LR Directions Research Articles

CBCT Guided Interfraction Absolute Displacement and Setup Error Assessment during Breast Prone Radiotherapy

To calculate interfraction absolute displacement/shift and setup error using cone beam computed tomography (CBCT) during breast prone radiotherapy. Fifty-nine patients undergoing prone whole breast-irradiation after breast-conserving surgery (BCS) were studied as part of an institutional review board- approved prospective trial. Setup precision was monitored using a daily online CBCT. Translational shifts in 3 axes (AP: anterior posterior; LR: left right; and SI: superior inferior) and 3 rotations (pitch, roll, and rtn) after CBCT were analyzed for 1062 treatment fractions. The random and systematic setup errors (SE) were calculated and were analyzed for time trends during the course of radiotherapy. For absolute inter-fractional shifts, the numbers of fractions exceeding 10 mm in the AP, LR, and SI directions were 6.5%, 17.42% and 8.92%, respectively; 0%, 0% and 1.31% fractions exceeded 3°for pitch, roll, and Rtn, respectively. The population systematic errors were 1.89/2.91/1.98 mm in AP/SI/LR directions, while the random error were 2.72/3.99/3.31 mm. In pitch, roll, rtn rotations, the population systematic error were 0.64°/0.49°/0.46°, and 0.89°/0.90°/0.93° for the random error. Without correction these would correspond to a clinical to planning target volume margin of 6.64/10.08/7.26 mm in AP/SI/LR directions and 2.22°/1.79°/1.8° in pitch, roll, rtn rotations. The magnitude of inter-fraction motion was not correlated with patient treatment time accept in AP direction (P = 0.000). SE was larger in prone position in breast cancer patients, attributable mostly to random errors, which emphasize the need for on-line imaging guidance in breast prone radiotherapy. 10mm margins would adequately cover the target volume and account for setup errors in the absence of IGRT.

Reducing the margin in prostate radiotherapy: optimizing radiotherapy with a general-purpose linear accelerator using an in-house position monitoring system.

To study the feasibility of optimizing the Clinical Target Volume to Planning Target Volume (CTV-PTV) margin in prostate radiotherapy(RT) with a general-purpose linear accelerator using an in-house developed position monitoring system, SeedTracker. A cohort of 30 patients having definitive prostate radiotherapy treated within an ethics-approved prospective trial was considered for this study. The intrafraction prostate motion and the position deviations were measured using SeedTracker system during each treatment fraction. Using this data the CTV-PTV margin required to cover 90% of the patients with a minimum of 95% of the prescription dose to CTV was calculated using van Herk's formula. The margin calculations were performed for treatment scenarios both with and without applying the position corrections for observed position deviations. The feasibility of margin reduction with real-time monitoring was studied by assessing the delivered dose that incorporates the actual target position during treatment delivery and comparing it with the planned dose. This assessment was performed for plans generated with reduced CTV-PTV margin in the range of 7mm-3mm. With real-time monitoring and position corrections applied the margin of 2.0mm, 2.1mm and 2.1mm in LR, AP and SI directions were required to meet the criteria of 90% population to receive 95% of the dose prescription to CTV. Without position corrections applied for observed position deviations a margin of 3.1mm, 4.0mm and 3.0mm was required in LR, AP and SI directions to meet the same criteria. A mean ± SD reduction of 0.5 ± 1.8% and 3 ± 7% of V60 for the rectum and bladder can be achieved for every 1mm reduction of PTV margin. With position corrections applied, the CTV D99 can be delivered within -0.2 ± 0.3 Gy of the planned dose for plans with a 3mm margin. Without applying corrections for position deviations the CTV D99 was reduced by a maximum of 1.1 ± 1.1 Gy for the 3mm margin plan and there was a statistically significant difference between planned and delivered dose for 3mm and 4mm margin plans. This study demonstrates the feasibility of reducing the margin in prostate radiotherapy with SeedTracker system without compromising the dose delivery accuracy to CTV while reducing dose to critical structures.

Assessment of Setup Errors in Gynecological Malignancies Treated With Radiotherapy Using Onboard Imaging.

Introduction Radiotherapy plays a vital role in the management of gynecological malignancies. However, maintaining patient position poses a challenge during daily radiotherapy treatment of these patients. This study identifies and calculates setup errors in interfraction radiotherapy and optimum clinical target volume-planning target volume (CTV-PTV) margins in patients with gynecological malignancies. Material and methods A total of 38 patients with gynecological malignancies were included in the study. They were treated with a dose of 50 Gy in 25 fractions for five weeks, followed by brachytherapy. All patients were immobilized using a 4-point thermoplastic cast. Anteroposterior and lateral images were taken thrice weekly for five weeks. Setup verification was done using kilovoltage images obtained using Varian On-board Imager (Varian Medical System, Inc., Palo Alto, CA). Manual matching was done utilizing bony landmarks such as the widest portion of the pelvic brim, anterior border of S1 vertebrae, and pubic symphysis in the X, Y, and Z axes, respectively. Results A total of 1140 images were taken. The individual systematic errors ranged from -0.24 to 0.17 cm (LR), -0.15 to 0.19 cm (AP) and -0.36 to 0.29 cm (CC) while the individual random errors ranged from 0.04 to 0.36 cm (LR), 0.06 to 0.33 cm (AP) and 0.10 to 0.29 cm (CC). The calculated CTV-PTV margins in LR, AP and CC directions were 0.17, 0.18, and 0.25 cm (ICRU-62); 0.28, 0.31 and 0.47 cm in LR, AP and CC directions (Stroom's), and 0.32, 0.36 and 0.55 cm (Van Herk) respectively. Conclusion Based on this study, the calculated CTV-PTV margin is 6 mm in gynecological malignancies, and the present protocol of 7 mm of PTV margin is optimum.

