Biologically Equivalent Dose Comparison Between Magnetic Resonance-Guided Adaptive and Computed Tomography-Guided Internal Target Volume-Based Stereotactic Body Radiotherapy for Liver Tumors.

Background and aim Magnetic resonance (MR) imaging has been increasingly adopted in the field of radiotherapy, and the most advanced MR image-guided radiotherapy is known as MR-guided online adaptive radiotherapy (MRgOART), which integrates MRI and linac systems. Few attempts have yet been made to directly compare treatment outcomes between the MRgOART and standard computed tomography (CT)-guided radiotherapy (CTgRT). Besides, it is reported that the biologically equivalent dose (BED) may be a good predictor of the local control (LC) and the overall survival (OS) for liver tumors. The purpose of this study is to compare the BEDs between the MRgOART and the CTgRT by way of virtual isotoxic planning for liver tumors. The hypothesis of this study is therefore that the MRgOART increases LC and OS as compared to the CTgRT. Materials and methods Using the five patient cases available, isotoxic planning was performed. For CTgRT, an internal target volume (ITV) was defined, and the planning target volume (PTV) was created by adding an isotropic margin of 10 mm. For MRgRT, a gross tumor volume (GTV) was defined, and the PTV was created by adding an isotropic margin of 5 mm. Each tumor size was virtually adjusted so that the CTgRT plans resulted in BED <100 Gy under the condition that the nearest organs at risk receive maximum tolerated doses. Subsequently, the BED was recalculated for MRgOART plans with the adjusted tumor size. Results and discussion It was found that the BEDs of the MRgOART plans always exceeded 100 Gy and were approximately 20 Gy larger than those of the corresponding CTgRT plans. Literature shows that superior overall survival rates for liver tumors were observed when BED was >100 Gy as compared to BED <100 Gy, suggesting that MR-guided adaptive planning may potentially lead to better treatment outcomes for liver tumors. We have also observed a case where the duodenum largely moved and abutted the liver after the CT images were acquired, indicating a significant disadvantage of the standard CTgRT because such abutting is not observable by the cone-beam CT immediately before treatment. Conclusion A highly accelerated evidence-creation procedure to suggest the clinical superiority of MRgOART has been arguably proposed with promising results. The sample size is small and limits the extent to which the findings in this study can be generalized. Further virtual clinical trials within the radiotherapy community are awaited with more clinical outcomes data.

Markerless Liver Tumor Localization Using Internal Liver Volume Delineated By Four-Dimensional Cone-Beam CT.

Markerless liver tumor localization has been proposed using an internal liver volume delineated by four-dimensional cone-beam CT (4D CBCT). Liver CT was performed under mid-ventilation breath hold, and transferred to a treatment planning system (TPS) to contour the gross target volume (GTV). Subsequently, liver 4D CBCT was performed and transferred to the TPS. After bone matching between the CT and the 4D CBCT, an internal liver volume was delineated on the liver CT volume as the union of liver volumes within a breathing cycle of the 4D CBCT volumes. Then, inhale liver volume was delineated on the 4D CBCT. Next, the internal target volume was defined by expanding the GTV by referring to the liver movement within the respiratory cycle of the 4D CBCT. Subsequently, all the delineated structures were transferred to the 4D CBCT unit. Immediately before treatment, 4D CBCT was performed again and the couch was repositioned so that the liver may move superiorly to the internal liver volume boundary and inferiorly to the inhale liver volume boundary during the respiratory cycle. The target localization accuracy of the proposed method was evaluated by comparing it to a published lipiodol-based technique. Both methods were applied to a single case in which lipiodol remained inside the tumor. 3D couch repositioning vectors for the two procedures were collected for 25 fraction data of the above same patient, and the differences in the vectors were calculated. The target localization deviations of the proposed method in reference to the lipiodol-based procedure were 0.7 mm ± 0.9 mm (SD) in the lateral direction, 2.0 mm ± 0.7 mm (SD) in the superior-inferior direction, and -2.1 mm ± 0.8 mm (SD) in the anterior-posterior direction. Markerless liver tumor localization is feasible by delineating the internal liver volume and the inhale liver volume using 4D CBCT.

