Superior-inferior Direction Research Articles

Degree of reduction in normal liver complication probability from free-breathing to breath-hold liver SBRT: a dose-escalation strategy using radiation dose-volume effect

Introduction. This study aimed to analyze the degree of reduction in normal liver complication probability (NTCP) from free-breathing (FB) to breath-hold (BH) liver SBRT. The effect of the radiation dose-volume on the mean liver dose (MLD) was also analyzed due to dose prescription, normal liver volume (NLV), and PTV. Materials and Methods. Thirty-three stereotactic body radiation therapy (SBRT) cases of hepatocellular carcinoma were selected, retrospectively. For FB, the treatments were planned on average intensity projection scan (CTavg), and patient-specific internal target volume (ITV) margins were applied. To simulate the BH treatment, computed tomography (CT) scan correspond to the 40%—50% of the respiratory cycle (CT40%-50%) was chosen, and an appropriate intrafraction margin of 2 mm, 1.5 mm, and 1.5 mm were given in craniocaudal (CC), superior-inferior (SI), and lateral direction to generate the final iGTV. As per RTOG 1112, all organs at risk (OAR’s) were considered during the optimization of treatment plans. NTCP was calculated using LKB fractionated model. Multivariate regression analysis was performed to see the effect of EQD2Gy, NLV, and PTV on MLD2Gy. Results. A significant dosimetric difference was observed in the normal liver (liver-ITV/iGTV). A reduction of 1.7% in NTCP was observed from FB to BH technique. The leverage of dose escalation is more in BH because MLD2Gy corresponds to 5%, 10%, 20%, and 50% NTCP was 0.099 Gy, 0.41 Gy, 1.21 Gy, and 3.432 Gy more in BH as compared to FB technique. In MVRA, the major factor which was attributed to a change in MLD2Gy is EQD2Gy. Conclusion. From FB to BH technique, a significant reduction in NTCP was observed. The dose prescription is a major factor attributed to the change in MLD2Gy. Advances in knowledge: If feasible, prefer BH treatment either for tumor dose escalation or for the reduction in NTCP.

Multi-contrast four-dimensional magnetic resonance imaging (MC-4D-MRI): Development and initial evaluation in liver tumor patients.

To develop a novel multi-contrast four-dimensional magnetic resonance imaging (MC-4D-MRI) technique that expands single image contrast 4D-MRI to a spectrum of native and synthetic image contrasts and to evaluate its feasibility in liver tumor patients. The MC-4D-MRI technique integrates multi-parametric MRI fusion, 4D-MRI, and deformable image registration (DIR) techniques. The fusion technique consists of native MRI as input, image pre-processing, fusion algorithm, adaptation, and fused multi-contrast MRI as output. Four-dimensional deformation vector fields (4D-DVF) were generated from an original T2/T1-w 4D-MRI by deforming end-of-inhalation (EOI) to nine other phase volumes via DIR. The 4D-DVF were applied to multi-contrast MRI to generate a spectrum of 4D-MRI in different image contrasts. The MC-4D-MRI technique was evaluated in five liver tumor patients on tumor contrast-to-noise ratio (CNR), internal target volume (ITV) contouring consistency, diaphragm motion range, and tumor motion trajectory; and in digital anthropomorphic phantoms on 4D-DIR introduced errors in tumor motion range, centroid location, extent, and volume. MC-4D-MRI consisting of 4D-MRIs in native image contrasts (T1-w, T2-w, and T2/T1-w) and synthetic image contrasts, such as tumor-enhanced contrast (TEC) were generated in five liver tumor patients. Patient tumor CNR increased from 2.6±1.8 in the T2/T1-w MRI, to -4.4±2.4, 6.6±3.0, and 9.6±3.9 in the T1-w, T2-w, and TEC MRI, respectively. Patient ITV inter-observer mean Dice similarity coefficient (mDSC) increased from 0.65±0.10 in the original T2/T1-w 4D-MRI, to 0.76±0.14, 0.77±0.12, and 0.86±0.05 in the T1-w, T2-w, and TEC 4D-MRI, respectively. Patient diaphragm motion range absolute differences between the three new 4D-MRIs and original T2/T1-w 4D-MRI were 1.2±1.3, 0.3±0.7, and 0.5±0.5mm, respectively. Patient tumor displacement phase-averaged absolute differences between the three 4D-MRIs and the original 4D-MRI were 0.72±0.33, 0.62±0.54, and 0.74±0.43mm in the superior-inferior (SI) direction, and 0.59±0.36, 0.51±0.30, and 0.50±0.24mm in the anterior-posterior (AP) direction, respectively. In the digital phantoms, phase-averaged absolute tumor centroid shift caused by the 4D-DIR were at or below 0.5mm in SI, AP, and left-right (LR) directions. We developed an MC-4D-MRI technique capable of expanding single image contrast 4D-MRI along a new dimension of image contrast. Initial evaluations in liver tumor patients showed enhancements in image contrast variety, tumor contrast, and ITV contouring consistencies using MC-4D-MRI. The technique might offer new perspectives on the image contrast of MRI and 4D-MRI in MR-guided radiotherapy.

Effect of Corneal Tilt on the Determination of Asphericity.