Quantifying 6D tumor motion and calculating PTV margins during liver stereotactic radiotherapy with fiducial tracking

Our study aims to estimate intra-fraction six-dimensional (6D) tumor motion with rotational correction and the related correlations between motions of different degrees of freedom (DoF), as well as quantify sufficient anisotropic clinical target volume (CTV) to planning target volume (PTV) margins during stereotactic body radiotherapy (SBRT) of liver cancer with fiducial tracking technique. A cohort of 12 patients who were implanted with 3 or 4 golden markers were included in this study, and 495 orthogonal kilovoltage (kV) pairs of images acquired during the first fraction were used to extract the spacial position of each golden marker. Translational and rotational motions of tumor were calculated based on the marker coordinates by using an iterative closest point (ICP) algorithm. Moreover, the Pearson product-moment correlation coefficients (r) were applied to quantify the correlations between motions with different degrees of freedom (DoFs). The population mean displacement ( ), systematic error (Σ) and random error (σ) were obtained to calculate PTV margins based on published recipes. The mean translational variability of tumors were 0.56, 1.24 and 3.38mm in the left-right (LR, X), anterior-posterior (AP, Y), and superior-inferior (SI, Z) directions, respectively. The average rotational angles θ X , θ Y and θ Z around the three coordinate axes were 0.88, 1.24 and 1.12, respectively. (|r|>0.4) was obtainted between Y -Z , Y - θ Z , Z -θ Z and θ X - θ Y . The PTV margins calculated based on 13 published recipes in X, Y, and Z directions were 1.08, 2.26 and 5.42mm, and the 95% confidence interval (CI) of them were (0.88,1.28), (1.99,2.53) and (4.78,6.05), respectively. The maximum translational motion was in SI direction, and the largest correlation coefficient of Y-Z was obtained. We recommend margins of 2, 3 and 7mm in LR, AP and SI directions, respectively.

Planning Margins Determined by Inter Fractional Errors for Patients with Hepatocarcinoma—A Real-World Practice

<h3>Purpose/Objective(s)</h3> With the progress of radiation techniques, radiotherapy is increasingly widely used in the treatment of hepatocarcinoma. Previous studies showed high radio-sensitivity and frequently motion of hepatobiliary zone. Although the application of magnetic resonance simulation (MR) and four-dimensional computed tomography (4D-CT) help us to clearly the gross tumor volume (GTV), accurate calculation of set-up margin is required to define the clinical to planning target volume margin. This study aimed at calculate the suitable margin between clinical target volume (CTV) and planning target volume (PTV) after the evaluation of the set-up errors using cone beam computed tomography (CBCT). <h3>Materials/Methods</h3> Patients aged 18-85 years old with pathologically diagnosed as primary hepatocarcinoma, Eastern Cooperative Oncology Group performance score ≤ 3 points, accepted the radiation recommendation by multi-disciplinary team, with only one tumor located in liver will be enrolled. Both 4D-CT and MR simulations were delivered to determine GTV. CTV was GTV plus a 5 mm margin in liver and GTV plus a 10mm margin along vessels if tumor embolus were included. PTV was CTV plus a 5mm margin. All the patients received image guided intensity modulated radiotherapy with conventional fractionation. CBCT was prescribed daily at the first week and then 2-3 times at every following week. Patients whose tumor could not be clearly classified in CBCT will be excluded. CBCT The setup errors in translational directions were recorded based on the image registration between the CBCT and the free breath CT images at simulation as reference images. Mean value and standard deviations (∑) of each patient were obtained. After that, we calculated the margin between CTV and PTV by a formula MPTV=2.5∑+0.7σ (σ means rootmeansquare of ∑). <h3>Results</h3> From November 2020 to April 2021, 35 patients fulfilled the criteria. The median age of these patients were 65 years old. The median value of GTV and PTV were 512.91 ml (45.09-1987.56 ml) and 1074.77ml (221.09-3028.40 ml), respectively. The median body mass index (BMI)was 23.98. The median CBCT times was 8 (5-12). After alignments by one senior radiation oncologist and one senior radiation therapist, we analyzed 284 sets of CBCT images. M±SD shifts in translational were 0.04±0.25 mm (LR), 0.05±0.51 mm (AP), and 0.06±0.26 mm (SI), respectively. Accordingly, the margins in LR, SI and AP directions were 5 mm, 6 mm and 10 mm, respectively. For patients whose BMI <24, the margins in LR, SI and AP directions were 4 mm, 5 mm and 12 mm. For patients whose BMI ≥24, the corresponding margins should be 6 mm,7 mm and 10 mm, respectively. <h3>Conclusion</h3> For patients with hepatocarcinoma, the setup errors were different in LR, SI and AP directions. We recommended that the margins from CTV to PTV should be 5mm, 6mm and 10 mm in LR, SI and AP directions. For patients with BMI≥24, larger margins in LR and SI directions are needed.

