ExacTrac System Research Articles

Data Driven Approach to Exactrac Setup Parameters after CBCT for Cranial Radiotherapy

The purpose of this study is to 1) Determine the congruence of shifts derived from Cone-Beam CT (CBCT) and from Exactrac dynamic KV X-rays 2) To setup action levels for Exactrac imaging to determine if patient potentially moved between CBCT and Exactrac acquisition and 3) To analyze if Exactrac alone can be reliably used for cranial setup forgoing the CBCT step MATERIALS/METHODS: Exactrac dynamic, uses in-room KV X-ray imaging that is mounted on the floor to monitor patient positioning during treatment. The congruence of imaging and radiation isocenter for the LINAC and Exactrac system was verified using the Winston-Lutz (WL) test over a period of one year. The WL pointer was set up and its coincidence with imaging and radiation isocenter was confirmed. Exactrac images were then acquired at various couch angles and the pointer deviation from Exactrac imaging isocenter is noted. Secondly, translation and rotation shifts for 175 consecutive cranial cases were examined to determine the deviation between shifts as denoted by CBCT and Exactrac. These patients were initially set up using CBCT and then once in treatment position, Exactrac KV images were acquired to check for any deviation. The deviation between the two systems was collected and a Kolomogorov-Smirnov (KS) test was performed on the data to check for the normality of the distribution. Statistical Process Control (SPC) was performed to derive action levels for Exactrac based shifts after CBCT. Based on repeated Winston-Lutz tests over a period of one year, the maximum deviation between radiation and imaging isocenters and Exactrac isocenter was 0.3mm and 0.3 deg over all couch angles. This number remained stable over the entire time period without necessitating any recalibration of Exactrac isocenter. Based on patient data for cranial cases, the mean and standard deviations for the largest shifts were (0.45 mm, 0.40 mm) for translations (range 0 - 2.03mm) and (0.44 deg and 0.32 deg) for rotations (range 0 - 2.21 degree). Using SPC, it was decided that the action level for translations and rotations could be set to 0.8mm and 0.5 deg respectively. Any deviation beyond this action level necessitates re-imaging to verify that the patient did not move in between the CBCT and Exactrac imaging acquisitions CONCLUSION: Exactrac system derived shifts show excellent agreement with CBCT. The isocentricity of the systems is maintained over a long period of time and shows no drift. Either system can be reliably used to set up cranial patients. With the added advantage of speed of acquisition, matching and shifts, the Exactrac system could replace CBCT for daily setup of patients undergoing cranial radiotherapy.

Quantification of six-degree-of-freedom motion during beam delivery in spine stereotactic body radiotherapy using intra-irradiation cone-beam computed tomography imaging technique

PurposeQuantifying intra-fractional six-degree-of-freedom (6DoF) residual errors or motion from approved patient setups is necessary for accurate beam delivery in spine stereotactic body radiotherapy. However, previously reported errors were not acquired during beam delivery. Therefore, we aimed to quantify the 6DoF residual errors and motions during arc beam delivery using a concurrent cone-beam computed tomography (CBCT) imaging technique, intra-irradiation CBCT. MethodsConsecutive 15 patients, 19 plans for various treatment sites, and 199 CBCT images were analyzed. Pre-irradiation CBCT was performed to verify shifts from the initial patient setup using the ExacTrac system. During beam delivery by two or three co-planar full-arc rotations, CBCT imaging was performed concurrently. Subsequently, an intra-irradiation CBCT image was reconstructed. Pre- and intra-irradiation CBCT images were rigidly registered to a planning CT image based on the bone to quantify 6DoF residual errors. Results6DoF residual errors quantified using pre- and intra-irradiation CBCTs were within 2.0 mm/2.0°, except for one measurement. The mean elapsed time (mean ± standard deviation [min:sec]) after pre-irradiation CBCT to the end of the last arc beam delivery was 6:08 ± 1:25 and 7:54 ± 2:14 for the 2- and 3-arc plans, respectively. Root mean squares of residual errors for several directions showed significant differences; however, they were within 1.0 mm/1.0°. Time-dependent analysis revealed that the residual errors tended to increase with elapsed time. ConclusionThe errors represent the optimal intra-fractional error compared with those acquired using the pre-, inter-beam, and post-6DoF image guidance and can be acquired within a standard treatment timeslot.