Purpose: To quantify the effect of levelling the corneal surface around the optical axis on the calculated values of corneal asphericity when conic and biconic models are used to fit the anterior corneal surface. Methods: This cross-sectional study starts with a mathematical simulation proving the concept of the effect that the eye’s tilt has on the corneal asphericity calculation. Spherical, conic and biconic models are considered and compared. Further, corneal asphericity is analysed in the eyes of 177 healthy participants aged 35.4 ± 15.2. The optical axis was determined using an optimization procedure via the Levenberg–Marquardt nonlinear least-squares algorithm, before fitting the corneal surface to spherical, conic and biconic models. The influence of pupil size (aperture radii of 1.5, 3.0, 4.0 and 5.0 mm) on corneal radius and asphericity was also analysed. Results: In computer simulations, eye tilt caused an increase in the apical radii of the surface with the increase of the tilt angle in both positive and negative directions and aperture radii in all models. Fitting the cornea to spherical models did not show a significant difference between the raw-measured corneal surfaces and the levelled surfaces for right and left eyes. When the conic models were fitted to the cornea, changes in the radii of the cornea among the raw-measured corneal surfaces’ data and levelled data were not significant; however, significant differences were recorded in the asphericity of the anterior surfaces at radii of aperture 1.5 mm (p < 0.01). With the biconic model, the posterior surfaces recorded significant asphericity differences at aperture radii of 1.5 mm, 3 mm, 4 mm and 5 mm (p = 0.01, p < 0.01, p < 0.01 & p < 0.01, respectively) in the nasal temporal direction of right eyes and left eyes (p < 0.01, p < 0.01, p < 0.01 & p < 0.01, respectively). In the superior–inferior direction, significant changes were only noticed at aperture radii of 1.5 mm for both right and left eyes (p = 0.05, p < 0.01). Conclusions: Estimation of human corneal asphericity from topography or tomography data using conic and biconic models of corneas are affected by eyes’ natural tilt. In contrast, the apical radii of the cornea are less affected. Using corneal asphericity in certain applications such as fitting contact lenses, corneal implant design, planning for refractive surgery and mathematical modelling when a geometrical centre of the eye is needed should be implemented with caution.

The Efficiency of Using Mirror Imaged Topography in Fellow Eyes Analyses of Pentacam HR Data

Purpose: To investigate the effectiveness of flipping left corneas topography and analysethem quantitively along with fellow right corneas based on the assumption that they are mirror images of each other. Methods: The study involved scanning both eyes of 177 healthy participants (aged 35.3 ± 15.8) and 75 keratoconic participants (aged 33.9 ± 17.8). Clinical tomography data were collected for both eyes using the Pentacam HR and processed by a fully automated custom-built MATLAB code. For every case, the right eye was used as a datum fixed surface while the left cornea was flipped around in the superior–inferior direction. In this position, the root-mean-squared difference (RMS) between the flipped left cornea and the right cornea was initially determined for both the anterior and posterior corneal surfaces. Next, the iterative closest point transformation algorithm was applied on the three-dimensional flipped cornea to allow the flipped left corneal anterior surface to translate and rotate, minimising the difference between it and the right corneal anterior surface. Then, the RMS differences were recalculated and compared. Results: A comparison of the dioptric powers showed a significant difference between the RMS of both the flipped left eyes and the right eyes in the healthy and the KC groups (p &lt; 0.001). The RMS of the surfaces of the flipped left corneas and the right corneas was 0.6 ± 0.4 D among the healthy group and 4.1 ± 2.3 among the KC group. After transforming the flipped left corneas, the RMS was recorded as 0.5 ± 0.3 D and 2.4 ± 2 D among the healthy and KC groups, respectively. Conclusions: Although fellow eyes are highly related in their clinical parameters, they should be treated with care when one eye topography is flipped and processed with the other eye topography in an optic-related research analysis where translation might be needed. In KC, an asymmetric disease, it was observed that a portion of the asymmetry was due to a corneal apex shift interfering with the image acquisition. Therefore, transforming the flipped left eyes by rotation and translation results in a fairer comparison between the fellow KC corneas.

Set-Up Errors, Organ Motion, Tumour Regression and its Implications on Internal Target Volume–Planning Target Volume During Cervical Cancer Radiotherapy: Results From a Prospective Study

AimsUterocervical motions and organ filling during cervical cancer conformal radiotherapy is complex. This prospective, observational study investigated set-up margins (clinical target vo, ume [CTV] to planning target volume [PTV]) for pelvic nodal CTV and internal margin (CTV to internal target volume [ITV]) expansions for uterocervical movements during cervical cancer radiotherapy. Materials and methodsDuring cervical cancer radiotherapy, a daily kilovoltage, cone-beam computed tomography (CBCT) scan was acquired. Bony anatomy-based rigid co-registration and matching to vessels/pelvic nodal region was carried out to document shifts, errors (systematic and random) and to calculate CTV to PTV margins. Subsequently, soft-tissue matching was carried out at the mid-cervical region and uterine fundus to record shifts, errors and to calculate CTV to ITV margins. ResultsIn 67 patients, 1380 CBCT scans were analysed. The mean (±standard deviation) couch shifts for CTV pelvic nodal region in all directions were within 4.5–5.3 mm, systematic and random errors 3.0–3.6 mm and set-up margins of within 10 mm (except anterior margin 10.3 mm). For the mid-cervical region, mean shifts were 4.5–5.5 mm, systematic and random errors 2–4 mm amounting to <10 mm internal margins (CTV–ITV for cervix) and for uterine fundus mean (±standard deviation) shifts were larger in the superior direction (12.1 mm) but 4.0–7.5 mm in other directions, systematic and random errors 2–7 mm amounting to anisotropic margins in various directions (10 mm in anterior–posterior and lateral directions, 12–20 mm in superior–inferior directions) (CTV–ITV for uterine fundus). ConclusionOur study suggests anisotropic CTV to ITV and CTV to PTV margins for cervical cancer radiotherapy.