Variability of Inter-Fraction Target Motion during Hypofractionated Lung Radiation Therapy

<h3>Purpose/Objective(s)</h3> Pre-treatment 4D-CTs are used in planning for motion management of lung RT. The target excursion (E) at the time of planning may differ between planning and treatment. Additionally, the tumor excursion can change between fraction deliveries. The objective of this study is to investigate patterns of excursion variability (∆E) during hypofractionated radiotherapy (HRT) and the clinical factors that correlate with ∆E. <h3>Materials/Methods</h3> 41 patients with primary or metastatic lung tumors underwent HRT. For each patient, planning was performed on 4D-CT imaging. Patients underwent daily pre- and post- treatment 4D-CBCT for setup conformation and motion assessment. Respiration-induced excursion (E) was quantified in the planning 4D-CT, and in each daily excursion 4D-CBCT. All excursion values were measured in 3 dimensions: AP, LR, and SI. ∆E was calculated as the difference between daily 4D-CBCT excursion values and the planning 4D-CT excursion value. The clinical statistics and excursion statistics were evaluated for all fractions. Finally, Pearson correlation was performed to associate clinical factors with ∆E. <h3>Results</h3> 377 4D-CBCTs were analyzed. ∆E was largest in the SI direction, with ∆E_avg in the SI, LR, and AP directions being 0.16, 0.05, and 0.08 cm respectively. Only 7.1% of all 4D-CBCT's displayed ∆E ≥ 4mm. The average age of patients with ∆E ≥ 4mm was 58.63 y/o, and that of patients with ∆E<4mm was 71.73 y/o (P<0.001). The average baseline weight of patients with ∆E ≥ 4mm and ∆E<4mm were 84.94 kg and 72.82 kg respectively (P<0.001). 71.1% of males and 28.9% of females gave ∆E ≥ 4mm (P=0.003). 58.3% of tumors with ∆E ≥ 4mm were in the right lower lobe (RLL) and 20.8% of tumors with ∆E ≥ 4mm were in the left lower lobe (LLL) (P<0.001). An average AP planning excursion of 0.34cm gave ∆E<4mm and that of 0.53 cm gave ∆E ≥ 4mm (P =0.008). The average Charlson Comorbidity Index (CCI) was 3.58 for tumors with ∆E<4mm, and 5.17 for tumors with ∆E ≥ 4mm (P=0.004). Pearson correlation between the left and right lungs didn't yield statistically significant results (P=0.76). <h3>Conclusion</h3> Large ∆E seems predictable, with ∆E ≥ 4mm seen more in young patients with higher baseline weight, male gender, tumors in the lower lobes, and with large 4D-CT excursion displayed in the AP direction. There was no correlation found between ∆E and right versus left lung. Additionally, CCI did not correlate with excursion variability. Finally, the largest degree of ∆E is displayed in the SI direction.

Demonstration of quasi-real-time intrafractional motion in esophageal cancer radiotherapy provided by ExacTrac X-ray snap verification.

To investigate the following hypotheses: (1) ExacTrac X-ray Snap Verification (ET-SV) is an alternative to CBCT for positioning patients with esophageal carcinoma (EC), (2) ET-SV can detect displacement in EC patients during radiotherapy (RT) and (3) EC patients can be feasibly monitored in quasi-real-time with ET-SV during RT. Anthropomorphic phantoms and 13 patients were included in this study. CBCT and ET-SV were both implemented before treatment delivery to detect displacement, and their correction results were compared. For the patient tests, positional correction in 3 translational directions and the yaw direction were applied using the ET-SV correction results. The residual error was detected immediately using ET-SV. Finally, to acquire the intrafractional motion, ET-SV was implemented when the gantry was at 0°, 90°, 180° and 270°, respectively. In phantom tests, the maximum value of the difference in displacement between the CBCT and ET systems was 1.16 mm for translation and 0.31° for yaw. According to Bland-Altman analysis of the patient test results, 5% (5/98), 5% (5/98), 5% (5/98), and 4% (4/98) of points were beyond the upper and lower limits of agreement in the AP, SI, LR and yaw directions, respectively. The mean residual error was -0.482 mm, 1.215 mm, 1.0 mm, -0.487°, 0.105°, and 0.003° in the AP, SI, LR, pitch, roll and yaw directions, respectively. The intrafractional displacement ranged from -0.21 mm to 0 mm for translation and from -0.63° to 0.21° for rotation. The mean total translational error for intrafractional motion increased from 0.47 mm to 1.14 mm during the treatment. The accuracy of ET-SV for EC RT positional correction is comparable to that of CBCT. Thus, Quasi-real-time intrafractional monitoring can be used to detect EC patient displacement during radiotherapy.

