Respiratory Motion Changes of Lung Tumors Over the Course of Radiation Therapy Based on Respiration-Correlated Four-Dimensional Computed Tomography Scans

Intra- and inter-fraction breath-hold variations and margins for radiotherapy of abdominal targets

Radiotherapy in expiration breath-hold (EBH) has the potential to reduce treatment volumes of abdominal targets compared to an internal target volume concept in free-breathing. The reproducibility of EBH and required safety margins were investigated to quantify this volumetric benefit. Pre- and post-treatment diaphragm position difference and the positioning variability were determined on computed tomography. Systematic and random errors for EBH position reproducibility and positioning variability were calculated, resulting in margins of 7 to 12 mm depending on the prescription isodose and fractionation. A reduced volume was shown for EBH for lesions with superior-inferior breathing motion above 4 to 8 mm.

Can bronchoscopically implanted anchored electromagnetic transponders be used to monitor tumor position and lung inflation during deep inspiration breath-hold lung radiotherapy?

PURPOSE

To evaluate the efficacy of using bronchoscopically implanted anchored electromagnetic transponders (EMTs) as surrogates for 1) tumor position and 2) repeatability of lung inflation during deep-inspiration breath-hold (DIBH) lung radiotherapy.

METHODS

41 patients treated with either hypofractionated (HF) or conventional (CF) lung radiotherapy on an IRB approved prospective protocol using coached DIBH were evaluated for this study. Three anchored EMTs were bronchoscopically implanted into small airways near or within the tumor. DIBH treatment was gated by tracking the EMT positions. Breath-hold cone-beam-CTs (CBCTs) were acquired prior to every HF treatment or weekly for CF patients. Retrospectively, rigid registrations between each CBCT and the breath-hold planning CT were performed to match to 1) spine 2) EMTs and 3) tumor. Absolute differences in registration between EMTs and spine were analyzed to determine surrogacy of EMTs for lung inflation. Differences in registration between EMTs and tumor were analyzed to determine surrogacy of EMTs for tumor position. The stability of the EMTs was evaluated by analyzing the difference between inter-EMT displacements recorded at treatment from that of the plan for the CF patients, as well as the geometric residual (GR) recorded at the time of treatment.

RESULTS

219 CBCTs were analyzed. The average differences between EMT centroid and spine registration among all CBCTs were 0.45±0.42cm, 0.29±0.28cm, and 0.18±0.15cm in superior-inferior (SI), anterior-posterior (AP) and lateral directions, respectively. Only 59% of CBCTs had differences in registration <0.5cm for EMT centroid compared to spine, indicating that lung inflation is not reproducible from simulation to treatment. The average differences between EMT centroid and tumor registration among all CBCTs were 0.13±0.13cm, 0.14±0.13cm and 0.12±0.12cm in SI, AP and lateral directions, respectively. 95% of CBCTs resulted in <0.5cm change between EMT centroid and tumor registration, indicating that EMT positions correspond well with tumor position during treatments. Six out of the 7 recorded CF patients had average differences in inter-EMT displacements to be ≤0.26cm and average GR ≤0.22cm, indicating that the EMTs are stable throughout treatment.

CONCLUSIONS

Bronchoscopically implanted anchored EMTs are good surrogates for tumor position and are reliable for maintaining tumor position when tracked during DIBH treatment, as long as the tumor size and shape are stable. Large differences in registration between EMTs and spine for many treatments suggest that lung inflation achieved at simulation is often not reproduced. This article is protected by copyright. All rights reserved.

Lung-CRNet: A convolutional recurrent neural network for lung 4DCT image registration

PURPOSE

Deformable image registration (DIR) of lung four-dimensional computed tomography (4DCT) plays a vital role in a wide range of clinical applications. Most of the existing deep learning-based lung 4DCT DIR methods focus on pairwise registration which aims to register two images with large deformation. However, the temporal continuities of deformation fields between phases are ignored. This paper proposes a fast and accurate deep learning-based lung 4DCT DIR approach that leverages the temporal component of 4DCT images.

METHODS

We present Lung-CRNet, an end-to-end convolutional recurrent registration neural network for lung 4DCT images and reformulate 4DCT DIR as a spatiotemporal sequence predicting problem in which the input is a sequence of three-dimensional computed tomography images from the inspiratory phase to the expiratory phase in a respiratory cycle. The first phase in the sequence is selected as the only reference image and the rest as moving images. Multiple convolutional gated recurrent units (ConvGRUs) are stacked to capture the temporal clues between images. The proposed network is trained in an unsupervised way using a spatial transformer layer. During inference, Lung-CRNet is able to yield the respective displacement field for each reference-moving image pair in the input sequence.

RESULTS

We have trained the proposed network using a publicly available lung 4DCT dataset and evaluated performance on the widely used the DIR-Lab dataset. The mean and standard deviation of target registration error are 1.56 ± 1.05 mm on the DIR-Lab dataset. The computation time for each forward prediction is less than 1 s on average.

CONCLUSIONS

The proposed Lung-CRNet is comparable to the existing state-of-the-art deep learning-based 4DCT DIR methods in both accuracy and speed. Additionally, the architecture of Lung-CRNet can be generalized to suit other groupwise registration tasks which align multiple images simultaneously.

Evaluation of lung tumor motion in a large sample: Target-related and clinical factors influencing tumor motion based on four-dimensional CT

Background and purpose

We aimed to analyze the influence of target‐related and clinical factors on lung tumor motion based on four‐dimensional CT (4DCT), and clarify the motion based on subgroups in lung stereotactic body radiation therapy.

Materials and methods

4DCT image data of 267 tumors from 246 patients were analyzed. The coordinates in the left–right (LR), anterior–posterior (AP), and cranial–caudal (CC) directions of the center of mass (COM) of the gross tumor volumes in 10 phases of 4DCT were measured. The peak‐to‐peak COM displacement in the LR, AP, CC, and 3D directions was calculated. The influence of target‐related and clinical factors on tumor motion was evaluated using multivariate analysis.

Results

The tumor segment location correlated with the tumor motion in each direction. Tumor size was predictive of tumor motion in the 3D (p = 0.023) and AP directions (p = 0.049). The tumor motion for metastatic tumors was smaller than that for primary tumors in the LR (p = 0.019) and AP directions (p = 0.008). The CC motion for pulmonary surgery recipients (3.8 ± 4.5 mm) was less than that for patients who had not undergone surgery (5.6 ± 5.4 mm), and no significant clinical factor was observed. BSA and BMI were positively correlated with the motion in the CC (p = 0.02) and LR directions (p = 0.002).

Conclusion

The tumor segment location was a good predictor of tumor motion. A larger tumor tends to have a smaller motion. Patients with metastatic tumors or those who have undergone pulmonary surgery exhibited smaller and more unpredictable tumor motions, which required individual assessments. Thus, clinical factors can potentially predict tumor motion.