An algorithm for real time minimum toe clearance estimation from signal of in-shoe motion sensor.

An algorithm has been constructed for estimating minimum toe clearance (MTC), an important gait parameter previously proven to be a critical indicator of tripping risk. It uses data from a previously reported in-shoe motion sensor (IMS) for detecting gait events. First, candidate feature points in the IMS signal for use in detecting MTC events were identified. Then, the temporal agreement between each feature point and target MTC event was evaluated. Next, the accuracy and precision of the MTC estimated using each feature point was evaluated using a reference value obtained using a 3-D optical motion-capture system. The MTC was estimated using a geometric model and the IMS signal corresponding to the predicted MTC event. Once the best candidate feature point was identified, a real-time MTC estimation algorithm for use with an IMS was constructed. The mean values and standard deviations of measured foot motions obtained in a previous study were used for evaluating accuracy and precision. The results suggest that MTC events can be estimated by detecting the crossing point between the acceleration waveforms in the anterior-posterior and superior-inferior directions in an accuracy of 2.0% gait cycle. Using this feature point enables the MTC to be estimated in real time with an accuracy of 8.6 mm, which will enable monitoring of MTC in daily living.

Anterior dislocation of sternoclavicular joint: A novel surgical technique

IntroductionAnterior sternoclavicular joint dislocation (SCJ) is a relative rare injury, related to high energy trauma. The objective of the present study is to present a novel suture technique for treatment of anterior SCJ traumatic dislocation and to report clinical outcomes from a small case series undergoing this procedure. Patients and MethodsPatients presenting with traumatic anterior SCJ disruption in our institution were eligible to participate. Surgical technique consisted of two bone tunnels drilled in vertical direction from the anterior to the posterior cortex of the manubrium. Analogous to the sternal side of the clavicle, two vertical bone tunnels were drilled from the anterior cortex towards the posterior cortex. A non-absorbable suture was passed though the four holes in a parallel configuration. Then, by pulling the free suture edges the posterior translation of the clavicle was performed. Two additional drill holes, the first in manubrium and the second in clavicle were performed from the anterior cortex to the posterior between the previous bone tunnels. A non-absorbable suture was placed in a simple configuration in order to stabilize the SCJ in the superior–inferior direction. The final follow up was 28.2 months. The mean QuickDASH was used for functional assessment. ResultsSeven patients (6 males and 1 female) with average age of 34,8 years were included in the present study. Two patients suffered from concomitant medial clavicle fracture. At final follow-up (none of the patients had experienced any symptoms of instability of SCJ, no side-to-side difference was observed, while the Mean QuickDASH score was 4.85. ConclusionThe reported technique for SCJ reconstruction in traumatic anterior SCJ dislocations with two sutures has theoretical advantages, since it stabilizes the SCJ in the antero–posterior, as well as the supero- inferior direction. Outcomes from this small case series are favorable. However, more research is desirable to compare different techniques and to conclude to the optimal surgical treatment.

Comparison of a First-in-Class LINAC-Integrated PET System and a Diagnostic PET/CT Scanner

The X1 system ("X1") integrates a fully functional Positron Emission Tomography (PET) combined with a fan-beam kVCT system into the architecture of a ring-gantry linear accelerator (LINAC) for biology-guided radiotherapy (BgRT) delivery. The integration of PET into a LINAC framework required design features that deviate from the diagnostic setting. For instance, the X1 houses two opposed 90° PET arcs that rotate at 60 RPM to generate sinogram reconstructed images rather than a static, circumferential array of detectors. To better understand X1 PET performance, we conducted a side-by-side comparison of images of a PET phantom obtained sequentially on the X1 PET subsystem and a traditional PET/CT scanner.An independent quality assurance phantom utilized a custom insert with two-insert targets filled with 18F-fluorodeoxyglucose (FDG) diluted water. Spherical target had a diameter of 22 mm and C-shape geometry consisted of an active volume of 26mm axial length and 255-degree partial annulus with major and minor radii of 27 mm x 12 mm, respectively. The targets were filled with an activity concentration of 9.3 kBq/ml to achieve a target to background ratio of 8:1 simulating the target uptake with hot background in a patient. The phantom was imaged within 30 minutes of preparation on the X1 PET and then immediately transferred to a PET/CT Biograph scanner for a second image. This experiment was repeated three times in separate days to assess reproducibility of PET acquisition. The PET images were visually assessed by the radiation oncologists to ensure the PET signal is within PTV. The PET signals of the targets were also evaluated using the full-width-half-max (FWHM) calculated on left-right (RL), anterior-posterior (AP) and superior-inferior (SI) directions for both targets and compared to the physical dimensions of the cavities.The imaging acquisition tests were successfully completed on all three days for both systems, as per the prescribed workflow. In all experiments, radiation oncologists verified that the PET avid area were within the PTV in both systems. Average FWHM results of the 22 mm spherical target on X1 PET were 24.5, 21.7 and 17.0 mm as compared to 20.0, 20.0 and 18.2 mm for PET/CT in LR, AP and SI directions, respectively. For the C-shape object X1 PET resulted in 18.8, 19.5, 21.8 and 25.0 mm whereas the PET/CT resulted in 12.7, 13.2, 13.2 and 24.7 mm FWHMs for the two RL C-shape legs, AP and SI directions, respectively as compared to 15-, 15-, 15- and 26-mm physical dimensions.The X1 PET subsystem - which utilizes dual PET arcs and a sinogram image reconstruction algorithm in order to integrate into a LINAC architecture - generates images that are qualitatively comparable to a traditional PET scanner. In general, X1 FWHM results were slightly larger and PET/CT ones were smaller than the physical dimensions in axial plane. Both systems slightly underestimated the target size in SI direction.