Combining a Customized Immobilization System with an Innovative Use of the ExacTrac System for Precise Volumetric Modulated Arc Therapy of Challenging Forearm Sarcomas

High-precision image-guided radiation therapy (RT) for tumors abutting the appendicular skeleton may mean technical difficulties and concerns among practitioners. This technical note addresses the specific challenge for normofractionated image-guided RT of a tumor target in a forearm through an unconventional use of a treatment verification system usually devoted to stereotactic RT.

Offline generator for digitally reconstructed radiographs of a commercial stereoscopic radiotherapy image-guidance system.

PurposeImage‐guided radiotherapy (IGRT) research sometimes involves simulated changes to patient positioning using retrospectively collected clinical data. For example, researchers may simulate patient misalignments to develop error detection algorithms or positioning optimization algorithms. The Brainlab ExacTrac system can be used to retrospectively “replay” simulated alignment scenarios but does not allow export of digitally reconstructed radiographs (DRRs) with simulated positioning variations for further analysis. Here we describe methods to overcome this limitation and replicate ExacTrac system DRRs by using projective geometry parameters contained in the ExacTrac configuration files saved for every imaged subject.MethodsTwo ExacTrac DRR generators were implemented, one with custom MATLAB software based on first principles, and the other using libraries from the Insight Segmentation and Registration Toolkit (ITK). A description of perspective projections for DRR rendering applications is included, with emphasis on linear operators in real projective space P3. We provide a general methodology for the extraction of relevant geometric values needed to replicate ExacTrac DRRs. Our generators were tested on phantom and patient images, both acquired in a known treatment position. We demonstrate the validity of our methods by comparing our generated DRRs to reference DRRs produced by the ExacTrac system during a treatment workflow using a manual landmark analysis as well as rigid registration with the elastix software package.ResultsManual landmarks selected between the corresponding DRR generators across patient and phantom images have an average displacement of 1.15 mm. For elastix image registrations, we found that absolute value vertical and horizontal translations were 0.18 and 0.35 mm on average, respectively. Rigid rotations were within 0.002 degrees.ConclusionCustom and ITK‐based algorithms successfully reproduce ExacTrac DRRs and have the distinctive advantage of incorporating any desired 6D couch position. An open‐source repository is provided separately for users to implement in IGRT patient positioning research.

Uncertainty in the positioning of patients receiving treatment for brain metastases and wearing surgical mask underneath thermoplastic mask during COVID-19 crisis.

Thermoplastic masks, used along with surgical masks, enable immobilization methods to reduce the risk of infection in patients undergoing intracranial stereotactic radiosurgery and stereotactic radiotherapy (SRS/SRT) during the COVID‐19 crisis. The purpose of this study was to investigate the feasibility of thermoplastic mask immobilization with a surgical mask using an ExacTrac system. Twelve patients each with brain metastases were immobilized using a thermoplastic mask and a surgical mask and only a thermoplastic mask. Two x‐ray images were acquired to correct (XC) and verify (XV) the patient’s position at a couch angle of 0°. Subsequently, the XC and XV images were acquired at each planned couch angle for non‐coplanar beams. When the position errors were detected after couch rotation for non‐coplanar beams, the errors were corrected at each planned couch angle until a clinically acceptable tolerance was attained. The position errors in the translational and rotational directions (vertical, lateral, longitudinal, pitch, roll, and yaw) were retrospectively investigated using data from the ExacTrac system database. A standard deviation of XC translational and rotational position errors with and without a surgical mask in the lateral (1.52 vs 2.07 mm), longitudinal (1.59 vs 1.87 mm), vertical (1.00 vs 1.73 mm), pitch (0.99 vs 0.79°), roll (1.24 vs 0.68°), and yaw (1.58 vs 0.90°) directions were observed at a couch angle of 0°. Most of patient positioning errors were less than 1.0 mm or 1.0° after the couch was rotated to the planned angle for non‐coplanar beams. The overall absolute values of the translational and rotational XV position errors with and without the surgical mask were less than 0.5 mm and 0.5°, respectively. This study showed that a thermoplastic mask with a surgical mask is a feasible immobilization technique for brain SRS/SRT patients using the ExacTrac system.

