difference was observed in dose profiles of the X6-1 probe placed in vertical orientation, which was attributed to inadequate modeling of the probe cable. Gamma analysis of the simulated and measured doses showed that over 96% of measurement points passed the 3%/3 mm criteria for both probes placed in horizontal orientation and for the X6-1 probe in vertical orientation. The X6-1 probe in vertical orientation caused the highest attenuation of the 6 and 15 MV beams, which at 10 cm depth accounted for 33% and 43% decrease compared to the respective (15 × 15) cm(2) open fields. The C5-2 probe in horizontal orientation, on the other hand, caused a dose increase of 10% and 53% for the 6 and 15 MV beams, respectively, in the buildup region at 0.5 cm depth. For the X6-1 probe in vertical orientation, the dose at 5 cm depth for the 3-cm diameter 6 MV and 5-cm diameter 15 MV beams was attenuated compared to the corresponding open fields to a greater degree by 65% and 43%, respectively.

CONCLUSIONS

MC models of two US probes used for real-time image guidance during radiotherapy have been built. Due to the high beam attenuation of the US probes, the authors generally recommend avoiding delivery of treatment beams that intersect the probe. However, the presented MC models can be effectively integrated into US-guided radiotherapy treatment planning in cases for which beam avoidance is not practical due to anatomy geometry.

An Ultrasound Image-Based Dynamic Fusion Modeling Method for Predicting the Quantitative Impact of In Vivo Liver Motion on Intraoperative HIFU Therapies: Investigations in a Porcine Model

Organ motion is a key component in the treatment of abdominal tumors by High Intensity Focused Ultrasound (HIFU), since it may influence the safety, efficacy and treatment time. Here we report the development in a porcine model of an Ultrasound (US) image-based dynamic fusion modeling method for predicting the effect of in vivo motion on intraoperative HIFU treatments performed in the liver in conjunction with surgery. A speckle tracking method was used on US images to quantify in vivo liver motions occurring intraoperatively during breathing and apnea. A fusion modeling of HIFU treatments was implemented by merging dynamic in vivo motion data in a numerical modeling of HIFU treatments. Two HIFU strategies were studied: a spherical focusing delivering 49 juxtapositions of 5-second HIFU exposures and a toroidal focusing using 1 single 40-second HIFU exposure. Liver motions during breathing were spatially homogenous and could be approximated to a rigid motion mainly encountered in the cranial-caudal direction (f = 0.20 Hz, magnitude > 13 mm). Elastic liver motions due to cardiovascular activity, although negligible, were detectable near millimeter-wide sus-hepatic veins (f = 0.96 Hz, magnitude < 1 mm). The fusion modeling quantified the deleterious effects of respiratory motions on the size and homogeneity of a standard "cigar-shaped" millimetric lesion usually predicted after a 5-second single spherical HIFU exposure in stationary tissues (Dice Similarity Coefficient: DSC < 45%). This method assessed the ability to enlarge HIFU ablations during respiration, either by juxtaposing "cigar-shaped" lesions with spherical HIFU exposures, or by generating one large single lesion with toroidal HIFU exposures (DSC > 75%). Fusion modeling predictions were preliminarily validated in vivo and showed the potential of using a long-duration toroidal HIFU exposure to accelerate the ablation process during breathing (from 0.5 to 6 cm3 · min(-1)). To improve HIFU treatment control, dynamic fusion modeling may be interesting for assessing numerically focusing strategies and motion compensation techniques in more realistic conditions.

Ultrasound Imaging in Radiation Therapy: From Interfractional to Intrafractional Guidance

External beam radiation therapy (EBRT) is included in the treatment regimen of the majority of cancer patients. With the proliferation of hypofractionated radiotherapy treatment regimens, such as stereotactic body radiation therapy (SBRT), interfractional and intrafractional imaging technologies are becoming increasingly critical to ensure safe and effective treatment delivery. Ultrasound (US)-based image guidance systems offer real-time, markerless, volumetric imaging with excellent soft tissue contrast, overcoming the limitations of traditional X-ray or computed tomography (CT)-based guidance for abdominal and pelvic cancer sites, such as the liver and prostate. Interfractional US guidance systems have been commercially adopted for patient positioning but suffer from systematic positioning errors induced by probe pressure. More recently, several research groups have introduced concepts for intrafractional US guidance systems leveraging robotic probe placement technology and real-time soft tissue tracking software. This paper reviews various commercial and research-level US guidance systems used in radiation therapy, with an emphasis on hardware and software technologies that enable the deployment of US imaging within the radiotherapy environment and workflow. Previously unpublished material on tissue tracking systems and robotic probe manipulators under development by our group is also included.