A Patient-Specific Correspondence Model to Track Tumor Location in Thorax During Radiation Therapy

<h3>Purpose/Objective(s)</h3> In radiation therapy, accurate localization of tumors is crucial to deliver the maximum dose to the tumor while sparing the organs at risk. However, respiration-induced motion creates challenges in the localization of the tumors in the thorax and abdomen. In this study, we present a patient-specific correspondence model to track tumors in the thorax during radiation therapy using surface displacement as the surrogate signal. <h3>Materials/Methods</h3> The proposed model is made prior to the treatment for each patient. Four-dimensional computed tomography (4D-CT) images of the patient are used as the knowledge of internal motion, and the displacement of two points on the patient's skin on the thoracic area is used as surrogate signals. The two types of data are acquired simultaneously. The 4D-CT images are sorted into ten respiratory phases by the amplitude-binning algorithm. A deformable image registration algorithm is applied to the 4D-CT images to extract the patient's internal motion data. The patient's average skin surface displacement at each respiratory phase is calculated from surrogate signals. Principal component analysis is used to fit the correspondence model. During radiation delivery, the model incorporates recorded surrogate signals as an input and delivers the 3D trajectory of the tumor or other anatomy of interest. We evaluated the accuracy of the proposed model on a respiratory phantom and five lung cancer patients. <h3>Results</h3> For the respiratory phantom, the location of the center of a target sphere inside the phantom during treatment was calculated in three directions: Left-Right (LR), Anterior-Posterior (AP) and, Superior-Inferior (SI). The error of localization was less than 1 mm in the LR and AP directions and less than 2 mm in the SI direction. For two patients, the location of the treatment isocenter and for the other three patients, the location of the apex of the diaphragm was calculated in three directions. The localization error in the LR and AP directions was less than 1 mm for all patients. For four of the patients, the localization error in the SI direction was less than 2 mm, and for one patient, it was less than 3 mm. <h3>Conclusion</h3> To reduce the unnecessary radiation exposure to the organs at risk, accurate localization of the tumors that have movement due to respiration is essential. In this study, we propose a patient-specific model to locate the tumor or any anatomy of interest in the thorax during radiation treatment. Our model is dose-free and accounts for hysteresis and irregular breathing.

Cardiac Motion and its Impact on Target Coverage During SBRT for Refractory Ventricular Tachycardia

<h3>Purpose/Objective(s)</h3> Refractory ventricular tachycardia (VT) is a common cause of death and morbidity in the heart failure population. Stereotactic body radiation therapy (SBRT) is a new and promising approach to treating VT, but there are practical concerns about SBRT delivery to the heart, which undergoes motion from both cardiac and respiratory cycles. In this study, we investigate motion magnitudes from both cycles and their impact on treatment margins and target coverage for VT SBRT. <h3>Materials/Methods</h3> Twenty-one patients were treated with SBRT for VT at our institution between April 2018 and December 2020 under an IRB-approved registry protocol. All patients had respiratory-correlated 4DCT for treatment planning and 7 patients had diagnostic cardiac-gated CT scans taken less than two weeks prior to treatment. Target structures were delineated on free-breathing CT and transferred to respiratory average (RAVG) CT via rigid registration. Dose was initially calculated on the RAVG per current institutional standard of care. For this retrospective study, deformable image registration (DIR) was applied between RAVG and the cardiac average CT (CAVG), systolic, and diastolic CTs from the cardiac dataset. Target contours were transferred to the registration target image. Motion magnitudes and distributions from the deformation vector fields were analyzed. Internal target volumes (ITVs) were created on the respiratory average images for quantitative analysis of target motion and coverage changes. From these 7 patients, 11 distinct target volumes were treated, with 8 being epicardial and 3 endocardial. Motion and dosimetry for all 11 targets were analyzed separately. <h3>Results</h3> The average (SD) target shift magnitudes (in mm) from the RAVG to CAVG CTs were 2.0 ± 1.0, 2.0 ± 1.3, and 4.0 ± 2.0 in the left-right (LR), anterior/posterior (AP), and superior/inferior (SI) directions. Endocardial targets had greater motion magnitude than epicardial targets, with mean shifts of 2.2 ± 1.0, 2.3 ± 1.9, and 4.2 ± 2.4 in the LR, AP, and SI directions versus 1.4 ± 0.9, 1.3 ± 0.8, and 3.6 ± 0.5. Cardiac cycle motion showed more variability for epicardial targets with shifts of 1.4 ± 2.5, 2.4 ± 3.0, and 2.5 ± 3.7 in the LR, AP and SI directions, versus 2.0 ± 0.6, 2.1 ± 0.8, and 1.6 ± 0.7 for endocardial targets. Creation of motion encompassing ITVs led to an average target size increase of 100% (25 cm³). V95 for initial target volumes was 99.9 ± 0.4% while the average V95 for ITVs was 84.1 ± 15.6%. Similarly, D95 for the target dropped from 108.2 ± 5.7% to 75.3 ± 28.4%. <h3>Conclusion</h3> In this cohort of patients undergoing SBRT for VT, cardiac shifts were location-dependent and were largest in the SI direction. These shifts typically exceeded the 1-5 mm margins used for SBRT treatment planning and ITV generation to encompass all target motion led to a doubling in target size. Our data suggest that cardiac-gated delivery may be helpful to minimize treatment volumes and spare cardiac tissue.