Comprehensive characterization of ExacTrac stereoscopic image guidance system using Monte Carlo and Spektr simulations

The purpose of this work is to develop accurate computational methods to comprehensively characterize and model the clinical ExacTrac imaging system, which is used as an image guidance system for stereotactic treatment applications.The Spektr toolkit was utilized to simulate the spectral and imaging characterization of the system. Since Spektr only simulates the primary beam (ignoring scatter), a full model of ExacTrac was also developed in Monte Carlo (MC) to characterize the imaging system. To ensure proper performance of both simulation models, Spektr and MC data were compared to the measured spectral and half value layers (HVLs) values.To validate the simulation results, x-ray spectra of the ExacTrac system were measured for various tube potentials using a CdTe spectrometer with multiple added narrow collimators. The raw spectra were calibrated using a 57Co source and corrected for the escape peaks and detector efficiency. HVLs in mm of Al for various energies were measured using a calibrated RaySafe detector. Spektr and MC HVLs were calculated and compared to the measured values. The patient surface dose was calculated for different clinical imaging protocols from the measured air kerma and HVL values following the TG-61 methodology. The x-ray focal spot was measured by slanted edge technique using gafchromic films. ExacTrac imaging system beam profiles were simulated for various energies by MC simulation and the results were benchmarked by experimentally acquired beam profiles using gafchromic films. The effect of 6D IGRT treatment couch on beam hardening, dynamic range of the flat panel detector and scatter effect were determined using both Spektr simulation and experimental measurements.The measured and simulated spectra (of both MC and Spektr) for various kVps were compared and agreed within acceptable error. As another validation, the measured HVLs agreed with the Spektr and MC simulated HVLs on average within 1.0% for all kVps. The maximum and minimum patient surface doses were found to be 1.06 mGy for shoulder (high) and 0.051 mGy for cranial (low) imaging protocols, respectively. The MC simulated beam profiles were well matched with experimental results and replicated the penumbral slopes, the heel effect, and out-of-field regions. Dynamic range of detector (in terms of air kerma at detector surface) was found to be in the range of [6.1 × 10−6, 5.3 × 10−3] mGy.Accurate MC and Spektr models of the ExacTrac image guidance system were successfully developed and benchmarked via experimental validation. While patient surface dose for available imaging protocols were reported in this study, the established MC model may be used to obtain 3D imaging dose distribution for real patient geometries.

A phantom study to evaluate three different registration platform of 3D/3D, 2D/3D, and 3D surface match with 6D alignment for precise image-guided radiotherapy.

PurposeTo evaluate two three‐dimensional (3D)/3D registration platforms, one two‐dimensional (2D)/3D registration method, and one 3D surface registration method (3DS). These three technologies are available to perform six‐dimensional (6D) registrations for image‐guided radiotherapy treatment.MethodsFiducial markers were asymmetrically placed on the surfaces of an anthropomorphic head phantom (n = 13) and a body phantom (n = 8), respectively. The point match (PM) solution to the six‐dimensional (6D) transformation between the two image sets [planning computed tomography (CT) and cone beam CT (CBCT)] was determined through least‐square fitting of the fiducial positions using singular value decomposition (SVD). The transformation result from SVD was verified and was used as the gold standard to evaluate the 6D accuracy of 3D/3D registration in Varian’s platform (3D3DV), 3D/3D and 2D/3D registration in the BrainLab ExacTrac system (3D3DE and 2D3D), as well as 3DS in the AlignRT system. Image registration accuracy from each method was quantitatively evaluated by root mean square of target registration error (rmsTRE) on fiducial markers and by isocenter registration error (IRE). The Wilcoxon signed‐rank test was utilized to compare the difference of each registration method with PM. A P < 0.05 was considered significant.Results rmsTRE was in the range of 0.4 mm/0.7 mm (cranial/body), 0.5 mm/1 mm, 1.0 mm/1.5 mm, and 1.0 mm/1.2 mm for PM, 3D3D, 2D3D, and 3DS, respectively. Comparing to PM, the mean errors of IRE were 0.3 mm/1 mm for 3D3D, 0.5 mm/1.4 mm for 2D3D, and 1.6 mm/1.35 mm for 3DS for the cranial and body phantoms respectively. Both of 3D3D and 2D3D methods differed significantly in the roll direction as compared to the PM method for the cranial phantom. The 3DS method was significantly different from the PM method in all three translation dimensions for both the cranial (P = 0.003–P = 0.03) and body (P < 0.001–P = 0.008) phantoms.Conclusion3D3D using CBCT had the best image registration accuracy among all the tested methods. 2D3D method was slightly inferior to the 3D3D method but was still acceptable as a treatment position verification device. 3DS is comparable to 2D3D technique and could be a substitute for X‐ray or CBCT for pretreatment verification for treatment of anatomical sites that are rigid.