A new method for tracking organ motion on diagnostic ultrasound images

PURPOSE

Respiratory-gated irradiation is effective in reducing the margins of a target in the case of abdominal organs, such as the liver, that change their position as a result of respiratory motion. However, existing technologies are incapable of directly measuring organ motion in real-time during radiation beam delivery. Hence, the authors proposed a novel quantitative organ motion tracking method involving the use of diagnostic ultrasound images; it is noninvasive and does not entail radiation exposure. In the present study, the authors have prospectively evaluated this proposed method.

METHODS

The method involved real-time processing of clinical ultrasound imaging data rather than organ monitoring; it comprised a three-dimensional ultrasound device, a respiratory sensing system, and two PCs for data storage and analysis. The study was designed to evaluate the effectiveness of the proposed method by tracking the gallbladder in one subject and a liver vein in another subject. To track a moving target organ, the method involved the control of a region of interest (ROI) that delineated the target. A tracking algorithm was used to control the ROI, and a large number of feature points and an error correction algorithm were used to achieve long-term tracking of the target. Tracking accuracy was assessed in terms of how well the ROI matched the center of the target.

RESULTS

The effectiveness of using a large number of feature points and the error correction algorithm in the proposed method was verified by comparing it with two simple tracking methods. The ROI could capture the center of the target for about 5 min in a cross-sectional image with changing position. Indeed, using the proposed method, it was possible to accurately track a target with a center deviation of 1.54±0.9 mm. The computing time for one frame image using our proposed method was 8 ms. It is expected that it would be possible to track any soft-tissue organ or tumor with large deformations and changing cross-sectional position using this method.

CONCLUSIONS

The proposed method achieved real-time processing and continuous tracking of the target organ for about 5 min. It is expected that our method will enable more accurate radiation treatment than is the case using indirect observational methods, such as the respiratory sensor method, because of direct visualization of the tumor. Results show that this tracking system facilitates safe treatment in clinical practice.

In vivo reproducibility of robotic probe placement for a novel ultrasound-guided radiation therapy system

Ultrasound can provide real-time image guidance of radiation therapy, but the probe-induced tissue deformations cause local deviations from the treatment plan. If placed during treatment planning, the probe causes streak artifacts in required computed tomography (CT) images. To overcome these challenges, we propose robot-assisted placement of an ultrasound probe, followed by replacement with a geometrically identical, CT-compatible model probe. In vivo reproducibility was investigated by implanting a canine prostate, liver, and pancreas with three 2.38-mm spherical markers in each organ. The real probe was placed to visualize the markers and subsequently replaced with the model probe. Each probe was automatically removed and returned to the same position or force. Under position control, the median three-dimensional reproducibility of marker positions was 0.6 to 0.7 mm, 0.3 to 0.6 mm, and 1.1 to 1.6 mm in the prostate, liver, and pancreas, respectively. Reproducibility was worse under force control. Probe substitution errors were smallest for the prostate (0.2 to 0.6 mm) and larger for the liver and pancreas (4.1 to 6.3 mm), where force control generally produced larger errors than position control. Results indicate that position control is better than force control for this application, and the robotic approach has potential, particularly for relatively constrained organs and reproducibility errors that are smaller than established treatment margins.