Anatomy Guided Thoracic Lymph Node Station Delineation in CT Using Deep Learning Model

Accurate thoracic lymph node station (LNS) contouring is an essential step for N staging and radiotherapy, especially for those with lung and esophageal cancers. Thoracic LNS contouring is often determined using a system of anatomical descriptions proposed by the International Association for the Study of Lung Cancer (IASLC). Given that manual delineation of LNS can be prohibitively time-consuming and lead to large interobserver variability, an automated and accurate LNS delineation method is clinically useful. However, only a handful of computerized approaches existed with limited performance. Hence, we aim to develop an automated deep learning segmentation and parsing model by incorporating the key anatomic information to accurately delineate the LNS in CT.We curated a dataset of contrast-enhanced CT scans with 12 manually annotated LNSs, ranging from station 1 to 8, from 97 esophageal cancer patients. Based on the IASLC protocol, the following key chest organs are also delineated: the arteries arch, ascendens and descendens, pulmonary artery, veins of inferior vena cava and superior vena cava, thyroid, trachea, bronchus, heart, lung, spine, and sternum. First, we train a nnUNet based deep model to segment the reference organs. Second, along the superior-inferior direction, we emulate radiation oncologist and use the first slice of the predicted sternum, the artery arch, and the artery pulmonary to divide the CT images into four sections. Third, we train another integrated segmentation model that combines the RTCT image, the segmented organ masks, and the four sections to delineate the LNS. The key organs provide the anatomical context in RTCT, while the four sections give references on the spatial locations of lymph node stations or zones. Four-fold cross-validation is used to fully evaluate the proposed deep network model.The proposed framework has achieved 0.81 ± 0.06 Dice score (DSC), 0.9 ± 0.6 mm average surface distance (ASD) and 10.0 ± 3.6 mm Hausdorff distance (HD) on LNS parsing. Detailed results are shown in the Table. Our anatomy-guided method significantly outperforms the deep segmentation model using only RTCT images by a large margin of 4.8% in DSC and 19.0 mm in HD.Our work demonstrates a well-calibrated deep learning framework that emulates radiation oncologists in LNS parsing with achieved good accuracy. We provide significant and tangible performance improvements, serving as a critical step towards deploying an automated, reliable and reproducible LNS parsing module into radiotherapy planning workflow.

Impact of Body Mass Index and Abdominal Subcutaneous Fat Thickness of Setup Errors in Postoperative Cervical Cancer Patients Underwent Daily Image-Guided Radiation Therapy

The purpose of this study was to investigate the relationship between body mass index (BMI) and abdominal subcutaneous fat thickness (SFT) and setup errors for postoperative cervical cancer patients treated with adjuvant radiotherapy, aiming to provide reference for precise individual radiotherapy.Ninety-two postoperative cervical cancer patients underwent volumetric-modulated arc therapy (VMAT) in Peking Union Medical College Hospital between August 2019 and August 2020 were enrolled. The BMI and SFT were measured before treatment. The daily shifts, systematic errors, and random errors in the anterior-posterior (AP), superior-inferior (SI), and left-right (LR) directions were recorded. Planning target volume (PTV) margins were calculated based on the equation of MPTV = 2.5Σ + 0.7δ among each BMI and SFT categories. Receiver operating characteristics (ROC) curve, linear regression, spearman rank correlation coefficient, and Kolmogorov-Smirnov test were employed.The median BMI for all patients was 22.1 kg/m2 (range, 15.2-29.7 kg/m2). Of the 92 patients, 9.8% (n = 9) were underweight; 67.4% (n = 62) were normal weight; 22.8% (n = 21) were excess weight. There was a strong correlation between BMI and SFT (P < 0.001). SFT level of underweight < 21.3 mm and SFT level of excess weight ≥ 26.3 mm were observed in 31 and 37 patients, respectively. On linear regression, mean absolute SI shifts were moderately correlated with BMI (P = 0.044) and SFT (P = 0.046). Mean absolute AP and LR shifts did not significantly correlate with BMI and SFT. The margin of AP direction was reduced to 4 mm to significantly diminish the inclusion of rectum in the PTV.For postoperative cervical cancer patients with excess weight or heavy abdominal SFT, a PTV margin of 10 mm in SI direction may be insufficient in the absence of image-guided radiation therapy (IGRT). A PTV margin of AP direction should be moderately reduced while internal target volume (ITV) is considered.D. Wang: None. K. Hu: None. F. Zhang: None.

Real-Time Motion Tracking Based on Orthogonal Cine MRI During MR-Guided Radiation Therapy for Prostate Cancer