A retrospective analysis of setup and intrafraction positional variation in stereotactic radiotherapy treatments.

PurposeThe aim of this study was to provide a comprehensive assessment of patient intrafraction motion in linac‐based frameless stereotactic radiosurgery (SRS) and radiotherapy (SRT).MethodsA retrospective review was performed on 101 intracranial SRS/SRT patients immobilized with the Klarity stereotactic thermoplastic mask (compatible with the Brainlab frameless stereotactic system) and aligned on a 6 Degree of Freedom (DoF) couch with the Brainlab ExacTrac image guidance system. Both pretreatment and intrafraction correction data are provided as observed by the ExacTrac system. The effects of couch angle and treatment duration on positioning outcomes are also explored.ResultsInitial setup data for patients is shown to vary by up to ±4.18 mm, ±2.97°, but when corrected with a single x‐ray image set with ExacTrac, patient positions are corrected to within ±2.11 mm, ±2.27°. Intrafraction patient motion is shown to be uniformly random and independent of both time and couch angle. Patient motion was also limited to within approximately 3 mm, 3° by the thermoplastic mask.ConclusionsOur results indicate that since patient intrafraction motion is unrelated to couch rotation and treatment duration, intrafraction patient monitoring in 6 DoF is required to minimize intracranial SRS/SRT margins.

Evaluation of Six-Dimensional Cranial Target Positioning Accuracy in Two Different Immobilization Methods Using Exactrac System

Introduction: The aim of this study was to determine the accuracy of two different immobilization methods in patient positioning in cranial radiotherapy. The six-dimensional (6D) target localization accuracy of using a dedicated stereotactic mask was compared with that of a conventional head mask by the ExacTrac system. Material and Methods: A total of 56 patients with cranial lesions were included in this study (26 patients with a dedicated stereotactic mask and 30 subjects with a conventional head mask). The ExacTrac image-guided positioning system was utilized to obtain daily translational and rotational patient positioning displacement from the intended position. The 6D setup data was analyzed to obtain population mean, systematic and random errors, and three-dimensional (3D) vector shifts in all the patients. Results: The population mean values of setup errors were comparable with both immobilization systems; however, the spread as indicated by population systematic and population random errors was more in the use of a conventional head mask. The mean values of the 3D vector shifts were 2.09±1.00 and 4.51±3.38 mm with the use of a dedicated stereotactic mask and conventional head mask, respectively. The frequency distribution of maximum rotational deviation and statistical analysis demonstrated a significant difference in immobilization accuracy between stereotactic immobilization and 3-clamp immobilization (p <0.05). Conclusion: The results revealed that there was a significant reduction in target positioning errors with a dedicated stereotactic mask, compared to that with a conventional cranial mask. Furthermore, a dedicated stereotactic mask is required to keep rotational deviations within system correctable limits.

Evaluating the positional uncertainty of intrafraction, adjacent fields, and daily setup with the BrainLAB ExacTrac system in patients who are receiving craniospinal irradiation.