4D ultrasound speckle tracking of intra-fraction prostate motion: a phantom-based comparison with x-ray fiducial tracking using CyberKnife

This study investigates the use of a mechanically-swept 3D ultrasound (3D-US) probe for soft-tissue displacement monitoring during prostate irradiation, with emphasis on quantifying the accuracy relative to CyberKnife® x-ray fiducial tracking. An US phantom, implanted with x-ray fiducial markers was placed on a motion platform and translated in 3D using five real prostate motion traces acquired using the Calypso system. Motion traces were representative of all types of motion as classified by studying Calypso data for 22 patients. The phantom was imaged using a 3D swept linear-array probe (to mimic trans-perineal imaging) and, subsequently, the kV x-ray imaging system on CyberKnife. A 3D cross-correlation block-matching algorithm was used to track speckle in the ultrasound data. Fiducial and US data were each compared with known phantom displacement. Trans-perineal 3D-US imaging could track superior-inferior (SI) and anterior-posterior (AP) motion to ≤0.81 mm root-mean-square error (RMSE) at a 1.7 Hz volume rate. The maximum kV x-ray tracking RMSE was 0.74 mm, however the prostate motion was sampled at a significantly lower imaging rate (mean: 0.04 Hz). Initial elevational (right-left; RL) US displacement estimates showed reduced accuracy but could be improved (RMSE <2.0 mm) using a correlation threshold in the ultrasound tracking code to remove erroneous inter-volume displacement estimates. Mechanically-swept 3D-US can track the major components of intra-fraction prostate motion accurately but exhibits some limitations. The largest US RMSE was for elevational (RL) motion. For the AP and SI axes, accuracy was sub-millimetre. It may be feasible to track prostate motion in 2D only. 3D-US also has the potential to improve high tracking accuracy for all motion types. It would be advisable to use US in conjunction with a small (∼2.0 mm) centre-of-mass displacement threshold in which case it would be possible to take full advantage of the accuracy and high imaging rate capability.

The road to clinical use of high-intensity focused ultrasound for liver cancer: technical and clinical consensus

Clinical use of high-intensity focused ultrasound (HIFU) under ultrasound or MR guidance as a non-invasive method for treating tumors is rapidly increasing. Tens of thousands of patients have been treated for uterine fibroid, benign prostate hyperplasia, bone metastases, or prostate cancer. Despite the methods' clinical potential, the liver is a particularly challenging organ for HIFU treatment due to the combined effect of respiratory-induced liver motion, partial blocking by the rib cage, and high perfusion/flow. Several technical and clinical solutions have been developed by various groups during the past 15 years to compensate for these problems. A review of current unmet clinical needs is given here, as well as a consensus from a panel of experts about technical and clinical requirements for upcoming pilot and pivotal studies in order to accelerate the development and adoption of focused ultrasound for the treatment of primary and secondary liver cancer.

Assessing Feasibility of Real-Time Ultrasound Monitoring in Stereotactic Body Radiotherapy of Liver Tumors

To monitor tumor motion during stereotactic body radiotherapy (SBRT) for patients with liver cancer, an integrated ultrasound and kilo-voltage cone-beam computed tomography (KV-CBCT) system has been proposed. The presence of an ultrasound probe may interfere with the radiation beams. The purpose of this study is to minimize this interference by altering orientations of the ultrasound probe and directions of radiation beams while not compromising the quality of SBRT plans. Ten patients, who received SBRT of liver cancer, were randomly selected for this study. To simulate the presence of an ultrasound probe, a virtual probe was oriented either parallel or vertical to the longitudinal axis of the patient's body and was added on the surface of the patient's body at the nearest location to the tumor. For both the parallel and vertical probe orientations, 2 new SBRT (Probe-Para and Probe-Vert) plans that minimize the interference between the probe and radiation beams were created for each patient. These SBRT plans were compared to the original clinically accepted SBRT plans, with a treatment goal of 37.5 Gy to the planning target volume (PTV) in 3 fractions. Specific dosimetric endpoints were evaluated, including doses to 95% (D95), of the PTV plan conformal index (CI), homogeneity index (HI), and relevant endpoint doses to organs at risk. For 2 patients with superficially located tumors, no clinically acceptable SBRT plans could be produced without the interference between the probe and radiation beams. For the remaining 8 patients, the Probe-Para plans allowed 7 patients to be treated with coplanar radiation beams (without moving the treatment couch during treatment) and 1 patient to be treated with non-coplanar beams (by moving the treatment couch during treatment). The Probe-Vert plans allowed 2 patients to be treated with coplanar beams and 6 patients to be treated with non-coplanar beams. The average D95 of the PTV were 38.63 Gy ± 0.14 ( p = 0.65) for Probe-Para plans, 38.48 Gy ± 0.31 ( p = 0.33) for Probe-Vert plans, and 38.72 Gy ± 0.14 for clinical SBRT plans. There were no significant differences ( p > 0.05) in CI and HI of all SBRT plans. The endpoint doses to the liver, heart, esophagus, right kidney, and stomach also had no significant differences ( p > 0.05). Except for superficial lesions, real-time ultrasound monitoring during liver SBRT is clinically feasible. Placing the ultrasound probe parallel to the longitudinal axis of the patient allows a greater probability of utilizing preferred coplanar beams.