A Novel Concept of a Phased-Array HIFU Transducer Optimized for MR-Guided Hepatic Ablation: Embodiment and First In-Vivo Studies.

PurposeHigh-intensity focused ultrasound (HIFU) is challenging in the liver due to the respiratory motion and risks of near-/far-field burns, particularly on the ribs. We implemented a novel design of a HIFU phased-array transducer, dedicated to transcostal hepatic thermo-ablation. Due to its large acoustic window and strong focusing, the transducer should perform safely for this application.Material and MethodsThe new HIFU transducer is composed of 256 elements distributed on 5 concentric segments of a specific radius (either 100, 111, or 125 mm). It has been optimally shaped to fit the abdominal wall. The shape and size of the acoustic elements were optimized for the largest emitting surface and the lowest symmetry. Calibration tests have been conducted on tissue-mimicking gels under 3-T magnetic resonance (MR) guidance. In-vivo MR-guided HIFU treatment was conducted in two pigs, aiming to create thermal ablation deep in the liver without significant side effects. Imaging follow-up was performed at D0 and D7. Sacrifice and post-mortem macroscopic examination occurred at D7, with the ablated tissue being fixed for pathology.ResultsThe device showed −3-dB focusing capacities in a volume of 27 × 46 × 50 mm3 as compared with the numerical simulation volume of 18 × 48 × 60 mm3. The shape of the focal area was in millimeter-range agreement with the numerical simulations. No interference was detected between the HIFU sonication and the MR acquisition. In vivo, the temperature elevation in perivascular liver parenchyma reached 28°C above physiological temperature, within one breath-hold. The lesion was visible on Gd contrast-enhanced MRI sequences and post-mortem examination. The non-perfused volume was found in pig #1 and pig #2 of 8/11, 6/8, and 7/7 mm along the LR, AP, and HF directions, respectively. No rib burns or other near-field side effects were visually observed on post-mortem gross examination. High-resolution contrast-enhanced 3D MRI indicated a minor lesion on the sternum.ConclusionThe performance of this new HIFU transducer has been demonstrated in vitro and in vivo. The transducer meets the requirement to perform thermal lesions in deep tissues, without the need for rib-sparing means.

Study on Motion Management of Pancreatic Cancer Treated by CyberKnife.

PurposeWe investigated the movement characteristics of pancreas and the clinical accuracy of tracking pancreas with the Synchrony Respiratory Tracking System (SRTS) during the CyberKnife treatment. These data provide a clinical data basis for the expansion margins of pancreatic tumor target.Methods and MaterialsForty-two patients with pancreatic cancer treated by CyberKnife were retrospectively studied. The pancreatic displacement calculated from the x-ray images collected during the time interval between two consecutive movements constituted a data set.ResultsThe total mean motion amplitudes and standard deviations of pancreatic tumors in SI, LR, AP, and radial directions were 3.66 ± 1.71 mm, 0.97 ± 0.62 mm, 1.52 ± 1.02 mm, and 1.36 ± 0.49 mm, respectively. The overall mean correlation errors and standard deviations were 0.82 ± 0.46 mm, 0.47 ± 0.33 mm, 0.41 ± 0.24 mm, and 0.98 ± 0.37 mm, respectively. The overall mean prediction errors and standard deviations were 0.57 ± 0.14 mm, 0.62 ± 0.28 mm, 0.39 ± 0.17 mm, and 1.58 ± 0.36 mm, respectively. The correlation errors and prediction errors of pancreatic tumors at different anatomical positions in SI, LR, and AP directions were statistically significant (p < 0.05).ConclusionsThe tumor motion amplitude, the tumor location, and the treatment time are the main factors affecting the tracking accuracy. The pancreatic tumors at different anatomical locations should be treated differently to ensure sufficient dose coverage of the pancreatic target area.

An Evaluation of Total Internal Motions of Locally Advanced Pancreatic Cancer during SABR Using Calypso® Extracranial Tracking, and Its Possible Clinical Impact on Motion Management.