Contrast media-assisted in-treatment cone beam CT during single-isocentre volumetric-modulated arc therapy for multiple brain metastases: a case study.

A direct visualization technique for verifying intrafractional localization accuracy of multiple brain metastases during single-isocentre volumetric-modulated arc therapy has been proposed using contrast media-assisted in-treatment cone beam CT (CBCT). Contrast-enhanced planning CT images were acquired immediately after intravenous bolus administration of iodized contrast media at a dose of 2 ml kg-1. Out of 41 nodules detected on the images, 8 lesions were contoured as high-risk gross tumour volumes (GTVs). Prior to each treatment, CBCT imaging was performed to match the skull structures with the planning CT images. Immediately after another intravenous bolus injection of the iodized contrast media at the same dose as administered for the planning CT imaging, contrast-enhanced CBCT images were acquired during volumetric-modulated arc therapy delivery, thereby providing direct verification of time-averaged tumour position during treatment. The planning target volume contours were overlaid with the in-treatment CBCT images, thereby allowing us to directly visualize the localization accuracy of each GTV when the beam delivery was completed. It was visually confirmed that each GTV was accurately localized inside each planning target volume during beam delivery.

Validation of planning target volume margins by analyzing intrafractional localization errors for 14 prostate cancer patients based on three-dimensional cross-correlation between the prostate images of planning CT and intrafraction cone-beam CT during volumetric modulated arc therapy.

Time-averaged intreatment prostate localization errors were calculated, for the first time, by three-dimensional prostate image cross-correlation between planning CT and intrafraction kilovoltage cone-beam CT (CBCT) during volumetric modulated arc therapy (VMAT). The intrafraction CBCT volume was reconstructed by an inhouse software after acquiring cine-mode projection images during VMAT delivery. Subsequently, the margin between a clinical target volume and a planning target volume (PTV) was obtained by applying the van Herk and variant formulas using the calculated localization errors. The resulting PTV margins were approximately 2 mm in lateral direction and 4 mm in craniocaudal and anteroposterior directions, which are consistent with the margin prescription employed in our facility.

Feasibility of single-isocenter, multi-arc non-coplanar volumetric modulated arc therapy for multiple brain tumors using a linear accelerator with a 160-leaf multileaf collimator: a phantom study.

The feasibility of single isocenter, multi-arc non-coplanar volumetric modulated arc therapy (VMAT) for multiple brain tumors was studied using an Elekta Synergy linear accelerator with an Agility multileaf collimator and a Monaco treatment planning system. Two VMAT radiosurgery plans consisting of a full arc and three half arcs were created with a prescribed dose of 20 Gy in a single fraction. After dose delivery to a phantom, ionization chambers and radiochromic films were used for dose measurement. The first VMAT radiosurgery plan had nine targets inside the phantom, and the doses were measured by the chambers at two different points and by the films on three sagittal and three coronal planes. The differences between the calculated dose and the dose measured by a Farmer ionization chamber and a pinpoint ionization chamber were <1.00% and <2.30%, respectively, and the average pass rates of gamma indices among the six planes under each of 3%/3 mm and 2%/2 mm criteria were 98.6% and 92.6%, respectively. The second VMAT radiosurgery plan was based on a clinical 14 brain metastases. Differences between calculated and film-measured doses were evaluated on two sagittal planes. The average pass rates of the gamma indices on the planes under each of 3%/3 mm and 2%/2 mm criteria were 97.8% and 88.8%, respectively. It was confirmed that single-isocenter, non-coplanar multi-arc VMAT radiosurgery for multiple brain metastases was feasible using Elekta Synergy with Agility and Monaco treatment planning systems. It was further shown that film dosimetry was accurately performed for a dose of up to nearly 25 Gy.