Concurrent chemoradiotherapy for stage III non-small-cell lung cancer: recent progress and future perspectives (a narrative review)

Concurrent chemoradiotherapy (CHRT) remains the therapeutic standard for locally advanced inoperable non-small-cell lung cancer (NSCLC). The median overall survival (OS) with this approach is in the range of 20–30 months, with five-year survival of approximately 30%. These outcomes have recently been further improved by supplementing CHRT with maintenance durvalumab, a monoclonal anti-PD-L1 agent. The progress in treatment outcomes of locally advanced NSCLC before the era of immunotherapy has been achieved mainly by virtue of developments in diagnostics and radiotherapy techniques. Routine implementation of endoscopic and endobronchial ultrasonography for mediastinal lymph nodes assessment, positron emission tomography/computed tomography and magnetic resonance imaging of the brain allows for more accurate staging of NSCLC and for optimizing treatment strategy. Thorough staging and respiratory motion control allows for higher conformity of radiotherapy and reduction of radiotherapy related toxicity. Dose escalation with prolonged overall treatment time does not improve treatment outcomes of CHRT. In consequence, 60 Gy in 2 Gy fractions or equivalent biological dose remains the standard dose for definitive CHRT in locally advanced NSCLC. However, owing to increased toxicity of CHRT, this option may not be applicable in a proportion of elderly or frail patients. This article summarizes recent developments in curative CHRT for inoperable stage III NSCLC, and presents perspectives for further improvements of this strategy

Investigation of inter-fraction target motion variations in the context of pencil beam scanned proton therapy in non-small cell lung cancer patients

Purpose

For locally advanced‐stage non‐small cell lung cancer (NSCLC), inter‐fraction target motion variations during the whole time span of a fractionated treatment course are assessed in a large and representative patient cohort. The primary objective is to develop a suitable motion monitoring strategy for pencil beam scanning proton therapy (PBS‐PT) treatments of NSCLC patients during free breathing.

Methods

Weekly 4D computed tomography (4DCT; 41 patients) and daily 4D cone beam computed tomography (4DCBCT; 10 of 41 patients) scans were analyzed for a fully fractionated treatment course. Gross tumor volumes (GTVs) were contoured and the 3D displacement vectors of the centroid positions were compared for all scans. Furthermore, motion amplitude variations in different lung segments were statistically analyzed. The dosimetric impact of target motion variations and target motion assessment was investigated in exemplary patient cases.

Results

The median observed centroid motion was 3.4 mm (range: 0.2–12.4 mm) with an average variation of 2.2 mm (range: 0.1–8.8 mm). Ten of 32 patients (31.3%) with an initial motion <5 mm increased beyond a 5‐mm motion amplitude during the treatment course. Motion observed in the 4DCBCT scans deviated on average 1.5 mm (range: 0.0–6.0 mm) from the motion observed in the 4DCTs. Larger motion variations for one example patient compromised treatment plan robustness while no dosimetric influence was seen due to motion assessment biases in another example case.

Conclusions

Target motion variations were investigated during the course of radiotherapy for NSCLC patients. Patients with initial GTV motion amplitudes of < 2 mm can be assumed to be stable in motion during the treatment course. For treatments of NSCLC patients who exhibit motion amplitudes of > 2 mm, 4DCBCT should be considered for motion monitoring due to substantial motion variations observed.

Quantifying day-to-day variations in 4DCBCT-based PCA motion models

The aim of this paper is to quantify the day-to-day variations of motion models derived from pre-treatment 4-dimensional cone beam CT (4DCBCT) fractions for lung cancer stereotactic body radiotherapy (SBRT) patients. Motion models are built by (1) applying deformable image registration (DIR) on each 4DCBCT image with respect to a reference image from that day, resulting in a set of displacement vector fields (DVFs), and (2) applying principal component analysis (PCA) on the DVFs to obtain principal components representing a motion model. Variations were quantified by comparing the PCA eigenvectors of the motion model built from the first day of treatment to the corresponding eigenvectors of the other motion models built from each successive day of treatment. Three metrics were used to quantify the variations: root mean squared (RMS) difference in the vectors, directional similarity, and an introduced metric called the Euclidean Model Norm (EMN). EMN quantifies the degree to which a motion model derived from the first fraction can represent the motion models of subsequent fractions. Twenty-one 4DCBCT scans from five SBRT patient treatments were used in this retrospective study. Experimental results demonstrated that the first two eigenvectors of motion models across all fractions have smaller RMS (0.00017), larger directional similarity (0.528), and larger EMN (0.678) than the last three eigenvectors (RMS: 0.00025, directional similarity: 0.041, and EMN: 0.212). The study concluded that, while the motion model eigenvectors varied from fraction to fraction, the first few eigenvectors were shown to be more stable across treatment fractions than others. This supports the notion that a pre-treatment motion model built from the first few PCA eigenvectors may remain valid throughout a treatment course. Future work is necessary to quantify how day-to-day variations in these models will affect motion reconstruction accuracy for specific clinical tasks.

Respiratory motion compensation in interventional liver SPECT using simultaneous fluoroscopic and nuclear imaging

PURPOSE

Quantitative accuracy of the single photon emission computed tomography (SPECT) reconstruction of the pretreatment procedure of liver radioembolization is crucial for dosimetry; visual quality is important for detecting doses deposited outside the planned treatment volume. Quantitative accuracy is limited by respiratory motion. Conventional gating eliminates motion by count rejection but increases noise, which degrades the visual reconstruction quality. Motion compensation using all counts can be performed if the motion signal and motion vector field over time are known. The measurement of the motion signal of a patient currently requires a device (such as a respiratory belt) attached to the patient, which complicates the acquisition. The motion vector field is generally extracted from a previously acquired four-dimensional scan and can differ from the motion in the scan performed during the intervention. The simultaneous acquisition of fluoroscopic and nuclear projections can be used to obtain both the motion vector field and the projections of the corresponding (moving) activity distribution. This eliminates the need for devices attached to the patient and provides an accurate motion vector field for SPECT reconstruction. Our approach to motion compensation would primarily be beneficial for interventional SPECT because the time-critical setting requires fast scans and no inconvenience of an external apparatus. The purpose of this work is to evaluate the performance of the motion compensation approach for interventional liver SPECT by means of simulations.

METHODS

Nuclear and fluoroscopic projections of a realistic digital human phantom with respiratory motion were generated using fast Monte Carlo simulators. Fluoroscopic projections were sampled at 1-5 Hz. Nuclear data were acquired continuously in list mode. The motion signal was extracted from the fluoroscopic projections by calculating the center-of-mass, which was then used to assign each photon to a corresponding motion bin. The fluoroscopic projections were reconstructed per bin and coregistered, resulting in a motion vector field that was used in the SPECT reconstruction. The influence of breathing patterns, fluoroscopic imaging dose, sampling rate, number of bins, and scanning time was studied. In addition, the motion compensation method was compared with conventional gating to evaluate the detectability of spheres with varying uptake ratios.

RESULTS

The liver motion signal was accurately extracted from the fluoroscopic projections, provided the motion was stable in amplitude and the sampling rate was greater than 2 Hz. The minimum total fluoroscopic dose for the proposed method to function in a 5-min scan was 10 µGy. Although conventional gating improved the quantitative reconstruction accuracy, substantial background noise was observed in the short scans because of the limited counts available. The proposed method similarly improved the quantitative accuracy, but generated reconstructions with higher visual quality. The proposed method provided better visualization of low-contrast features than when using gating.

CONCLUSION

The proposed motion compensation method has the potential to improve SPECT reconstruction quality. The method eliminates the need for external devices to measure the motion signal and generates an accurate motion vector field for reconstruction. A minimal increase in the fluoroscopic dose is required to substantially improve the results, paving the way for clinical use.

Reconstruction of a high-quality volumetric image and a respiratory motion model from patient CBCT projections

PURPOSE

To develop and evaluate a method of reconstructing a patient- and treatment day- specific volumetric image and motion model from free-breathing cone-beam projections and respiratory surrogate measurements. This Motion-Compensated Simultaneous Algebraic Reconstruction Technique (MC-SART) generates and uses a motion model derived directly from the cone-beam projections, without requiring prior motion measurements from 4DCT, and can compensate for both inter- and intrabin deformations. The motion model can be used to generate images at arbitrary breathing points, which can be used for estimating volumetric images during treatment delivery.