A learning-based approach for fast and robust vessel tracking in long ultrasound sequences

We propose a learning-based method for robust tracking in long ultrasound sequences for image guidance applications. The framework is based on a scale-adaptive block-matching and temporal realignment driven by the image appearance learned from an initial training phase. The latter is introduced to avoid error accumulation over long sequences. The vessel tracking performance is assessed on long 2D ultrasound sequences of the liver of 9 volunteers under free breathing. We achieve a mean tracking accuracy of 0.96 mm. Without learning, the error increases significantly (2.19 mm, p<0.001).

Real-time tumour tracking in particle therapy: technological developments and future perspectives

A key challenge in radiation oncology is accurate delivery of the prescribed dose to tumours that move because of respiration. Tumour tracking involves real-time target localisation and correction of radiation beam geometry to compensate for motion. Uncertainties in tumour localisation are important in particle therapy (proton therapy, carbon-ion therapy) because charged particle beams are highly sensitive to geometrical and associated density and radiological variations in path length, which will affect the treatment plan. Target localisation and motion compensation methods applied in x-ray photon radiotherapy require careful performance assessment for clinical applications in particle therapy. In this Review, we summarise the efforts required for an application of real-time tumour tracking in particle therapy, by comparing and assessing competing strategies for time-resolved target localisation and related clinical outcomes in x-ray radiation oncology.

[Image-guided radiation therapy]

Radiotherapy technology has improved rapidly over the past two decades. New imaging modalities, such as positron emission (computed) tomography (PET, PET-CT) and high-resolution morphological and functional magnetic resonance imaging (MRI) have been introduced into the treatment planning process. Image-guided radiation therapy (IGRT) with 3D soft tissue depiction directly imaging target and normal structures, is currently replacing patient positioning based on patient surface markers, frame-based intracranial and extracranial stereotactic treatment and partially also 2D field verification methods. On-line 3D soft tissue-based position correction unlocked the full potential of new delivery techniques, such as intensity-modulated radiotherapy, by safely delivering highly conformal dose distributions that facilitate dose escalation and hypofractionation. These strategies have already resulted in better clinical outcomes, e.g. in prostate and lung cancer and are expected to further improve radiotherapy results.

Hypofractionated image-guided breath-hold SABR (stereotactic ablative body radiotherapy) of liver metastases--clinical results

Purpose

Stereotactic Ablative Body Radiotherapy (SABR) is a non-invasive therapy option for inoperable liver oligometastases. Outcome and toxicity were retrospectively evaluated in a single-institution patient cohort who had undergone ultrasound-guided breath-hold SABR.

Patients and methods

19 patients with liver metastases of various primary tumors consecutively treated with SABR (image-guidance with stereotactic ultrasound in combination with computer-controlled breath-hold) were analysed regarding overall-survival (OS), progression-free-survival (PFS), progression pattern, local control (LC), acute and late toxicity.

Results

PTV (planning target volume)-size was 108 ± 109cm³ (median 67.4 cm³). BED2 (Biologically effective dose in 2 Gy fraction) was 83.3 ± 26.2 Gy (median 78 Gy). Median follow-up and median OS were 12 months. Actuarial 2-year-OS-rate was 31%. Median PFS was 4 months, actuarial 1-year-PFS-rate was 20%. Site of first progression was predominantly distant. Regression of irradiated lesions was observed in 84% (median time to detection of regression was 2 months). Actuarial 6-month-LC-rate was 92%, 1- and 2-years-LC-rate 57%, respectively. BED2 influenced LC. When a cut-off of BED2 = 78 Gy was used, the higher BED2 values resulted in improved local control with a statistical trend to significance (p = 0.0999). Larger PTV-sizes, inversely correlated with applied dose, resulted in lower local control, also with a trend to significance (p-value = 0.08) when a volume cut-off of 67 cm³ was used.