In contrast to radiographic imaging with implanted radio-opaque fiducials, the high soft tissue contrast and noninvasive nature of MRI are advantageous for real-time motion tracking of prostate motion. The purpose of this study is to investigate the feasibility of tracking the intrafraction prostate motion using an intensity-based template registration technique between real-time 2D cine and 3D MRI acquired during MR-guided Adaptative Radiation Therapy (MRgART).The motion tracking algorithm includes two steps: (1) creating a template by rigidly registering a set of cine images with a 3D reference MRI, and (2) tracking the target motion by registering incoming (i.e., real-time) cine with the template with a known relationship to the 3D volume. The daily 3D T2 MRI and 2D T2/T1 bFFE real-time cine MRI acquired on a 1.5T MR-Linac during MRgART for 6 prostate cancer patients were selected to test the algorithm limits. The cine images were acquired with a temporal resolution of 0.2 s for a total duration of 9-11 minutes interleaved between coronal and sagittal orientations. A 2D template image set was generated in the coronal and sagittal planes using the cine data acquired during the first 20 s registered to the 3D volume using in-plane rigid-body image registration based on a region of interest defined by the prostate contour on 3D T2 MRI. Motion tracking of the prostate was performed by registering the rest of the 2D cine series with the corresponding template. Manually delineated contours on sampled cine frames were used as the ground truth. Tracking accuracy was defined as the standard deviation of the error (SDE) between the center of mass (CM) of the ground-truth and tracked motions in 1 min. Prostate drift was evaluated by tracking the CM shift with the algorithm every 8-9 s for 9-11 min.The prostate tracking algorithm was qualitatively validated by visual assessment of 6 prostate cases. Overall, the tracking was found robust to implant-induced image artefacts in two cases, large change in bladder size in one case, and body motion in one other case. The tracking algorithm was also validated quantitatively on 2 cases for which ground truth contours were available and reliable. Mean SDE for the two cases was 0.45, 0.32, and 0.31 mm in superior-inferior, left-right, and anterior-posterior directions, respectively. A continuous motion drift of the prostate was observed over a 600 s period for one of these two cases and was accurately tracked by the technique.We have demonstrated the feasibility to track prostate motion based on real-time orthogonal cine MRI without implanting fiducials. With further investigation on more patients and following careful workflow, the real-time motion tracking technique may be implemented in MRgART to manage intrafraction motion.H. Jassar: None. A. Tai: None. X. Chen: None. T. Keiper: None. E.S. Paulson: Research Grant; Siemens Healthineers, Elekta Instruments AB.F. Lathuiliere: None. S. Bériault: None. D. Cooper: None. F. Hebert: None. L. Savard: None. S. Cloake: None. W.A. Hall: Research Grant; Elekta, National Cancer Institute, American Cancer Society. C.A. Lawton: None. A. Li: Research Grant; Elekta AB, Accuray Inc, Siemens Healthineers, Manteia Med. Honoraria; Elekta AB, Accuray Inc. Patent/License Fees/Copyright; Manteia Med.

Siamese-Based Deep Learning for Markerless Lung Tumor Tracking During Stereotactic Radiotherapy

<h3>Purpose/Objective(s)</h3> During stereotactic lung radiotherapy we want to monitor the target position to ensure it remains inside the planning target volume (PTV). This ensures accurate treatment delivery and may allow for margin reduction. Many LINAC systems have a kilo-voltage (kV) imaging system mounted orthogonally to the MV beam. This can acquire images during volumetric modulated arc therapy delivery. The 2D-kV images can be used to identify the tumor position, e.g., using template matching and triangulation (TMT). However, small lung targets are difficult to track, due to e.g., low tumor contrast, occlusion (e.g., by spine), or noisy images. We investigated deep learning (DL) as an alternative method for lung tumor tracking. <h3>Materials/Methods</h3> Clinical data from 6 patients (P1-P6) with small lung tumors (0.5-3 cm diameter) and a 3D-printed anthropomorphic phantom (P0), including a low-density lung tumor, was used. For each patient, data included 4D-CT scans, fluoroscopic kV images and breathing-related surface motion (Real-Time Position Management, RPM). For the phantom, a 3D-CT scan and kV images taken with a static and moving couch (regular and irregular motion) were available. Due to lack of ground truth tumor locations for the kV images, a patient-specific DL model is trained on Digitally Reconstructed Radiographs (DRRs), generated from the planning CT scans. To enhance the tumor's visual diversity in DRRs, we augmented the training data with locally deformed CT scans. A Siamese-based tracking by similarity method is hypothesized to be better suited for the tumor tracking task than other DL methods, due to its more localized search space and the use of previous frames (temporal information). <h3>Results</h3> Table 1 shows the superior-inferior (SI) direction results obtained by the Siamese model and TMT, on all kV images. For the patient data, the <b>Correlation coefficient</b> between estimated tumor trajectory and RPM was computed, due to lack of ground truth locations. For the phantom, the <b>Mean Absolute Distance</b> (MAD) between detected and ground truth center coordinates was computed. For all 6 patients and the phantom, the Siamese model outperformed TMT. <h3>Conclusion</h3> A novel approach for real-time markerless lung tumor tracking is presented, based on deep Siamese networks. The model compares favorably to the template matching/triangulation-based technique and merits further investigation. The current analysis presents only SI tracking. Reliable 3D tracking, with sub-mm accuracy, is the ultimate goal. Further work is needed to achieve this.

Influence of Clinical and Tumor Factors on Interfraction Setup Errors With Rotation Correction for Vacuum Cushion in Lung Stereotactic Body Radiation Therapy.