METHODS

The MC-SART was formulated using simultaneous image reconstruction and motion model estimation. For image reconstruction, projections were first binned according to external surrogate measurements. Projections in each bin were used to reconstruct a set of volumetric images using MC-SART. The motion model was estimated based on deformable image registration between the reconstructed bins, and least squares fitting to model parameters. The model was used to compensate for motion in both projection and backprojection operations in the subsequent image reconstruction iterations. These updated images were then used to update the motion model, and the two steps were alternated between. The final output is a volumetric reference image and a motion model that can be used to generate images at any other time point from surrogate measurements.

RESULTS

A retrospective patient dataset consisting of eight lung cancer patients was used to evaluate the method. The absolute intensity differences in the lung regions compared to ground truth were 50.8 ± 43.9 HU in peak exhale phases (reference) and 80.8 ± 74.0 in peak inhale phases (generated). The 50th percentile of voxel registration error of all voxels in the lung regions with >5 mm amplitude was 1.3 mm. The MC-SART was also applied to measured patient cone-beam projections acquired with a linac-mounted CBCT system. Results from this patient data demonstrate the feasibility of MC-SART and showed qualitative image quality improvements compared to other state-of-the-art algorithms.

CONCLUSION

We have developed a simultaneous image reconstruction and motion model estimation method that uses Cone-beam computed tomography (CBCT) projections and respiratory surrogate measurements to reconstruct a high-quality reference image and motion model of a patient in treatment position. The method provided superior performance in both HU accuracy and positional accuracy compared to other existing methods. The resultant reference image and motion model can be combined with respiratory surrogate measurements to generate volumetric images representing patient anatomy at arbitrary time points.

Longitudinal assessment of anchored transponder migration following lung stereotactic body radiation therapy

Purpose

To assess the long‐term stability of the anchored radiofrequency transponders and compare displacement rates with other commercially available lung fiducial markers. We also sought to describe late toxicity attributable to fiducial implantation or migration.

Materials and methods

The transponder cohort was comprised of 17 patients at our institution who enrolled in a multisite prospective clinical trial and underwent bronchoscopic implantation of three anchored transponders into small (2–2.5 mm) airways. We generated a comparison cohort of 34 patients by selecting patients from our institutional lung SBRT database and matching 2:1 based on the lobe containing tumor and proximity to the bronchial tree. Assessment of migration was performed by rigidly registering the most recent follow‐up CT scan to the simulation scan, and assessing whether the relative geometry of the fiducial markers had changed by more than 5 mm. Toxicity outcomes of interest were hemoptysis and pneumothorax.

Results

The median follow‐up of patients in the transponder cohort was 25.3 months and the median follow‐up in the comparison cohort was 21.7 months. When assessing the most recent CT, all fiducial markers were within 5 mm of their position at CT simulation in 11 (65%) patients in the transponder group as compared to 23 (68%) in the comparison group (P = 0.28). One case of hemoptysis was identified in the transponder cohort, and bronchoscopy confirmed bleeding from recurrent tumor; no cases of hemoptysis were noted in the comparison cohort. No case of pneumothorax was noted in either group.

Conclusion

No significant difference in the rates of fiducial marker retention and migration were noted when comparing patients who had anchored transponders placed into small airways and a 2:1 matched cohort of patients who had other commercially available lung fiducial markers placed. In both groups, no late or chronic toxicity appeared to be related to the implanted fiducial markers.

Advances in the use of motion management and image guidance in radiation therapy treatment for lung cancer

The development of advanced radiation technologies, including intensity-modulated radiation therapy (IMRT), stereotactic body radiation therapy (SBRT) and proton therapy, has resulted in increasingly conformal radiation treatments. Recent evidence for the importance of minimizing dose to normal critical structures including the heart and lungs has led to incorporation of these advanced treatment modalities into radiation therapy (RT) for non-small cell lung cancer (NSCLC). While such technologies have allowed for improved dose delivery, implementation requires improved target accuracy with treatments, placing increasing importance on evaluating tumor motion at the time of planning and verifying tumor position at the time of treatment. In this review article, we describe issues and updates related both to motion management and image guidance in the treatment of NSCLC.

Open Source 3D Printed Lung Tumor Movement Simulator for Radiotherapy Quality Assurance

In OECD (Organization for Economic Co-operation and Development) countries, cancer is one of the main causes of death, lung cancer being one of the most aggressive. There are several techniques for the treatment of lung cancer, among which radiotherapy is one of the most effective and least invasive for the patient. However, it has associated difficulties due to the moving target tumor. It is possible to reduce the side effects of radiotherapy by effectively tracking a tumor and reducing target irradiation margins. This paper presents a custom electromechanical system that follows the movement of a lung tumor. For this purpose, a hysteresis loop of human lung movement during breathing was studied to obtain its characteristic movement equation. The system is controlled by an Arduino, steppers motors and a customized 3D printed mechanism to follow the characteristic human breathing, obtaining an accurate trajectory. The developed device helps the verification of individualized radiation treatment plans and permits the improvement of radiotherapy quality assurance procedures.

Optimizing immobilization, margins, and imaging for lung stereotactic body radiation therapy

The simultaneous advancement of technologies for the delivery of precisely targeted radiation therapy and the paradigm shift to substantial hypofractionation have led to significant improvements in the treatment of early stage non-small cell lung cancer (ES-NSCLC). Stereotactic body radiation therapy (SBRT) has become a well-established option for the treatment of ES-NSCLC and is now becoming widely available within the radiation oncology community. Implementation of this technique, however, requires highly accurate target delineation, thorough evaluation of tumor motion, and improved on-board imaging at the time of treatment for patient alignment, each of which is critical for successful tumor control and mitigation of risks to normal tissues. In this article, we review updates and issues related to immobilization and image guidance for SBRT in the treatment of ES-NSCLC.

Feasibility of proton pencil beam scanning treatment of free-breathing lung cancer patients

BACKGROUND

The interplay effect might degrade the dose of pencil beam scanning proton therapy to a degree that free-breathing treatment might be impossible without further motion mitigation techniques, which complicate and prolong the treatment. We assessed whether treatment of free-breathing patients without motion mitigation is feasible.

MATERIAL AND METHODS

For 40 lung cancer patients, 4DCT datasets and individual breathing patterns were used to simulate 4D dynamic dose distributions of 3D treatment plans over 33 fractions delivered with an IBA universal nozzle. Evaluation was done by assessing under- and overdosage in the target structure using the parameters V90, V95, V98, D98, D2, V107 and V110. The impact of using beam-specific target volumes and the impact of changes in motion and patient anatomy in control 4DCTs were assessed.

RESULTS

Almost half of the patients had tumour motion amplitudes of less than 5 mm. Under- and overdosage was significantly smaller for patients with tumour motion below 5 mm compared to patients with larger motion (2% vs. 13% average absolute reduction of V95, 2% vs. 8% average increase in V107, p < .01). Simulating a 33-fraction treatment, the dose degradation was reduced but persisted for patients with tumour motion above 5 mm (average ΔV95 of <1% vs. 3%, p < .01). Beam-specific target volumes reduced the dose degradation in a fractionated treatment, but were more relevant for large motion. Repeated 4DCT revealed that changes in tumour motion during treatment might result in unexpected large dose degradations.