No local relapse was observed at PTV-sizes < 67 cm³ and BED2 > 78 Gy. No acute clinical toxicity > °2 was observed. Late toxicity was also ≤ °2 with the exception of one gastrointestinal bleeding-episode 1 year post-SABR. A statistically significant elevation in the acute phase was observed for alkaline-phosphatase; in the chronic phase for alkaline-phosphatase, bilirubine, cholinesterase and C-reactive protein.

Conclusions

A trend to statistically significant correlation of local progression was observed for BED2 and PTV-size. Dose-levels BED2 > 78 Gy cannot be reached in large lesions constituting a significant fraction of this series. Image-guided SABR (igSABR) is therefore an effective non-invasive treatment modality with low toxicity in patients with small inoperable liver metastases.

Potential use of ultrasound speckle tracking for motion management during radiotherapy: preliminary report

We prospectively evaluated real-time ultrasound speckle tracking for monitoring soft tissue motion for image-guided radiotherapy. Two human volunteers and 1 patient with a proven hepatocellular carcinoma, who was being prepared for radiation therapy treatment, were scanned using a clinical ultrasound scanner modified to acquire and store radiofrequency signals. Scans were performed of the liver in the volunteers and the patient. In the patient, the speckle-tracking results were compared to those measured on a treatment-planning 4-dimensional computed tomogram with tumors contoured manually in each phase and with estimates made by hand on gray scale ultrasound images. The surface of the right lung and the prostate were scanned in a volunteer. The liver and lung surface were scanned during respiration. To simulate prostate motion, the ultrasound probe was rocked in an anterior-posterior direction. The correlation coefficients of all motion measurements were significantly correlated at all sites (P < .00001 for all sites) with 0 time delays. Ultrasound speckle-tracking motion estimates of tumor motion were within 2 mm of estimates made by hand tracking on gray scale ultrasound images and the 4-dimensional computed tomogram. The total tumor motion was greater than 20 mm. The angular displacement of the prostate was within 0.02 radians (1.1°) with displacements measured by hand. Speckle tracking could be used to monitor organ motion during radiotherapy.

Feasibility of intrafraction whole-body motion tracking for total marrow irradiation

With image-guided tomotherapy, highly targeted total marrow irradiation (TMI) has become a feasible alternative to conventional total body irradiation. The uncertainties in patient localization and intrafraction motion of the whole body during hour-long TMI treatment may pose a risk to the safety and accuracy of targeted radiation treatment. The feasibility of near-infrared markers and optical tracking system (OTS) is accessed along with a megavoltage scanning system of tomotherapy. Three near-infrared markers placed on the face of a rando phantom are used to evaluate the capability of OTS in measuring changes in the markers' positions as the rando is moved in the translational direction. The OTS is also employed to determine breathing motion related changes in the position of 16 markers placed on the chest surface of human volunteers. The maximum uncertainty in locating marker position with the OTS is 1.5 mm. In the case of normal and deep breathing motion, the maximum marker position change is observed in anterior-posterior direction with the respective values of 4 and 12 mm. The OTS is able to measure surface changes due to breathing motion. The OTS may be optimized to monitor whole body motion during TMI to increase the accuracy of treatment delivery and reduce the radiation dose to the lungs.

Anniversary paper: evolution of ultrasound physics and the role of medical physicists and the AAPM and its journal in that evolution

Ultrasound has been the greatest imaging modality worldwide for many years by equipment purchase value and by number of machines and examinations. It is becoming increasingly the front end imaging modality; serving often as an extension of the physician's fingers. We believe that at the other extreme, high-end systems will continue to compete with all other imaging modalities in imaging departments to be the method of choice for various applications, particularly where safety and cost are paramount. Therapeutic ultrasound, in addition to the physiotherapy practiced for many decades, is just coming into its own as a major tool in the long progression to less invasive interventional treatment. The physics of medical ultrasound has evolved over many fronts throughout its history. For this reason, a topical review, rather than a primarily chronological one is presented. A brief review of medical ultrasound imaging and therapy is presented, with an emphasis on the contributions of medical physicists, the American Association of Physicists in Medicine (AAPM) and its publications, particularly its journal Medical Physics. The AAPM and Medical Physics have contributed substantially to training of physicists and engineers, medical practitioners, technologists, and the public.