PurposeTo explore the influence of clinical and tumor factors over interfraction setup errors with rotation correction for non-small cell lung cancer (NSCLC) stereotactic body radiation therapy (SBRT) patients immobilized in vacuum cushion (VC) to better understand whether patient re-setup could further be optimized with these parameters.Materials and MethodsThis retrospective study was conducted on 142 NSCLC patients treated with SBRT between November 2017 to July 2019 in the local institute. Translation and rotation setup errors were analyzed in 732 cone-beam computed tomography (CBCT) scans before treatment. Differences between groups were analyzed using independent sample t-test. Logistic regression test was used to analyze possible correlations between patient re-setup and clinical and tumor factors.ResultsMean setup errors were the largest in anterior–posterior (AP) direction (3.2 ± 2.4 mm) compared with superior–inferior (SI) (2.8 ± 2.1 mm) and left–right (LR) (2.5 ± 2.0 mm) directions. The mean values were similar in pitch, roll, and rtn directions. Of the fractions, 83.7%, 90.3%, and 86.6% satisfied setup error tolerance limits in AP, SI, and LR directions, whereas 95% had rotation setup errors of <2° in the pitch, roll, or rtn directions. Setup errors were significantly different in the LR direction when age, body mass index (BMI), and “right vs. left” location parameters were divided into groups. Both univariate and multivariable model analyses showed that age (p = 0.006) and BMI (p = 0.002) were associated with patient re-setup.ConclusionsAge and BMI, as clinical factors, significantly influenced patient re-setup in the current study, whereas all other clinical and tumor factors were not correlated with patient re-setup. The current study recommends that more attention be paid to setup for elderly patients and patients with larger BMI when immobilized using VC, especially in the left–right direction.

Using 4DCBCT simulation and guidance to evaluate inter-fractional tumor variance during SABR for lung tumor within the lower lobe

Inter-fractional tumor variance would lead to insufficient dosage or overdose in tumor region during lung cancer radiotherapy. However, previous works have not considered influence of inter-fractional tumor amplitude variance at treatment position due to lack of effective evaluation method during radiotherapy, especially for lung tumor within the lower lobe. Our objective was to investigate inter-fractional tumor baseline shift and amplitude variance due to respiratory motion with 4DCBCT simulation and guidance during stereotactic ablative body radiotherapy (SABR) for lung tumor. Subject included 19 patients with lung tumor within the lower lobe. 4DCBCT-simulated images at treatment position were acquired sequentially to determine internal tumor volume (ITV) and reference tumor motion at simulation process. Compared with reference tumor motion, 95 4DCBCT-guided images were acquired during each treatment to evaluate inter-fractional tumor baseline shift and amplitude variance, which were − 0.0 ± 1.3 mm and − 0.2 ± 1.4 mm in left–right(LR) direction, 0.9 ± 2.3 mm and 0.4 ± 2.9 mm in superior-inferior (SI) direction, 0.1 ± 1.5 mm and − 0.4 ± 2.0 mm in anterior–posterior (AP) direction. ITV margin were 3.5 mm, 7.5 mm and 5.3 mm in LR, SI and AP directions with van Herk’s (Int J Radiat Oncol Biol Phys 52(5):1407–1422, 2002) formula. 4DCBCT simulation and guidance is a reliable method to evaluate inter-fractional tumor variance during SABR for lung tumor within the lower lobe. ITV margin of 3.5 mm, 7.5 mm and 5.3 mm in LR, SI and AP directions would ensure greater tumor coverage during SABR for lung tumor within the lower lobe.

First clinical experience of correcting phantom-based image distortion related to gantry position on a 0.35T MR-Linac.

MR‐guided radiotherapy requires strong imaging spatial integrity to deliver high quality plans and provide accurate dose calculation. The MRI system, however, can be compromised by the integrated linear accelerator (Linac), resulting in inaccurate imaging isocenter position and geometric distortion. Dependence on gantry position further complicates the correction of distortions. This work presents a new clinical application of a commercial phantom and software system that quantifies isocenter alignment and geometric distortion, as well as providing a deformation vector field (DVF). A large distortion phantom and a smaller grid phantom were imaged at multiple gantry angles from 0 to 330° on a 0.35 T integrated MR‐Linac. The software package was used to assess geometric distortion and generate DVFs to correct distortions within the phantom volume. The DVFs were applied to the grid phantom with resampling software then evaluated using structural similarity index measure (SSIM). Scans were also performed with a ferromagnetic clip near the phantom to investigate the correction of more severe artifacts. The mean magnitude isocenter shift was 0.67 mm, ranging from 0.25 to 1.04 mm across all angles. The DVF had a mean component value of 0.27 ± 0.02, 0.24 ± 0.01, and 0.19 ± 0.01 mm in the right‐left (RL), anterior‐posterior (AP), and superior‐inferior (SI) directions. The ferromagnetic clip increased isocenter position error from 1.98 mm to 2.20 mm and increased mean DVF component values in the RL and AP directions. The resampled grid phantom had an increased SSIM for all gantry angles compared to original images, increasing from 0.26 ± 0.001 to 0.70 ± 0.004. Through this clinical assessment, we were able to correct geometric distortion and isocenter shift related to gantry position on a 0.35 T MR‐Linac using the distortion phantom and software package. This provides encouragement that it could be used for quality assurance and clinically to correct systematic distortion caused by imaging at different gantry angles.

Suture Tape Augmentation of Screw Fixation Reduces Fragment Migration in Tibial Tubercle Osteotomy: A Biomechanical Study.