(1) Background: the aims of this study were to determine the total extent of pancreatic cancer’s internal motions, using Calypso® extracranial tracking, and to indicate possible clinical advantages of continuous intrafractional fiducial-based tumor motion tracking during SABR. (2) Methods: thirty-four patients were treated with SABR for LAPC using Calypso® for motion management. Planning MSCTs in FB and DBH, and 4D-CTs were performed. Using data from Calypso® and 4D-CTs, the movements of the lesions in the CC, AP and LR directions, as well as the volumes of the 4D-CT-based ITV and the volumes of the Calypso®-based ITV were compared. (3) Results: significantly larger medians of tumor excursions were found with Calypso® than with 4D-CT: CC: 29 mm (p < 0.001); AP: 14 mm (p < 0.001) and LR: 11 mm (p < 0.039). The median volume of the Calypso®-based ITV was significantly larger than that of the 4D-CT based ITV (p < 0.001). (4) Conclusion: beside known respiratory-induced internal motions, pancreatic cancer seems to have significant additional motions which should be considered during respiratory motion management. Only direct and continuous intrafractional fiducial-based motion tracking seems to provide complete coverage of the target lesion with the prescribed isodose, which could allow for safe tumor dose escalation.

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.

Feasibility of Ablative SBRT Treatment of Pancreas Patients on an MR-Linac

To report on our early experience of treating patients with pancreas ductal adenocarcinoma (PDAC) with ablative SBRT (50 Gy in 5 fractions) on the Elekta Unity MR-Linac.Ten PDAC patients were treated with abdominal compression (AC) and daily online plan adaptation using Adapt-to-Shape (ATS) workflow. AC is the method of choice to reduce breathing motion since automatic breath-hold/gating or other motion management options are not yet available on Unity MR-Linac. Three tumors were located in the head of the pancreas and seven were in the body. Seven patients were considered inoperable per surgeon and six were node positive. Average GTV volume during simulation was 42.5 ± 24.8 cc. Three orthogonal plane cine MRI were acquired to assess AC belt pressure during MR simulation as well as during each treatment fraction to assess stability of AC in minimizing tumor motion. Three sets of 3D T2w MR scans, pre-treatment (MRIpre), verification (MRIver) and post-treatment (MRIpost) MRI, were acquired for online planning. To assess the dosimetric impact of intrafraction organ motion, a post-treatment quality assurance (QA) was performed before the next fraction by propagating pre-treatment plan and structures to both MRIver and MRIpost, editing the contours and recalculating dose. GTV coverage and selected organs-at-risk (OAR) dose constraints (e.g., < 33 Gy to 0.035 cc, < 25 Gy to 2 cc and < 25 Gy to 5cc volume) were evaluated on MRIver and MRIpost.Median total treatment time was 75.5(49-132) minutes. Average contouring, planning, physics QA and beam-on time was 25.1 ± 11.9, 12.2 ± 6.0, 1.9 ± 1.5 and 14.1 ± 3.7 mins respectively. Average AP, LR and SI motion in FB and with compression was 0.3 ± 0.1, 0.6 ± 0.2 and 0.7 ± 0.2 and 0.2 ± 0.1, 0.2 ± 0.1, 0.4 ± 0.1 cm respectively, indicating that compression belt was effective in minimizing patient breathing motion. Average tumor motion in AC belt for all fractions was 0.2 ± 0.1, 0.2 ± 0.1 and 0.4 ± 0.2 cm in AP, LR and SI direction. Median GTV coverage was 78.7% of Rx dose for all fractions. Average GTV Dmax and GTV mean dose for all the fractions was 57.4 ± 0.7 and 51.2 ± 1.6 Gy respectively. Average 0.035 cc, D2cc and D5cc stomach dose was 34.4 ± 5.4, 26.7 ± 2.9 and 24.3 ± 2.1 Gy on MRIver and 35.3 ± 6.2, 27.0 ± 3.3 and 24.6 ± 2.5 Gy on MRIpost. Average 0.035 cc, D2cc and D5cc small bowel dose was 35.1 ± 6.3, 27.1 ± 3.6 and 24.4 ± 2.6 Gy on MRIver and 35.5 ± 6.5, 27.3 ± 3.9 and 24.6 ± 2.5 Gy on MRIpost.MR-guided adaptive RT enables delivery of curative dose to pancreatic tumors by taking into account interfraction motion of gastrointestinal OARs in daily adaptive planning. To manage intrafraction motion in our current clinical workflow, the organ motion is assessed for first two fractions and conservative strategies (e.g., bigger PRV margins or change the directive to keep stomach 0.035 cc dose to < 30 Gy) are employed for later fractions. ATS workflow with AC and the post-treatment QA allow safe delivery of ablative radiation doses for selected cases of PDAC on Unity MR-Linac.N. Tyagi: Honoraria; Elekta. Travel Expenses; Elekta; ISMRM.J. Liang: None. S. Burleson: None. E. Subashi: None. P. Godoy Scripes: None. K.R. Tringale: None. P.B. Romesser: Travel Expenses; Elekta. M. Reyngold: None. C.H. Crane: Honoraria; Elekta. Travel Expenses; Elekta.

Geometric and Dosimetric Changes in Tumor and Lung Tissue During Radiotherapy for Lung Cancer With Atelectasis.