Anatomical imaging for radiotherapy

The goal of radiation therapy is to achieve maximal therapeutic benefit expressed in terms of a high probability of local control of disease with minimal side effects. Physically this often equates to the delivery of a high dose of radiation to the tumour or target region whilst maintaining an acceptably low dose to other tissues, particularly those adjacent to the target. Techniques such as intensity modulated radiotherapy (IMRT), stereotactic radiosurgery and computer planned brachytherapy provide the means to calculate the radiation dose delivery to achieve the desired dose distribution. Imaging is an essential tool in all state of the art planning and delivery techniques: (i) to enable planning of the desired treatment, (ii) to verify the treatment is delivered as planned and (iii) to follow-up treatment outcome to monitor that the treatment has had the desired effect. Clinical imaging techniques can be loosely classified into anatomic methods which measure the basic physical characteristics of tissue such as their density and biological imaging techniques which measure functional characteristics such as metabolism. In this review we consider anatomical imaging techniques. Biological imaging is considered in another article. Anatomical imaging is generally used for goals (i) and (ii) above. Computed tomography (CT) has been the mainstay of anatomical treatment planning for many years, enabling some delineation of soft tissue as well as radiation attenuation estimation for dose prediction. Magnetic resonance imaging is fast becoming widespread alongside CT, enabling superior soft-tissue visualization. Traditionally scanning for treatment planning has relied on the use of a single snapshot scan. Recent years have seen the development of techniques such as 4D CT and adaptive radiotherapy (ART). In 4D CT raw data are encoded with phase information and reconstructed to yield a set of scans detailing motion through the breathing, or cardiac, cycle. In ART a set of scans is taken on different days. Both allow planning to account for variability intrinsic to the patient. Treatment verification has been carried out using a variety of technologies including: MV portal imaging, kV portal/fluoroscopy, MVCT, conebeam kVCT, ultrasound and optical surface imaging. The various methods have their pros and cons. The four x-ray methods involve an extra radiation dose to normal tissue. The portal methods may not generally be used to visualize soft tissue, consequently they are often used in conjunction with implanted fiducial markers. The two CT-based methods allow measurement of inter-fraction variation only. Ultrasound allows soft-tissue measurement with zero dose but requires skilled interpretation, and there is evidence of systematic differences between ultrasound and other data sources, perhaps due to the effects of the probe pressure. Optical imaging also involves zero dose but requires good correlation between the target and the external measurement and thus is often used in conjunction with an x-ray method. The use of anatomical imaging in radiotherapy allows treatment uncertainties to be determined. These include errors between the mean position at treatment and that at planning (the systematic error) and the day-to-day variation in treatment set-up (the random error). Positional variations may also be categorized in terms of inter- and intra-fraction errors. Various empirical treatment margin formulae and intervention approaches exist to determine the optimum strategies for treatment in the presence of these known errors. Other methods exist to try to minimize error margins drastically including the currently available breath-hold techniques and the tracking methods which are largely in development. This paper will review anatomical imaging techniques in radiotherapy and how they are used to boost the therapeutic benefit of the treatment.

Effect of ultrasound probe on dose delivery during real-time ultrasound-guided tumor tracking

Ultrasound is a noninvasive and less costly modality for real-time imaging of soft tissues. It has the capability of tracking soft tissue at levels of submillimeter precision even in the presence of radiation beams. The effect of a transducer on radiation dose is not fully known. The best imaging location for an ultrasound transducer happens to coincide with the path of an anterior-posterior beam in intensity modulated radiation therapy (IMRT). This study indicates a significant change in dose when this juxtaposition occurs. If the anterior-posterior beam is avoided in IMRT planning, however, the effect of the transducer on radiotherapy is found to be negligible.