PurposeTo investigate the daily setup, interfraction motion, variability in the junction areas, and dosimetric effect in craniospinal irradiation (CSI) patients.MethodsFifteen CSI patients who had undergone split‐field IMRT were followed in the study. Previous, middle, and posttreatment, each target volume position was evaluated using the ExacTrac system. Interfraction and intrafraction motions, the margin of the junction in adjacent targets volumes, and the dosimetric effect of the longitudinal residual error were analyzed.ResultsThe lowest attainment rate within the tolerance of the initial setup error was 66.79% in six directions. The values of the initial error were within 15 mm (SD 4.5 mm) in the translation direction and 5° (SD 1.3°) in the rotation direction after the transposition of the treatment isocenter. With the guidance of the ExacTrac system, the interfraction and intrafraction residual errors were almost within the tolerance after correction, the margin of CTV‐to‐PCTV was in the range of target expansion criteria. The residual longitudinal errors resulted in only slight changes in the mean doses of PGTV and PCTV, while the maximum dose of the spinal cord increased by 16.1%. The patients did not exhibit any side‐effects by the overall treatment during the follow‐up period.ConclusionsPosition correction is necessary after setup and the transposition of the treatment isocenter. Intra‐fraction motion in the lateral direction should be monitored throughout treatment. The position errors in junction areas are almost within the tolerance after correction. The patients did not exhibit any side‐effects by the overall treatment.

Impact of patient positioning uncertainty in noncoplanar intracranial stereotactic radiotherapy.

The aim of this study is to evaluate the patient positioning uncertainty in noncoplanar stereotactic radiosurgery or stereotactic radiotherapy (SRS/SRT) for intracranial lesions with the frameless 6D ExacTrac system. In all, 28 patients treated with SRS/SRT of 70 treatment plans at our institution were evaluated in this study. Two X‐ray images with the frameless 6D ExacTrac system were first acquired to correct (XC) and verify (XV) the patient position at a couch angle of 0º. Subsequently, the XC and XV images were also acquired at each planned couch angle for using noncoplanar beams to detect position errors caused by rotating a couch. The translational XC and XV shift values at each couch angle were calculated for each plan. The percentages of the translational XC shift values within 1.0 mm for each planned couch angle for using noncoplanar beams were 77.86%, 72.26%, and 98.47% for the lateral, longitudinal, and vertical directions, respectively. Those within 2.0 mm were 98.22%, 97.96%, and 99.75% for the lateral, longitudinal, and vertical directions, respectively. The maximum absolute values of the translational XC shifts among all planned couch angles for using noncoplanar beams were 2.69, 2.45, and 2.17 mm for the lateral, longitudinal, and vertical directions, respectively. The overall absolute values of the translational XV shifts were less than 1.0 mm for all directions except for one case in the longitudinal direction. The patient position errors were detected after couch rotation for using noncoplanar beams, and they exceeded a planning target volume (PTV) margin of 1.0–2.0 mm used commonly in SRS/SRT treatment. These errors need to be corrected at each planned couch angle, or the PTV margin should be enlarged.

Evaluating the use of exactrac software for CBCT analysis during the development of an oligometastatic spinal SABR service

Keywords: Cone Beam CT; Exactrac; Brainlab; Oligomets; Spinal Stereotactic Body Radiotherapy. Introduction: The purpose of this service development was to evaluate Exactrac cone beam CT (CBCT) verification, utilising both 6 degrees of freedom (DoF) and patient motion management system, for the clinical implementation of spinal stereotactic ablative body radiotherapy (SABR) for oligometastatic disease. The Exactrac system has been utilised extensively at the Edinburgh Cancer Centre for many years as part of a well established pre-treatment verification process. A core radiographer team within the department worked towards evaluating the process and creating the patient pathway for this group of patients. Method and materials: 5 palliative patients requiring irradiation of the spine who would otherwise routinely undergo kilo voltage (Kv) imaging only, were authorised for CBCT volumetric imaging by the treating consultant. Images were acquired using the CBCT on board imager (OBI) and evaluated in the Exactrac system, utilising the patient motion management system throughout the process. Automatic image matching was applied and radiographers competent in both image interpretation and the Exactrac system examined the resultant image. The CBCT’s were analysed to consider clinical acceptability looking at bony and soft tissue contrast. Results: Currently all 5 patients CBCT’s have been clinically acceptable allowing both bony registration and analysis of soft tissue. During the process a few issues were identified including issues surrounding bed height and gantry direction. These were subsequently resolved enhancing our patient pathway. Conclusion/ Discussion: The team have proven the Exactrac system to be a safe and effective process for evaluating and correcting for set up variations in this group of patients. To confirm the robustness of the audit, a further 10 patients undergoing radiotherapy to the spine will be authorised for volumetric imaging. Indentifying potential difficulties will enable the creation of a concise protocol for differing sites and patient shapes.