Background:Tibial tubercle osteotomy (TTO) is a complex surgical procedure with a significant risk of complications, which include nonunion and tibial fracture.Purpose:To determine whether an additional suture tape augmentation can provide better biomechanical stability compared with standard screw fixation.Study Design:Controlled laboratory study.Methods:Five matched pairs of human cadaveric knees were divided into 2 groups: the first group underwent standard TTO fixation with 2 parallel screws (standard group). The second group underwent a novel fixation technique, in which a nonabsorbable suture tape (FiberTape) in a figure-of-8 construct was added to the standard screw fixation for extra stabilization in the inferior-superior direction (augmented group). The specimens were biomechanically tested using a multistep cyclic loading protocol from 400 N up to 800 N to simulate the rehabilitation process. Tubercular fragment migration of >50% of the initial distalization length was defined as clinical failure. A pull-to-failure test was applied to the specimens that survived cyclic loading. Tubercular fragment displacement during cyclic loading and pull-to-failure force were recorded and compared between the 2 groups.Results:Two specimens of the standard group exhibited clinical failure during cyclic loading to 400 N. All other specimens survived cyclic loading to 800 N. The augmented group showed less cyclic tubercular fragment displacement after every load level compared with the standard group, with statistically significant differences starting from 500 N (P < .05; power > 0.8). Mean ± standard deviation tubercular fragment displacement at the end of cyclic loading was 2.56 ± 0.82 mm for the augmented group and 5.21 ± 0.51 mm for the standard group. Mean ultimate failure load after the pull-to-failure test was 2475 ± 554 N for the augmented group and 1475 ± 280 N for the standard group.Conclusion:The specimens that underwent suture tape augmentation showed less tubercular fragment displacement during cyclic loading and higher ultimate failure forces compared with those that underwent standard screw fixation.Clinical Relevance:The augmentation technique could potentially increase the success of a TTO.

Radiomics analysis of EPID measurements for patient positioning error detection in thyroid associated ophthalmopathy radiotherapy.

Electronic portal imaging detector (EPID)-based patient positioning verification is an important component of safe radiotherapy treatment delivery. In computer simulation studies, learning-based approaches have proven to be superior to conventional gamma analysis in the detection of positioning errors. To approximate a clinical scenario, the detectability of positioning errors via EPID measurements was assessed using radiomics analysis for patients with thyroid-associated ophthalmopathy. Treatment plans of 40 patients with thyroid-associated ophthalmopathy were delivered to a solid anthropomorphic head phantom. To simulate positioning errors, combinations of 0-, 2-, and 4-mm translation errors in the left-right (LR), superior-inferior (SI), and anterior-posterior (AP) directions were introduced to the phantom. The positioning errors-induced dose differences between measured portal dose images were used to predict the magnitude and direction of positioning errors. The detectability of positioning errors was assessed via radiomics analysis of the dose differences. Three classification models-support vector machine (SVM), k-nearest neighbors (KNN), and XGBoost-were used for the detection of positioning errors (positioning errors larger or smaller than 3mm in an arbitrary direction) and direction classification (positioning errors larger or smaller than 3mm in a specific direction). The receiver operating characteristic curve and the area under the ROC curve (AUC) were used to evaluate the performance of classification models. For the detection of positioning errors, the AUC values of SVM, KNN, and XGBoost models were all above 0.90. For LR, SI, and AP direction classification, the highest AUC values were 0.76, 0.91, and 0.80, respectively. Combined radiomics and machine learning approaches are capable of detecting the magnitude and direction of positioning errors from EPID measurements. This study is a further step toward machine learning-based positioning error detection during treatment delivery with EPID measurements.

Performance of a newly designed end‐to‐end phantom compatible with magnetic resonance‐guided radiotherapy systems

In this study, we report on our proposed phantom based on the new end-to-end (E2E) methodology and its results. In addition, we verify whether the proposed phantom can replace conventional phantoms. The hexagonal-shaped newly designed phantom has pockets on each side for a film dosimeter of size 80×90mm2 , which is easily removable, considering the 60 Co penumbra. The new phantom comprises water, shell, and auxiliary shell phantoms. The shell and auxiliary shell materials are Solid Water HE. A mock tumor (aluminum oxide) was attached by a single prop in the water phantom and placed at the center of the new phantom. The results of a conventional E2E test were compared with those of the novel E2E test using the newly designed phantom. The irradiated film dosimeter in the novel E2E test was scanned in a flatbed scanner and analyzed using an in-house software developed with MATLAB. The irradiated field center, laser center, and mock tumor center were calculated. In the novel image-matching E2E (IM-E2E) test, image matching is performed by aligning the laser center with ruled lines. In the novel irradiation-field E2E (IF-E2E) test, the displacement of the irradiation-field center was defined as its distance from the laser center. In the composite E2E test, the overall displacement, which included the accuracy of the irradiated field and image matching, was defined as the distance between the irradiated field center and mock tumor center. In addition, using the newly designed phantom, the overall irradiation accuracy of the machine was evaluated by calculating the three-dimensional (3D) center of the irradiated field, phantom, and laser. The composite E2E test could be performed using the newly designed phantom only. In the IM-E2E test, the results of the conventional and novel IM-E2E tests were significantly different in each direction (left-right direction: p-value<<0.05, anterior-posterior direction: p-value=0.002, and superior-inferior direction: p-value=0.002). The displacement directions were the same in both the conventional and novel IM-E2E tests. In the analysis of the IF-E2E test, no significant difference was evident between the results in each direction. Moreover, the displacement directions were the same in the conventional and novel IF-E2E tests, except for the left-right lateral direction of head three. In addition, the 3D analysis results of the novel IF-E2E test were less than 1mm in all directions. In the analysis of the composite E2E test, the maximum displacement was 1.4mm in all directions. In addition, almost all results of 3D analysis for the composite E2E test were less than 1mm in all directions. The newly designed E2E phantom simplifies the E2E test for MRIdian, and is a possible alternative to the conventional E2E test. Furthermore, we can perform the previously unfeasible composite E2E tests that include the entire treatment process.