CONCLUSION

Tumour motion amplitude is an indicator of dose degradation caused by the interplay effect. Fractionation reduces the dose degradation allowing the unmitigated treatment of patients with small tumour motions of less than 5 mm. The beam-specific target approach improves the dose coverage. The tumour motion and position needs to be assessed during treatment for all patients, to quickly react to possible changes, which might require treatment adaptation.

Observation of different tumor motion magnitude within liver and estimate of internal motion margins in postoperative patients with hepatocellular carcinoma

Aims

To assess motion magnitude in different parts of the liver through surgical clips in postoperative patients with hepatocellular carcinoma and to examine the correlation between the clip and diaphragm motion.

Methods

Four-dimensional computed tomography images from 30 liver cancer patients under thermoplastic mask immobilization were selected for this study. Three to seven surgical clips were placed in the resection cavity of each patient. The liver volume on computed tomography image was divided into the right upper (RU), right middle (RM), right lower (RL), hilar, and left lobes. Agreement between the clip and diaphragm motion was assessed by calculating intraclass correlation coefficient, and Bland–Altman analysis (Diff). Furthermore, population-based and patient-specific margins for internal motion were evaluated.

Results

The clips located in the RU lobe showed the largest motion, (7.5±1.6) mm, which was significantly more than in the RM lobe (5.7±2.8 mm, p=0.019), RL lobe (4.8±3.3 mm, p=0.017), and hilar lobe (4.7±2.7 mm, p<0.001) in the cranial–caudal direction. The mean intraclass correlation coefficient values between the clip and diaphragm motion were 0.915, 0.735, 0.678, 0.670, and the mean Diff values between them were 0.1±0.8 mm, 2.3±1.4 mm, 3.1±2.0 mm, 2.4±1.5 mm, when clips were located in the RU lobe, RM lobe, RL lobe, and hilar lobe, respectively. The clip and diaphragm motions had high concordance when clips were located in the RU lobe. Internal margin can be reduced from 5 mm in the cranial–caudal direction based on patient population average and to 3 mm based on patient-specific margins.

Conclusions

The motion magnitude of clips varied significantly depending on their location within the liver. The diaphragm was a more appropriate surrogate for tumor located in the RU lobe than for other lobes.

Development of patient-controlled respiratory gating system based on visual guidance for magnetic-resonance image-guided radiation therapy

PURPOSE

The aim of this study is to develop a visual guidance patient-controlled (VG-PC) respiratory gating system for respiratory-gated magnetic-resonance image-guided radiation therapy (MR-IGRT) and to evaluate the performance of the developed system.

METHODS

The near-real-time cine planar MR image of a patient acquired during treatment was transmitted to a beam projector in the treatment room through an optical fiber cable. The beam projector projected the cine MR images inside the bore of the ViewRay system in order to be visible to a patient during treatment. With this visual information, patients voluntarily controlled their respiration to put the target volume into the gating boundary (gating window). The effect of the presence of the beam projector in the treatment room on the image quality of the MRI was investigated by evaluating the signal-to-noise ratio (SNR), uniformity, low-contrast detectability, high-contrast spatial resolution, and spatial integrity with the VG-PC gating system. To evaluate the performance of the developed system, we applied the VG-PC gating system to a total of seven patients; six patients received stereotactic ablative radiotherapy (SABR) and one patient received conventional fractionated radiation therapy.

RESULTS

The projected cine MR images were visible even when the room light was on. No image data loss or additional time delay during delivery of image data were observed. Every indicator representing MRI quality, including SNR, uniformity, low-contrast detectability, high-contrast spatial resolution, and spatial integrity exhibited values higher than the tolerance levels of the manufacturer with the VG-PC gating system; therefore, the presence of the VG-PC gating system in the treatment room did not degrade the MR image quality. The average beam-off times due to respiratory gating with and without the VG-PC gating system were 830.3 ± 278.2 s and 1264.2 ± 302.1 s respectively (P = 0.005). Consequently, the total treatment times excluding the time for patient setup with and without the VG-PC gating system were 1453.3 ± 297.3 s and 1887.2 ± 469.6 s, respectively, on average (P = 0.005). The average number of beam-off events during whole treatment session was reduced from 457 ± 154 times to 195 ± 90 times by using the VG-PC gating system (P < 0.001).

CONCLUSIONS

The developed system could improve treatment efficiency when performing respiratory-gated MR-IGRT. The VG-PC gating system could be applied to any kind of bore-type radiotherapy machine.

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.

An autotuning respiration compensation system based on ultrasound image tracking

The purpose of this study was to develop an ultrasound image tracking algorithm (UITA) for extracting the exact displacement of internal organs caused by respiratory motion. The program can track organ displacements in real time, and analyze the displacement signals associated with organ displacements via a respiration compensating system (RCS). The ultrasound imaging system is noninvasive and has a high spatial resolution and a high frame rate (around 32 frames/s), which reduces the radiation doses that patients receive during computed tomography and X-ray observations. This allows for the continuous noninvasive observation and compensation of organ displacements simultaneously during a radiation therapy session.This study designed a UITA for tracking the motion of a specific target, such as the human diaphragm. Simulated diaphragm motion driven by a respiration simulation system was observed with an ultrasound imaging system, and then the induced diaphragm displacements were calculated by our proposed UITA. These signals were used to adjust the gain of the RCS so that the amplitudes of the compensation signals were close to the target movements. The inclination angle of the ultrasound probe with respect to the surface of the abdomen affects the results of ultrasound image displacement tracking. Therefore, the displacement of the phantom was verified by a LINAC with different inclination-angle settings of the ultrasound probe. The experimental results indicate that the best inclination angle of the ultrasound probe is 40 degrees, since this results in the target displacement of the ultrasound images being close to the actual target motion. The displacement signals of the tracking phantom and the opposing displacement signals created by the RCS were compared to assess the positioning accuracy of our proposed ultrasound image tracking technique combined with the RCS.When the ultrasound probe was inclined by 40 degrees in simulated respiration experiments using sine waves, the correlation between the target displacement on the ultrasound images and the actual target displacement was around 97%, and all of the compensation rates exceeded 94% after activating the RCS. Furthermore, the diaphragm movements on the ultrasound images of three patients could be captured by our image tracking technique. The test results show that our algorithm could achieve precise point locking and tracking functions on the diaphragm. This study has demonstrated the feasibility of the proposed ultrasound image tracking technique combined with the RCS for compensating for organ displacements caused by respiratory motion.This study has shown that the proposed ultrasound image tracking technique combined with the RCS can provide real-time compensation of respiratory motion during radiation therapy, without increasing the overall treatment time. In addition, the system has modest space requirements and is easy to operate.

Diaphragm height varies with arm position: comparison between angiography and CT

PURPOSE

To investigate how elevation of the arms affects diaphragm height.

MATERIALS AND METHODS

We retrospectively reviewed angiography and computed tomography (CT) portography data from 44 patients who were treated for hepatocellular carcinoma at our institution from July 2013 to May 2014. Diaphragm height was determined independently by two radiologists as the distance from the upper edge of the first lumbar vertebra to the highest point of the right diaphragm. The differences in height between angiography and CT images were compared using a paired t-test. We also evaluated the influence of table height and distance between X-ray tube and flat panel detector [source-image distance (SID)] on a phantom model.

RESULTS

Diaphragm height was higher on CT images [mean ± standard deviation (SD), 113.2 ± 27.2 mm] than on angiography images (105.5 ± 27.8 mm; P < 0.001). Inter-rater correlation was excellent both in angiography (R = 0.920; P < 0.001) and CT (R = 0.950; P < 0.001) measurements. Table height and SID had no influence on diaphragm height measurements (P = 0.33).