Analysis of the setup errors and residual errors for ExacTrac X-ray image-guidance system for radiotherapy of somal tumors

Objective To retrospectively analyze the setup error in radiotherapy of somal tumors and body metastases using the ExacTrac X-ray portal image, and to evaluate the feasibility and effectiveness of 6D setup error correction in body radiotherapy. Methods The translational and rotational setup errors were calculated by registering the bony structures on the ExacTrac X- setup images to that of the digitally reconstructed setup images, and the corresponding residual errors were calculated together. Results The translational and rotational setup errors in the x (left-right), y (superior-inferior), z (anterior-posterior) and Rx (sagittal), Ry (transverse), Rz (coronal) directions were(2.27±2.02)mm, (4.49±2.52)mm, (2.27±1.37) mm and (1.02±0.73)°, (0.67±0.68)°, (0.76±0.84)°, respectively. The residual translational and rotational setup errors in the x(r), y(r), z(r) and Rx(r), Ry(r), Rz(r) directions were(0.27±0.48)mm, (0.37±0.45)mm, (0.22±0.30)mm and (0.17±0.33)°, (0.14±0.34)°, (0.16±0.28)°respectively. Conclusions Besides the translational setup errors, a certain amount of rotational setup errors exist in radiotherapy of somal tumors and body metastases. By using the 6D setup error correction of the ExacTrac system, a translational less than 0.4 mm and rotational setup errors less than 0.2° could be achieved. Key words: ExacTrac X-ray image-guidance system; 6 degree-of-freedom treatment couch; Setup errors; Residual errors

Application of the ExacTrac System in Respiratory Gated Radiotherapy for Lung Cancer Patients

Application of the ExacTrac System in Respiratory Gated Radiotherapy for Lung Cancer Patients

197. Stereotactic Body Radiotherapy: Comparison between two IGRT systems

Purpose The aim of this work is to compare the residual setup errors measured with ExacTrac X-ray 6 degree-of-freedom (6D) and kilo-voltage cone-beam computed tomography (CBCT) for patients receiving stereotactic body radiotherapy (SBRT) at Fondazione IOM in collaboration with REM Radioterapia. The first device employs an infrared optical system that performs patients pre-positioning through body markers and isocenter coordinates, and an X-ray imaging system that verifies target position using bone anatomy. CBCT imaging allows positioning verification using not only bone anatomy but also soft tissues. Methods On a TrueBeam-Novalis STx treatment unit, from January to November 2017, 45 patients with extracranial tumours were treated with SBRT for a total of 53 lesions (some patients having more than one lesion) and 152 treatment sessions. Prescribing doses ranged from 24 to 60 Gy in 3 fractions or 20 Gy in a single fraction and planning techniques were dynamic conformal arc therapy or hybrid intensity modulated radiotherapy, with no-coplanar fields. All patients were initially located using personalized immobilization systems and setup corrections were determined and corrected by means of registrations of ExacTrac X-ray images with the corresponding digitally reconstructed radiographs using the ExacTrac 6D fusion. At the end of each treatment session, with the couch at 0°, the residual setup errors were determined by means of registrations of CBCT images with the planning CT using online 3D fusion and for each session, displacements were evaluated to analyse the residual setup errors. Results A modest difference in residual setup errors was found between ExacTrac system and CBCT (figure below). The average of the observed residual displacements for the three space directions, in absolute value, was 1.5 mm in vertical and in lateral directions, 1.2 mm in longitudinal direction. Conclusion This work showed the good agreement on the setup accuracy between ExacTrac X-ray and CBCT for patients treated with SBRT. The two systems present comparable precision, but ExacTrac offers additional benefits such the capability to quantify all rotational errors, fastest automated positioning in 6D even for no-coplanar fields and smaller doses, representing a valid alternative to CBCT with complementary information in image-guided stereotactic radiotherapy.