Background and PurposeThis article retrospectively characterized the geometric and dosimetric changes in target and normal tissues during radiotherapy for lung cancer patients with atelectasis.Materials and MethodsA total of 270 cone beam computed tomography (CBCT) scans of 18 lung patients with atelectasis were collected. The degree and time of resolution or expansion of the atelectasis were recorded. The geometric, dosimetric, and biological changes in the target and lung tissue were also quantified.ResultsThere were two patients with expansion, four patients with complete regression, six patients with partial regression, and six patients with no change. The time of resolution or expansion varied. The tumor volume increased by 3.8% in the first seven fractions, then decreased from the 9th fraction, and by 33.4% at the last CBCT. In the LR direction, the average center of mass (COM), boundaries of the tumors gradually shifted mediastinally. In the AP direction, the COM of the tumors was shifted slightly in the posterior direction and then gradually shifted to the anterior direction; the boundaries of the tumors all moved mediastinally. In the SI direction, the COM of the tumors on the right side of the body was substantially shifted toward the head direction. The boundaries of the tumors varied greatly. D2, D98, Dmean, V95, V107, and TCP of the PTV were reduced during radiotherapy and were reduced to their lowest values during the last two fractions. The volume of the ipsilateral lung tended to increase gradually. The V5, V10, V20, V30, V40, and NTCP of the total lung gradually increased with the fraction.ConclusionsFor most patients, regression of the atelectasis occurred, and the volume of the ipsilateral lung tended to increase while the tumor volume decreased, and the COM and boundary of the tumors shifted toward mediastinum, which caused an insufficient dose to the target and an overdose to the lungs. Regression or expansion may occur for any fraction, and it is therefore recommended that CBCT be performed at least every other day.

Fiducial-based image-guided SBRT for pancreatic adenocarcinoma: Does inter-and intra-fraction treatment variation warrant adaptive therapy?

PurposeVariation in target positioning represents a challenge to set-up reproducibility and reliability of dose delivery with stereotactic body radiation therapy (SBRT) for pancreatic adenocarcinoma (PDAC). While on-board imaging for fiducial matching allows for daily shifts to optimize target positioning, the magnitude of the shift as a result of inter- and intra-fraction variation may directly impact target coverage and dose to organs-at-risk. Herein, we characterize the variation patterns for PDAC patients treated at a high-volume institution with SBRT.MethodsWe reviewed 30 consecutive patients who received SBRT using active breathing coordination (ABC). Patients were aligned to bone and then subsequently shifted to fiducials. Inter-fraction and intra-fraction scans were reviewed to quantify the mean and maximum shift along each axis, and the shift magnitude. A linear regression model was conducted to investigate the relationship between the inter- and intra-fraction shifts.ResultsThe mean inter-fraction shift in the LR, AP, and SI axes was 3.1 ± 1.8 mm, 2.9 ± 1.7 mm, and 3.5 ± 2.2 mm, respectively, and the mean vector shift was 6.4 ± 2.3 mm. The mean intra-fraction shift in the LR, AP, and SI directions were 2.0 ± 0.9 mm, 2.0 ± 1.3 mm, and 2.3 ± 1.4 mm, respectively, and the mean vector shift was 4.3 ± 1.8 mm. A linear regression model showed a significant relationship between the inter- and intra-fraction shift in the AP and SI axis and the shift magnitude.ConclusionsClinically significant inter- and intra-fraction variation occurs during treatment of PDAC with SBRT even with a comprehensive motion management strategy that utilizes ABC. Future studies to investigate how these variations could lead to variation in the dose to the target and OAR should be investigated. Strategies to mitigate the dosimetric impact, including real time imaging and adaptive therapy, in select cases should be considered.