CONCLUSION

The diaphragm elevation was observed on CT with arm elevation compared with angiography without arm elevation.

Asymmetric margin setting at the cranial and caudal sides in respiratory gated and non-gated stereotactic body radiotherapy for lung cancer

OBJECTIVE

To evaluate total errors, including setup errors, and tumour motion changes with a electronic portal imaging device (EPID) cine at the cranial and caudal sides in respiratory gated and non-gated radiotherapy.

METHODS

Co-ordinates of the tumour centres (TCs) in the craniocaudal direction were obtained by using four-dimensional CT (4DCT) for each bin and EPID cine frame. During the 100% duty cycle (DC100), 50% duty cycle (DC50) and 30% duty cycle (DC30), both centred on the 50 phase, the co-ordinates of the TCs were compared at the most cranial and caudal positions on both 4DCT and EPID cine.

RESULTS

During DC100, total errors were -0.2 ± 2.1 and 1.1 ± 2.6 mm at the cranial and caudal sides, respectively. During DC50, the corresponding values were -0.2 ± 2.1 and 1.7 ± 3.2 mm, respectively; during DC30, they were -0.1 ± 2.1 and 1.8 ± 2.9 mm, respectively. The tumour motion changes at the caudal side were strongly correlated with tumour motion observed on 4DCT during DC100 (R(2) = 0.59).

CONCLUSION

Total errors and tumour motion changes on the caudal side were larger than on the cranial side because of the patients' breathing levels. Owing to variations of the TCs at beam-trigger events, the larger margin was required at the caudal side in gated radiotherapy.

ADVANCES IN KNOWLEDGE

Variations of the TCs were evaluated at the cranial and caudal sides separately. Providing some margins to compensate for tumour motion changes was a significant requirement at the caudal side in gated and non-gated radiotherapy.

Technical Note: Intrafractional changes in time lag relationship between anterior-posterior external and superior-inferior internal motion signals in abdominal tumor sites

PURPOSE

To investigate constancy, within a treatment session, of the time lag relationship between implanted markers in abdominal tumors and an external motion surrogate.

METHODS

Six gastroesophageal junction and three pancreatic cancer patients (IRB-approved protocol) received two cone-beam CTs (CBCT), one before and one after treatment. Time between scans was less than 30 min. Each patient had at least one implanted fiducial marker near the tumor. In all scans, abdominal displacement (Varian RPM) was recorded as the external motion signal. Purpose-built software tracked fiducials, representing internal signal, in CBCT projection images. Time lag between superior-inferior (SI) internal and anterior-posterior external signals was found by maximizing the correlation coefficient in each breathing cycle and averaging over all cycles. Time-lag-induced discrepancy between internal SI position and that predicted from the external signal (external prediction error) was also calculated.

RESULTS

Mean ± standard deviation time lag, over all scans and patients, was 0.10 ± 0.07 s (range 0.01-0.36 s). External signal lagged the internal in 17/18 scans. Change in time lag between pre- and post-treatment CBCT was 0.06 ± 0.07 s (range 0.01-0.22 s), corresponding to 3.1% ± 3.7% (range 0.6%-10.8%) of gate width (range 1.6-3.1 s). In only one patient, change in time lag exceeded 10% of the gate width. External prediction error over all scans of all patients varied from 0.1 ± 0.1 to 1.6 ± 0.4 mm.

CONCLUSIONS

Time lag between internal motion along SI and external signals is small compared to the treatment gate width of abdominal patients examined in this study. Change in time lag within a treatment session, inferred from pre- to post-treatment measurements is also small, suggesting that a single measurement of time lag at the session start is adequate. These findings require confirmation in a larger number of patients.

Respiratory motion variability of primary tumors and lymph nodes during radiotherapy of locally advanced non-small-cell lung cancers

Background and purpose

The need for target adjustment due to respiratory motion variation and the value of carina as a motion surrogate is evaluated for locally advanced non-small-cell lung cancer.

Material and methods

Using weekly 4D CTs (with audio-visual biofeedback) of 12 patients, respiratory motion variation of primary tumors (PT), lymph nodes (LN) and carina (C) were determined.

Results

Mean (SD) 3D respiratory motion ranges of PT, LN and C were 4 (3), 5 (3) and 5 (3) mm. PT and LN (p = 0.003), and LN and C motion range were correlated (p = 0.03). Only 20 %/5 % of all scans had variations >3 mm/5 mm. Large respiratory motion range on the initial scan was associated with larger during-treatment variations for PT (p = 0.03) and LN (p = 0.001).

Mean (SD) 3D relative displacements of PT-C, LN-C and PT-LN were each 6 (2) mm. Variations of displacements >3 mm/5 mm were observed in 28 %/6 % of scans for PT-LN, 20 %/9 % for PT-C, and 20 %/8 % for LN-C.

Conclusions

Motion reassessment is recommended in patients with large initial motion range. Relative motion-related displacements between PT and LN were larger than PT and LN motion alone. Both PT and C appear to be comparable surrogates for LN respiratory motion.

Baseline correction of a correlation model for improving the prediction accuracy of infrared marker-based dynamic tumor tracking

We previously found that the baseline drift of external and internal respiratory motion reduced the prediction accuracy of infrared (IR) marker‐based dynamic tumor tracking irradiation (IR Tracking) using the Vero4DRT system. Here, we proposed a baseline correction method, applied immediately before beam delivery, to improve the prediction accuracy of IR Tracking. To perform IR Tracking, a four‐dimensional (4D) model was constructed at the beginning of treatment to correlate the internal and external respiratory signals, and the model was expressed using a quadratic function involving the IR marker position (x) and its velocity (v), namely function F(x,v). First, the first 4D model, F1st(x,v), was adjusted by the baseline drift of IR markers (BDIR) along the x‐axis, as function F′(x,v). Next, BDdetect, that defined as the difference between the target positions indicated by the implanted fiducial markers (Pdetect) and the predicted target positions with F′(x,v) (Ppredict) was determined using orthogonal kV X‐ray images at the peaks of the Pdetect of the end‐inhale and end‐exhale phases for 10 s just before irradiation. F′(x,v) was corrected with BDdetect to compensate for the residual error. The final corrected 4D model was expressed as Fcor(x,v)=F1st{(x−BDIR),v}−BDdetect. We retrospectively applied this function to 53 paired log files of the 4D model for 12 lung cancer patients who underwent IR Tracking. The 95th percentile of the absolute differences between Pdetect and Ppredict (|Ep|) was compared between F1st(x,v) and Fcor(x,v). The median 95th percentile of |Ep| (units: mm) was 1.0, 1.7, and 3.5 for F1st(x,v), and 0.6, 1.1, and 2.1 for Fcor(x,v) in the left–right, anterior–posterior, and superior–inferior directions, respectively. Over all treatment sessions, the 95th percentile of |Ep| peaked at 3.2 mm using Fcor(x,v) compared with 8.4 mm using F1st(x,v). Our proposed method improved the prediction accuracy of IR Tracking by correcting the baseline drift immediately before irradiation.

PACS number: 87.19.rs, 87.19.Wx, 87.56.‐v, 87.59.‐e, 88.10.gc

A technique for estimating 4D-CBCT using prior knowledge and limited-angle projections

PURPOSE

To develop a technique to estimate onboard 4D-CBCT using prior information and limited-angle projections for potential 4D target verification of lung radiotherapy.