4D registration and 4D verification of lung tumor position for stereotactic volumetric modulated arc therapy using respiratory-correlated cone-beam CT.

We propose a clinical workflow of stereotactic volumetric modulated arc therapy (VMAT) for a lung tumor from planning to tumor position verification using 4D planning computed tomography (CT) and 4D cone-beam CT (CBCT). A 4D CT scanner, an Anzai belt and a BodyFix were employed to obtain 10-phase respiratory-correlated CT data for a lung patient under constrained breathing conditions. A planning target volume (PTV) was defined by adding a 5-mm margin to an internal target volume created from 10 clinical target volumes, each of which was delineated on each of the 10-phase planning CT data. A single-arc VMAT plan was created with a D(95) prescription dose of 50 Gy in four fractions on the maximum exhalation phase CT images. The PTV contours were exported to a kilovoltage CBCT X-ray Volume Imaging (XVI) equipped with a linear accelerator (linac). Immediately before treatment, 10-phase 4D CBCT images were reconstructed leading to animated lung tumor imaging. Initial bone matching was performed between frame-averaged 4D planning CT and frame-averaged 4D CBCT datasets. Subsequently, the imported PTV contours and the animated moving tumor were simultaneously displayed on the XVI monitor, and a manual 4D registration was interactively performed on the monitor until the moving tumor was symmetrically positioned inside the PTV. A VMAT beam was delivered to the patient and during the delivery further 4D CBCT projection data were acquired to verify the tumor position. The entire process was repeated for each fraction. It was confirmed that the moving tumor was positioned inside the PTV during the VMAT delivery.

4D digitally reconstructed radiography for verifying a lung tumor position during volumetric modulated arc therapy.

We have proposed four dimensional (4D) digitally reconstructed radiography (DRR) for verifying a lung tumor position during volumetric modulated arc therapy (VMAT). An internal target volume (ITV) was defined based on two clinical target volumes (CTVs) delineated on maximum exhalation and maximum inhalation images acquired by 4D planning computed tomography (CT). A planning target volume (PTV) was defined by adding a margin of 5 mm to the ITV on the maximum exhalation 3D CT images. A single-arc VMAT plan was created on the same CT data using Pinnacle SmartArc with a maximum multi-leaf collimator leaf speed of 1 mm/degree, thereby resulting in quasi-conformal field shapes while optimizing each beam intensity for each gantry angle. During VMAT delivery, cone-beam CT (CBCT) projection data were acquired by an on-board kilovoltage X-ray unit and a flat panel 2D detector. Four CBCT image sets with different respiratory phases were reconstructed using in-house software, where respiratory phases were extracted from the projection data. Subsequently a CTV was delineated on each of the 4D CBCT images by an oncologist. Using the resulting 4D CBCT data including the CTV contours, 4D DRRs during the VMAT delivery were calculated as a function of gantry angle. It was confirmed that the contoured CTV was within the radiation field during the four-fraction lung VMAT delivery. The proposed 4D DRR may facilitate the verification of the position of a respiratory moving lung tumor during VMAT delivery on each treatment day.

Single-arc volumetric modulated arc therapy planning for left breast cancer and regional nodes.

We have successfully created a single arc volumetric modulated arc therapy (VMAT) plan for treating post-surgical left breast/chest wall and regional nodes using Elekta multileaf collimator (MLC). Dose volume histograms (DVHs) were compared between the VMAT plans and conventional tangential beam plans using a field-in-field technique, leading to significant DVH advantages in the VMAT plans. The difference between Elekta VMAT and Varian RapidArc due to different MLC designs was discussed in terms of the number of arcs required to cover a large target, highlighting a single arc capability of Elekta VMAT for a large target volume which may be less sensitive to unexpected organ motion during dose delivery.