Margin Reduction by Virtual Couch Shift in Prostate SBRT on a 1.5T MR-Linac

We treat localized prostate cancer on a 1.5T MR-Linac using plan re-optimization to the anatomy of the day in each fraction. We investigated the efficacy of a 2-stage, clinically available workflow to reduce intrafraction motion during the 40-minute patient on-couch time and potential margin reduction. Patients with intermediate risk prostate cancer (NCNN classification) were treated with 5 x 7.25 Gy on an MRL. Each fraction, an initial T2 weighed MR scan (‘INI’, 2 min scan time) was acquired, and pre-treatment contours were propagated and manually adapted to this scan, thus correcting for prostate rotation and organ deformation. Next, dose re-optimization (7 beam IMRT) was applied (Monaco treatment planning system), with a 5 mm CTV-PTV margin. Because the average time between the initial scan and beam on is approximately 30 minutes, prostate intrafraction motion can be considerable. Therefore, during the last minutes of this process, a position verification (‘PV’) MR scan was obtained to determine this motion. In our initial treatments (group 1, 26 patients) a virtual couch shift (VCS) was applied if part of the prostate was outside the PTV, (i.e., a prostate shift > 5 mm). The VCS involved a translation of the planned dose according to the PV to INI scan registration of the prostate, using a fast re-optimization method. In the subsequent 25 patients (group 2), the VCS was always applied, also for shifts below 5 mm, to further reduce the impact of pre-delivery intrafraction motion. Next, the plan was delivered in typically 7 minutes, while 3D cine-MR dynamics were acquired simultaneously (balanced turbo field echo). Each 3D dynamic spanned 9.4 seconds, and consisted of 448x448x45 voxels of 1.0x1.0x2.2mm. The prostate in all dynamics was automatically registered to INI and PV scans using a previously published method (D M de Muinck Keizer et al 2019 Phys. Med. Biol. 64 235008). In group 1, 24% of the patients required a VCS. In this group, intrafraction motion shifts relative to the treatment plan anatomy during dose delivery were 0.9±0.9 mm, 1.8±1.5 mm and 1.6±1.4 mm for respectively LR, AP and CC directions (mean ± SD of absolute values). In group 2, the VCS reduced the corresponding values to 0.4±0.4 mm, 1.4±1.2 mm and 1.3±1.3 mm. The average time between PV scan and beam on was 5 min, of which the VCS calculation time was below 1 min. An analysis of the required PTV margins in the corresponding directions showed that in group 2 margins could be safely reduced to 2, 4 and 4 mm in the LR, CC and AP directions. Daily VCS shortly before dose delivery allowed for a PTV margin reduction below conventionally applied 5 mm margins. However, due to the continuous and stochastical nature of intrafraction motion, margin reduction below 4 mm would require intrafraction plan adaptation. Presently, we develop such methods based on the high quality of the 1.5T 3D cineMR.

Dosimetric Impact of Deep Inspiration Breath Hold Uncertainty on Pancreas Stereotactic Body Radiotherapy

Deep inspiration breath hold (DIBH) is often used to immobilize pancreatic tumor for SBRT. Geometric reproducibility of tumor positioning between breath hold have been reported, but the corresponding dosimetric impact is not well understood. We evaluated dosimetric reproducibility of tumor coverage and OAR doses for inter-breath hold variation at treatment. Ten pancreatic cancer patients who underwent DIBH SBRT with 660cGy x 5 fractions (Rx) were selected for the study. The implanted fiducial markers were used as tumor motion surrogate at treatment. At CT sim, the contrast planning CT was acquired with 2-mm slice thickness, immediately followed by three additional CT sets. The patient specific breath-hold variation measured from the four sim CT sets was added as a margin to GTV in addition to a 2-mm set-up margin. At treatment, 4.6 ± 1.5 breath-hold CBCTs were acquired for each patient and each fraction. The first CBCT of each fraction was used as the reference to quantify the geometric and dosimetric inter-breath hold variations on the subsequent CBCT sets in the treatment planning system. The geometric variation of the GTV was quantified as the variation of fiducial centroids. The metrics used for the dosimetric variation were GTV D95 ≥ Rx, PTV D95 ≥ Rx, PTV Dmax < 4290 cGy (130% Rx), Paddick conformity index (pCI), and the ratio of the volume of the 50% isodose volume and the PTV volume (R50%) for tumor, V20Gy < 20% for duodenum, stomach and bowel, V12% < 50% for liver, and V12% < 25% for combined kidneys, in addition to Dmax for each OAR. The tumor position showed an average variation of 0.0 ± 1.6 mm, -0.3 ± 1.5 mm, and -0.4 ± 2.6 mm in the LR, AP, and SI directions, respectively. The GTV D95, PTV D95, and PTV Dmax were changed by 0.8 ± 5.3%, -3.1 ± 14.0%, and -0.1 ± 3.5%. The pCI and R50% varied by 1.7 ± 6.5% and 1.3 ± 10.8%. The V20Gy of duodenum, stomach, and bowel varied by -0.2 ± 3.1%, -0.3 ± 2.5%, and -0.2 ± 4.8%. There was no sizable difference with V12% for liver and combined kidneys. The Dmax for duodenum, stomach, bowel, liver, and combined kidneys were changed by -0.7 ± 4.8%, -0.4 ± 8.2%, 0.8 ± 5.6%, 0.4 ± 4.3%, and -0.6 ± 3.5%, respectively. Inter-breath hold variation in pancreatic tumor position affects tumor coverage, dose conformity, dose fall-off and OAR doses, more significantly for Dmax of duodenum, stomach and bowel which are closely located to the tumor. Future work is to evaluate whether a robust optimization can account for the inter-breath hold variation to minimize the corresponding dosimetric deviations.

Comparison of methods for determining shear modulus of wood

In this study, the relatively new picture frame method applied to wood is compared with three established shear test methods, namely the experimental modal analysis, the square plate twist method and the torsion test. For the investigations, the wood species European beech (Fagus sylvatica L.) and spruce (Picea abies L. Karst.) were used and the shear tests were conducted in LR and RL direction. The results show comparable shear moduli for beech and spruce in the range of 931–1289 Nmm−2 and 495–842 Nmm−2, respectively. In contrast to the theory of linear elastic orthotropic materials, significant differences in the results of the picture frame method between LR and RL direction were observed for spruce.