METHODS

Each phase of onboard 4D-CBCT is considered as a deformation from one selected phase (prior volume) of the planning 4D-CT. The deformation field maps (DFMs) are solved using a motion modeling and free-form deformation (MM-FD) technique. In the MM-FD technique, the DFMs are estimated using a motion model which is extracted from planning 4D-CT based on principal component analysis (PCA). The motion model parameters are optimized by matching the digitally reconstructed radiographs of the deformed volumes to the limited-angle onboard projections (data fidelity constraint). Afterward, the estimated DFMs are fine-tuned using a FD model based on data fidelity constraint and deformation energy minimization. The 4D digital extended-cardiac-torso phantom was used to evaluate the MM-FD technique. A lung patient with a 30 mm diameter lesion was simulated with various anatomical and respirational changes from planning 4D-CT to onboard volume, including changes of respiration amplitude, lesion size and lesion average-position, and phase shift between lesion and body respiratory cycle. The lesions were contoured in both the estimated and "ground-truth" onboard 4D-CBCT for comparison. 3D volume percentage-difference (VPD) and center-of-mass shift (COMS) were calculated to evaluate the estimation accuracy of three techniques: MM-FD, MM-only, and FD-only. Different onboard projection acquisition scenarios and projection noise levels were simulated to investigate their effects on the estimation accuracy.

RESULTS

For all simulated patient and projection acquisition scenarios, the mean VPD (±S.D.)∕COMS (±S.D.) between lesions in prior images and "ground-truth" onboard images were 136.11% (±42.76%)∕15.5 mm (±3.9 mm). Using orthogonal-view 15°-each scan angle, the mean VPD∕COMS between the lesion in estimated and "ground-truth" onboard images for MM-only, FD-only, and MM-FD techniques were 60.10% (±27.17%)∕4.9 mm (±3.0 mm), 96.07% (±31.48%)∕12.1 mm (±3.9 mm) and 11.45% (±9.37%)∕1.3 mm (±1.3 mm), respectively. For orthogonal-view 30°-each scan angle, the corresponding results were 59.16% (±26.66%)∕4.9 mm (±3.0 mm), 75.98% (±27.21%)∕9.9 mm (±4.0 mm), and 5.22% (±2.12%)∕0.5 mm (±0.4 mm). For single-view scan angles of 3°, 30°, and 60°, the results for MM-FD technique were 32.77% (±17.87%)∕3.2 mm (±2.2 mm), 24.57% (±18.18%)∕2.9 mm (±2.0 mm), and 10.48% (±9.50%)∕1.1 mm (±1.3 mm), respectively. For projection angular-sampling-intervals of 0.6°, 1.2°, and 2.5° with the orthogonal-view 30°-each scan angle, the MM-FD technique generated similar VPD (maximum deviation 2.91%) and COMS (maximum deviation 0.6 mm), while sparser sampling yielded larger VPD∕COMS. With equal number of projections, the estimation results using scattered 360° scan angle were slightly better than those using orthogonal-view 30°-each scan angle. The estimation accuracy of MM-FD technique declined as noise level increased.

CONCLUSIONS

The MM-FD technique substantially improves the estimation accuracy for onboard 4D-CBCT using prior planning 4D-CT and limited-angle projections, compared to the MM-only and FD-only techniques. It can potentially be used for the inter/intrafractional 4D-localization verification.

The potential role of respiratory motion management and image guidance in the reduction of severe toxicities following stereotactic ablative radiation therapy for patients with centrally located early stage non-small cell lung cancer or lung metastases

Image guidance allows delivery of very high doses of radiation over a few fractions, known as stereotactic ablative radiotherapy (SABR). This treatment is associated with excellent outcome for early stage non-small cell lung cancer and metastases to the lungs. In the delivery of SABR, central location constantly poses a challenge due to the difficulty of adequately sparing critical thoracic structures that are immediately adjacent to the tumor if an ablative dose of radiation is to be delivered to the tumor target. As of current, various respiratory motion management and image guidance strategies can be used to ensure accurate tumor target localization prior and/or during daily treatment, which allows for maximal and safe reduction of set up margins. The incorporation of both may lead to the most optimal normal tissue sparing and the most accurate SABR delivery. Here, the clinical outcome, treatment related toxicities, and the pertinent respiratory motion management/image guidance strategies reported in the current literature on SABR for central lung tumors are reviewed.

The effectiveness of combined gating and re-scanning for treating mobile targets with proton spot scanning. An experimental and simulation-based investigation

Organ motion is one of the major obstacles in radiotherapy and charged particle therapy. Even more so, the theoretical advantages of dose distributions in scanned ion beam therapy may be lost due to the interplay between organ motion and beam scanning. Several techniques for dealing with this problem have been devised. In re-scanning, the target volume is scanned several times to average out the motion effects. In gating and breath-hold, dose is only delivered if the tumour is in a narrow window of position. Experiments have been performed to verify if gating and re-scanning are effective means of motion mitigation. Dose distributions were acquired in a lateral plane of a homogeneous phantom. For a spherical target volume and regular motion gating was sufficient. However, for realistic, irregular motion or a patient target volume, gating did not reduce the interplay effect to an acceptable level. Combining gating with re-scanning recovered the dose distributions. The simplest re-scanning approach, where a treatment plan is duplicated several times and applied in sequence, was not efficient. Simulations of different combinations of gating window sizes and re-scanning schemes revealed that reducing the gating window is the most efficient approach. However, very small gating windows are not robust for irregular motion.

Strategies of dose escalation in the treatment of locally advanced non-small cell lung cancer: image guidance and beyond

Radiation dose in the setting of chemo-radiation for locally advanced non-small cell lung cancer (NSCLC) has been historically limited by the risk of normal tissue toxicity and this has been hypothesized to correlate with the poor results in regard to local tumor recurrences. Dose escalation, as a means to improve local control, with concurrent chemotherapy has been shown to be feasible with three-dimensional conformal radiotherapy in early phase studies with good clinical outcome. However, the potential superiority of moderate dose escalation to 74 Gy has not been shown in phase III randomized studies. In this review, the limitations in target volume definition in previous studies; and the factors that may be critical to safe dose escalation in the treatment of locally advanced NSCLC, such as respiratory motion management, image guidance, intensity modulation, FDG-positron emission tomography incorporation in the treatment planning process, and adaptive radiotherapy, are discussed. These factors, along with novel treatment approaches that have emerged in recent years, are proposed to warrant further investigation in future trials in a more comprehensive and integrated fashion.

Four-dimensional versus 3-dimensional computed tomographic planning for gastric mucosa associated lymphoid tissue lymphoma

PURPOSE

This study compares dosimetric parameters of 4-dimensional (4D) and 3-dimensional (3D) computed tomographic (CT) planning for gastric mucosa-associated lymphoid tissue (MALT) lymphoma in an attempt to identify any potential benefit of 4DCT planning.

METHODS AND MATERIALS

We identified 18 patients who received definitive 4DCT radiation planning from September 2006 to September 2011 for gastric MALT lymphoma at our institution. In addition to the kidneys and liver, we contoured an internal target volume (ITV) and static clinical target volume (sCTV) for each patient based on the 4D and 3D images, respectively, to develop 3D conformal radiation plans. Using the static and motion plans, we measured the volume of ITV covered by at least 95% of the prescribed dose (V95), the minimum dose received by 95% of the ITV (D95), and the volume of organs receiving at least 20 Gy or 30 Gy (V20 or V30).

RESULTS

Volumes of the ITV, motion liver, left kidney, and right kidney were significantly larger than their static counterparts. The static plan significantly lowered the ITV V95 and D95 compared with the motion plan. However, this undercoverage was significantly associated with the superior-inferior (SI) respiratory excursions. A V95 of >98% was observed in 92% of patients with SI excursions <15 mm versus 33% of patients with SI excursions >15 mm (P = .02). When compared with the motion plan, the static plan also significantly lowered the liver V30 and left kidney V20.