Technical note: Extension of Van Herk's treatment margin model for anisotropic systematic positioning errors in cartesian coordinate systema.

PURPOSE: A coefficient of a treatment margin model for anisotropic systematic positioning errors has been calculated in Cartesian coordinate system based on van Herk's analytical formulation. METHODS: Three-dimensional (3D) patient population distribution was formulated in Cartesian coordinate system to model anisotropic systematic positioning errors. Analytical 3D integration with anisotropic standard deviations Sigma's and the following Newton's method yielded the coefficient of van Herk's systematic positioning error model in Cartesian coordinate system. RESULTS: The treatment margins for the anisotropic systematic errors in Cartesian coordinate system were 2.1 Sigma for 90% patient population coverage and 2.4 Sigma for 95% patient population coverage. CONCLUSIONS: It was found that the treatment margins for anisotropic systematic positioning errors in Cartesian coordinate system were smaller than those for the isotropic model in spherical coordinate system for a given patient population coverage probability.

Patient setup error and day-to-day esophageal motion error analyzed by cone-beam computed tomography in radiation therapy.

UNLABELLED: Little has been reported on the errors of setup and daily organ motion that occur during radiation therapy (RT) for esophageal cancer. The purpose of this paper was to determine the margins of esophageal motion during RT. METHODS AND MATERIALS: The shift of the esophagus was analyzed in 20 consecutive patients treated with RT for esophageal cancer from November 2007. CT images for RT planning were used as the primary image series. Computed tomography (CT) images were acquired using an Elekta Synergy System, equipped with a kilovoltage-based cone-beam CT (CBCT) unit. The subsequent CBCT image series used for daily RT setup were compared with the primary image series to analyze esophageal motion. CBCT was performed before treatment sessions a total of 10 times in each patient twice a week. The outer esophageal wall was contoured on the CBCT images of all 200 sets. RESULTS: In the 200 sets of CBCT images, the mean (absolute) +/- standard deviation (SD) of setup errors were 2 +/- 2 mm (max, 8 mm) in the lateral direction, 4 +/- 3 mm (max, 11 mm) in the longitudinal direction, and 4 +/- 3 mm (max, 13 mm) in the vertical direction. Additionally, the mean +/- SD values of daily esophageal motion comparing the CBCT with RT planning CT were 5 +/- 3 mm (max, 15 mm) in the lateral direction and 5 +/- 3 mm (max, 15 mm) in the vertical direction. CONCLUSIONS: Our data support the use of target margins (between the clinical target volume and planning target volume) of 9 mm for day-to-day esophageal motion and 8 mm for patient setup in all directions, respectively.

Quality assurance of volumetric modulated arc therapy using Elekta Synergy.

PURPOSE. Recently, Elekta has supplied volumetric modulated arc therapy (VMAT) in which multi-leaf collimator (MLC) shape, jaw position, collimator angle, and gantry speed vary continuously during gantry rotation. A quality assurance procedure for VMAT delivery is described. METHODS AND MATERIALS. A single-arc VMAT plan with 73 control points (CPs) and 5-degree gantry angle spacing for a prostate cancer patient has been created by ERGO + + treatment planning system (TPS), where MLC shapes are given by anatomic relationship between a target and organs at risk and the monitor unit for each CP is optimized based on given dose prescriptions. Actual leaf and jaw positions, gantry angles and dose rates during prostate VMAT delivery were recorded in every 0.25 seconds, and the errors between planned and actual values were evaluated. The dose re-calculation using these recorded data has been performed and compared with the original TPS plan using the gamma index. RESULTS. Typical peak errors of gantry angles, leaf positions, and jaw positions were 3 degrees, 0.6 mm, and 1 mm, respectively. The dose distribution obtained by the TPS plan and the recalculated one agreed well under 2%-2 mm gamma index criteria. CONCLUSIONS. Quality assurance for prostate VMAT delivery has been performed with a satisfied result.