Development of a stereoscopic CT metal artifact management algorithm using gantry angle tilts for head and neck patients.

Dental amalgams are a common source of artifacts in head and neck (HN) images. Commercial artifact reduction techniques have been offered, but are substantially ineffectual at reducing artifacts from dental amalgams, can produce additional artifacts, provide inaccurate HU information, or require extensive computation time, and thus offer limited clinically utility. The goal of this work was to define and validate a novel algorithm and provide a phantom‐based testing as proof of principle. An initial clinical comparison to a vendor's current solution was also performed. The algorithm uses two‐angled CT scans in order to generate a single image set with minimal artifacts posterior to the metal implants. The algorithm was evaluated using a phantom simulating a HN patient with dental fillings. Baseline (no artifacts) geometrical measurements of the phantom were taken in the anterior–posterior, left–right, and superior–inferior directions and compared to the metal‐corrected images using our algorithm to evaluate possible distortion from application of the algorithm. Mean HU numbers were also compared between the baseline scan and corrected image sets. A similar analysis was performed on the vendor's algorithm for comparison. The algorithm developed in this work successfully preserved the image geometry and HU and corrected the CT metal artifacts in the region posterior to the metal. The average total distortion for all gantry angles in the AP, LR, and SI directions was 0.17, 0.12, and 0.14 mm, respectively. The HU measurements showed significant consistency throughout the different reconstructed images when compared to the baseline image sets. The vendor's algorithm also showed no geometrical distortion but performed inferiorly in the HU number analysis compared to our technique. Our novel metal artifact management algorithm, using CT gantry angle tilts, provides a promising technique for clinical management of metal artifacts from dental amalgam.

A novel dynamic robotic moving phantom system for patient-specific quality assurance in real-time tumor-tracking radiotherapy.

In this study, we assess a developed novel dynamic moving phantom system that can reproduce patient three‐dimensional (3D) tumor motion and patient anatomy, and perform patient‐specific quality assurance (QA) of respiratory‐gated radiotherapy using SyncTraX. Three patients with lung cancer were enrolled in a study. 3D printing technology was adopted to obtain individualized lung phantoms using CT images. A water‐equivalent phantom (WEP) with the 3D‐printed plate lung phantom was set at the tip of the robotic arm. The log file that recorded the 3D positions of the lung tumor was used as the input to the dynamic robotic moving phantom. The WEP was driven to track 3D respiratory motion. Respiratory‐gated radiotherapy was performed for driving the WEP. The tracking accuracy was calculated as the differences between the actual and measured positions. For the absolute dose and dose distribution, the differences between the planned and measured doses were calculated. The differences between the planned and measured absolute doses were <1.0% at the isocenter and <4.0% for the lung region. The gamma pass ratios of γ3 mm/3% and γ2 mm/2% under the conditions of gating and no‐gating were 99.9 ± 0.1% and 90.1 ± 8.5%, and 97.5 ± 0.9% and 68.6 ± 17.8%, respectively, for all the patients. Furthermore, for all the patients, the mean ± SD of the root mean square values of the positional error were 0.11 ± 0.04 mm, 0.33 ± 0.04 mm, and 0.20 ± 0.04 mm in the LR, AP, and SI directions, respectively. Finally, we showed that patient‐specific QA of respiratory‐gated radiotherapy using SyncTraX can be performed under realistic conditions using the moving phantom.

Evaluation of interbreath-hold lung tumor position reproducibility with vector volume histogram using the breath-hold technique

Tumor geometric reproducibility for lung stereotactic body radiotherapy (SBRT) is an important issue in the breath-hold (BH) technique. We investigated the inter-BH reproducibility of the tumor position in expiratory BH using our proposed vector volume histogram (VVH) method. Subjects comprising 14 patients with lung cancer who were treated with lung SBRT under expiratory BH conditions were monitored by the Abches system. Multiple computed tomography (CT) scans were performed to evaluate the inter-BH reproducibility of the tumor position at the expiratory BH in the simulation session. Gross tumor volume was delineated by a physician. Deformable image registration was used to deform the images from the 3 expiratory BH-CTs to the treatment planning expiratory BH-CT. To evaluate the inter-BH reproducibility of the tumor positions, we measured the largest motion extent within the organ of 3 dimensions (left-right, LR; anterior-posterior, AP; cranio-caudal, CC) and a 3D vector using the VVH method. The average and standard deviations of the inter-BH reproducibility of the tumor position in the LR, AP, and CC directions, and the 3D vector were 1.7 ± 0.5, 2.0 ± 0.7, 2.1 ± 0.7, and 2.7 ± 0.7 mm, respectively. Ten patients exhibited inter-BH displacements of the lung tumor >3 mm in the 3D vector. No displacement >5 mm was observed in any direction for all patients. Our study indicated that the inter-BH variation of the tumor position was small for lung cancer patients, using the Abches system and the VVH method.