CONCLUSIONS

The 3DCT planning can result in undercoverage of the ITV and altered estimation of doses to normal structures. However, in patients with limited respiratory excursions (<15 mm), 4D and 3D images generated similar ITV coverage.

Assessment of tumor motion reproducibility with audio-visual coaching through successive 4D CT sessions

This study aimed to compare combined audio-visual coaching with audio coaching alone and assess their respective impact on the reproducibility of external breathing motion and, one step further, on the internal lung tumor motion itself, through successive sessions. Thirteen patients with NSCLC were enrolled in this study. The tumor motion was assessed by three to four successive 4D CT sessions, while the breathing signal was measured from magnetic sensors positioned on the epigastric region. For all sessions, the breathing was regularized with either audio coaching alone (AC, n = 5) or combined with a real-time visual feedback (A/VC, n = 8) when tolerated by the patients. Peak-to-peak amplitude, period and signal shape of both breathing and tumor motions were first measured. Then, the correlation between the respiratory signal and internal tumor motion over time was evaluated, as well as the residual tumor motion for a gated strategy. Although breathing and tumor motions were comparable between AC and AV/C groups, A/VC approach achieved better reproducibility through sessions than AC alone (mean tumor motion of 7.2 mm ± 1 vs. 8.6 mm ± 1.8 mm, and mean breathing motion of 14.9 mm ± 1.2 mm vs. 13.3mm ± 3.7 mm, respectively). High internal/external correlation reproducibility was achieved in the superior-inferior tumor motion direction for all patients. For the anterior posterior tumor motion direction, better correlation reproducibility has been observed when visual feedback has been used. For a displacement-based gating approach, A/VC might also be recommended, since it led to smaller residual tumor motion within clinically relevant duty cycles. This study suggests that combining real-time visual feedback with audio coaching might improve the reproducibility of key characteristics of the breathing pattern, and might thus be considered in the implementation of lung tumor radiotherapy.

Interfraction displacement of primary tumor and involved lymph nodes relative to anatomic landmarks in image guided radiation therapy of locally advanced lung cancer

PURPOSE

To analyze primary tumor (PT) and lymph node (LN) position changes relative to each other and relative to anatomic landmarks during conventionally fractionated radiation therapy for patients with locally advanced lung cancer.

METHODS AND MATERIALS

In 12 patients with locally advanced non-small cell lung cancer PT, LN, carina, and 1 thoracic vertebra were manually contoured on weekly 4-dimensional fan-beam CT scans. Systematic and random interfraction displacements of all contoured structures were identified in the 3 cardinal directions, and resulting setup margins were calculated. Time trends and the effect of volume changes on displacements were analyzed.

RESULTS

Three-dimensional displacement vectors and systematic/random interfraction displacements were smaller for carina than for vertebra both for PT and LN. For PT, mean (SD) 3-dimensional displacement vectors with carina-based alignment were 7 (4) mm versus 9 (5) mm with bony anatomy (P<.0001). For LN, smaller displacements were found with carina- (5 [3] mm, P<.0001) and vertebra-based (6 [3] mm, P=.002) alignment compared with using PT for setup (8 [5] mm). Primary tumor and LN displacements relative to bone and carina were independent (P>.05). Displacements between PT and bone (P=.04) and between PT and LN (P=.01) were significantly correlated with PT volume regression. Displacements between LN and carina were correlated with LN volume change (P=.03).

CONCLUSIONS

Carina-based setup results in a more reproducible PT and LN alignment than bony anatomy setup. Considering the independence of PT and LN displacement and the impact of volume regression on displacements over time, repeated CT imaging even with PT-based alignment is recommended in locally advanced disease.

Investigation of gated cone-beam CT to reduce respiratory motion blurring

PURPOSE

Methods of reducing respiratory motion blurring in cone-beam CT (CBCT) have been limited to lung where soft tissue contrast is large. Respiration-correlated cone-beam CT uses slow continuous gantry rotation but image quality is limited by uneven projection spacing. This study investigates the efficacy of a novel gated CBCT technique.

METHODS

In gated CBCT, the linac is programmed such that gantry rotation and kV image acquisition occur within a gate around end expiration and are triggered by an external respiratory monitor. Standard CBCT and gated CBCT scans are performed in 22 patients (11 thoracic, 11 abdominal) and a respiration-correlated CT (RCCT) scan, acquired on a standard CT scanner, from the same day serves as a criterion standard. Image quality is compared by calculating contrast-to-noise ratios (CNR) for tumors in lung, gastroesophageal junction (GEJ) tissue, and pancreas tissue, relative to surrounding background tissue. Congruence between the object in the CBCT images and that in the RCCT is measured by calculating the optimized normalized cross-correlation (NCC) following CBCT-to-RCCT rigid registrations.

RESULTS

Gated CBCT results in reduced motion artifacts relative to standard CBCT, with better visualization of tumors in lung, and of abdominal organs including GEJ, pancreas, and organs at risk. CNR of lung tumors is larger in gated CBCT in 6 of 11 cases relative to standard CBCT. A paired two-tailed t-test of lung patient mean CNR shows no statistical significance (p = 0.133). In 4 of 5 cases where CNR is not increased, lung tumor motion observed in RCCT is small (range 1.3-5.2 mm). CNR is increased and becomes statistically significant for 6 out of 7 lung patients with > 5 mm tumor motion (p = 0.044). CNR is larger in gated CBCT in 5 of 7 GEJ cases and 3 of 4 pancreas cases (p = 0.082 and 0.192). Gated CBCT yields improvement with lower NCC relative to standard CBCT in 10 of 11, 7 of 7, and 3 of 4 patients for lung, GEJ, and pancreas images, respectively (p = 0.0014, 0.0030, 0.165).

CONCLUSIONS

Gated CBCT reduces image blurring caused by respiratory motion. The gated gantry rotation yields uniformly and closely spaced projections resulting in improved reconstructed image quality. The technique is shown to be applicable to abdominal sites, where image contrast of soft tissues is low.

Tumor tracking based on correlation models in scanned ion beam therapy: an experimental study

Accurate dose delivery to extra-cranial lesions requires tumor motion compensation. An effective compensation can be achieved by real-time tracking of the target position, either measured in fluoroscopy or estimated through correlation models as a function of external surrogate motion. In this work, we integrated two internal/external correlation models (a state space model and an artificial neural network-based model) into a custom infra-red optical tracking system (OTS). Dedicated experiments were designed and conducted at GSI (Helmholtzzentrum für Schwerionenforschung). A robotic breathing phantom was used to reproduce regular and irregular internal target motion as well as external thorax motion. The position of a set of markers placed on the phantom thorax was measured with the OTS and used by the correlation models to infer the internal target position in real-time. Finally, the estimated target position was provided as input for the dynamic steering of a carbon ion beam. Geometric results showed that the correlation models transversal (2D) targeting error was always lower than 1.3 mm (root mean square). A significant decrease of the dosimetric error with respect to the uncompensated irradiation was achieved in four out of six experiments, demonstrating that phase shifts are the most critical irregularity for external/internal correlation models.

Building motion models of lung tumours from cone-beam CT for radiotherapy applications

A method is presented to build a surrogate-driven motion model of a lung tumour from a cone-beam CT scan, which does not require markers. By monitoring an external surrogate in real time, it is envisaged that the motion model be used to drive gated or tracked treatments. The motion model would be built immediately before each fraction of treatment and can account for inter-fraction variation. The method could also provide a better assessment of tumour shape and motion prior to delivery of each fraction of stereotactic ablative radiotherapy. The two-step method involves enhancing the tumour region in the projections, and then fitting the surrogate-driven motion model. On simulated data, the mean absolute error was reduced to 1 mm. For patient data, errors were determined by comparing estimated and clinically identified tumour positions in the projections, scaled to mm at the isocentre. Averaged over all used scans, the mean absolute error was under 2.5 mm in superior-inferior and transverse directions.

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.

Patient specific respiratory motion modeling using a 3D patient's external surface

PURPOSE

Respiratory motion modeling of both tumor and surrounding tissues is a key element in minimizing errors and uncertainties in radiation therapy. Different continuous motion models have been previously developed. However, most of these models are based on the use of parameters such as amplitude and phase extracted from 1D external respiratory signal. A potentially reduced correlation between the internal structures (tumor and healthy organs) and the corresponding external surrogates obtained from such 1D respiratory signal is a limitation of these models. The objective of this work is to describe a continuous patient specific respiratory motion model, accounting for the irregular nature of respiratory signals, using patient external surface information as surrogate measures rather than a 1D respiratory signal.

METHODS

Ten patients were used in this study having each one 4D CT series, a synchronized RPM signal and patient surfaces extracted from the 4D CT volumes using a threshold based segmentation algorithm. A patient specific model based on the use of principal component analysis was subsequently constructed. This model relates the internal motion described by deformation matrices and the external motion characterized by the amplitude and the phase of the respiratory signal in the case of the RPM or using specific regions of interest (ROI) in the case of the patients' external surface utilization. The capability of the different models considered to handle the irregular nature of respiration was assessed using two repeated 4D CT acquisitions (in two patients) and static CT images acquired at extreme respiration conditions (end of inspiration and expiration) for one patient.

RESULTS

Both quantitative and qualitative parameters covering local and global measures, including an expert observer study, were used to assess and compare the performance of the different motion estimation models considered. Results indicate that using surface information [correlation coefficient (CC): 0.998 ± 0.0006 and model error (ME): 1.35 ± 0.21 mm] is superior to the use of both motion phase and amplitude extracted from a 1D respiratory signal (CC and ME of 0.971 ± 0.02 and 1.64 ± 0.28 mm). The difference in performance was more substantial compared to the use of only one parameter (phase or amplitude) used in the motion model construction. Similarly, the patient surface based model was better in estimating the motion in the repeated 4D CT acquisitions and those CT images acquired at the full inspiration (FI) and the full expiration (FE). Once more, within this context the use of both amplitude and phase in the model building was substantially more robust than the use of phase or amplitude only.

CONCLUSIONS

The present study demonstrates the potential of using external patient surfaces for the construction of patient specific respiratory motion models. Such information can be obtained using different devices currently available. The use of external surface information led to the best performance in estimating internal structure motion. On the other hand, the use of both amplitude and phase parameters derived from an 1D respiration signal led to largely superior model performance relative to the use of only one of these two parameters in the model building process.

Quasi-breath-hold technique using personalized audio-visual biofeedback for respiratory motion management in radiotherapy

PURPOSE

To introduce a respiratory motion management technique, so called quasi-breath-hold (QBH) technique and evaluate its feasibility. As a hybrid technique combining free-breathing-based gating (denoted as gating for convenience) and breath-hold (BH), the QBH is designed to overcome typical limitations existing in either one such as phase-shift, residual motion, complexity, and discomfort.

METHODS

The QBH is realized using an audio-visual biofeedback system (AVBFS) and a respiratory motion management program (RMMP). The AVBFS, consisting of two infra-red stereo cameras and a head mounted display, monitors respiratory motion and provides dynamic feedback to patients. The RMMP establishes a personalized respiration model based on deep free breathing. The model is further processed to generate a QBH model by inserting a short breath-hold period into the end point of the-end-of-expiration phase. Then the patient is guided to follow the QBH model through the AVBFS. A simulation study with ten volunteers was performed to evaluate the feasibility of the proposed technique. In the simulation, an in-house developed macro program automatically controlled the QBH procedure to virtually deliver an intensity modulated radiation therapy (IMRT) plan. For each volunteer subject, three QBH maneuvers with different breath-hold times of 3, 5, and 7s (denoted as QBH3s, QBH5s, and QBH7s, respectively) and a conventional gating maneuver with 30% duty cycle (for comparison purpose) were applied. External respiration motion signals obtained during the gating window were analyzed to obtain mean absolute error (MAE) between the measured and guiding curve, mean absolute deviation (MAD) of the measured curve, and an inverse uncertainty time histogram (IUTH).

RESULTS

Every volunteer successfully performed all of the four maneuvers (1 gating and 3 QBH patterns). The average treatment times were 466.8, 452.3, and 430.8 s for the QBH3s, QBH5s, and QBH7s, respectively, compared to 530.4 s for the gating technique. The mean absolute errors between measured and guiding curve during the gating window were 0.9 +/- 0.7, 0.8 +/- 0.6, 0.7 +/- 0.6, and 0.6 +/- 0.7 mm for the gating, QBH3s, QBH5s, and QBH7s, respectively. The mean absolute deviations of the measured curve during the gating window were 0.7 +/- 0.7, 0.5 +/- 0.5, 0.5 +/- 0.4, and 0.5 +/- 0.6 mm for the gating, QBH3s, QBH5s, and QBH7s, respectively. In the analysis of the IUTH during the gating window, the QBH simulations showed similar (QBH3s) or less (QBH5s and QBH7s) motion uncertainties compared to the gating simulation.

CONCLUSIONS

The proposed QBH technique with personalized audio-visual biofeedback was feasible for respiratory motion management. It showed equivalent or less motion uncertainty and shorter treatment time than the conventional free-breathing-based gating technique did. The technique is expected to optimally compromise between patient comfort and treatment efficiency.

Technical advances of radiation therapy for thymic malignancies

Radiation therapy often plays a critical role in the treatment of thymic malignancies. However, because of the location of these tumors, historically patients have been at a significant risk for radiation-related toxicity such as pericardial effusions, radiation pneumonitis, long-term pulmonary fibrosis, and occasional long-term esophageal stricture, particularly for unresectable thymoma. Recent advancements in technology have provided the treating radiation oncologist with the ability to more accurately target the region at risk while sparing normal structures. In this review, we provide an overview of key advances in radiation techniques for thymoma over the past two decades. These techniques include 3D conformal therapy, intensity-modulated radiation therapy, 4D treatment planning, adaptive radiation therapy, and proton therapy. Each advancement has brought with it unique advantages in maintaining long-term disease control while improving quality of life in this manageable disease.

Adaptive radiation for lung cancer

The challenges of lung cancer radiotherapy are intra/inter-fraction tumor/organ anatomy/motion changes and the need to spare surrounding critical structures. Evolving radiotherapy technologies, such as four-dimensional (4D) image-based motion management, daily on-board imaging and adaptive radiotherapy based on volumetric images over the course of radiotherapy, have enabled us to deliver higher dose to target while minimizing normal tissue toxicities. The image-guided radiotherapy adapted to changes of motion and anatomy has made the radiotherapy more precise and allowed ablative dose delivered to the target using novel treatment approaches such as intensity-modulated radiation therapy, stereotactic body radiation therapy, and proton therapy in lung cancer, techniques used to be considered very sensitive to motion change. Future clinical trials using real time tracking and biological adaptive radiotherapy based on functional images are proposed.