Intrafraction Verification Using ExacTrac 6D Imaging for Frameless Cranial Radiosurgery

Background: Radiosurgery has been an integral component in the management of intracranial tumors. It involves the administration of a high dose to the tumor. An essential aspect of this kind of treatment delivery is accurate localization of the tumor target that is precisely reproducible during treatment. To achieve this, the methods of positioning and immobilization of the patient play a critical role. Conventionally, the patient has been immobilized with invasive head frame and the location of internal anatomy deduced by external coordinates. This method has some drawbacks. - It works on the assumption that the external coordinates represent the isocentre location correctly. - It depends on the spatial relationships between the frame and the skull. Slippage and deformity of the frame, although rare, can cause an error in positioning. - From the patient's perspective, the placement of the frame is a source of discomfort. It also puts them at risk for bleeding and infection. Frameless SRS has come into use in recent times. This is a noninvasive method. However, giving up the use of an invasive head frame leads to increased intrafraction motion. To overcome this, we require constant verification of the patient's positioning at every step of the treatment. Aim: The objective of this study is to compare the precision and accuracy between Brainlab ExacTrac imaging and CBCT and evaluate the intrafraction motion of patient undergoing intracranial SRT. Methods: For this study we included 10 patients with intracranial tumors who are eligible to undergo SRS. A frameless stereotactic head frame mask was prepared to immobilize the head during SRS. During treatment the patients were immobilized with the same frame and the 6D positional accuracy was checked for every single field before the treatment execution by taking verification image using 6D ExacTrac and Onboard Imagers. The 6D ExacTrac tolerance were set to rotation &lt; 1.00 and translational shift &lt; 0.7 mm. We make sure of intrafarction movement by taking frequent ExacTrac images during the treatment of every 30 s. After the treatment the registration between the planning CT image and CBCT image taken before treatment were cross verified 6 dimensionally using offline review. The difference in shift between ExacTrac and CBCT are evaluated by means of RMS. Results: The RMS of the difference in the translational dimensions between the ExacTrac System and CBCT was &lt; 1.0 for online matching and &lt; 1.10 for offline matching. Furthermore, the RMS of the difference in the rotational dimensions between the ExacTrac System and CBCT were &lt; 0.81 for online matching and &lt; 0.96 for offline matching. Conclusion: It is concluded that the discrepancies in residual set-up error between the ExacTrac 6D x-ray and the CBCT were minor. ExacTrac 6D imaging provides additional advantage of patient intrafraction movement control by means of imaging during treatment.

Implementation of a voluntary deep inspiration breath hold technique (vDIBH) using BrainLab ExacTrac infrared optical tracking system.

BackgroundVoluntary deep inspiration breath hold technique (vDIBH) is considered as the key to achieving the widest cardiac sparing in whole breast irradiation. Several techniques have been implemented to achieve a reproducible, fast and friendly treatment. The aim of the present study is to implement vDIBH using the ExacTrac (BrainLAB AG, Germany) monitoring system.MethodsWomen with left-sided breast cancer, younger than 50 years or with cardiac disease, underwent whole breast RT with vDIBH using the ExacTrac (BrainLAB AG, Germany) monitoring system. Simulations were performed with patients positioned supine on a breast board with both arms raised above the head. Five optical markers were placed on the skin around the border of the left breast gland and their position was referenced with ink marking. Each patient received a training session to find the individual deep inspiration level. Finally, a vDIBH CT was taken. All patients were also studied in free breathing (FB) in order to compare the dose distribution for PTV, heart and left anterior descending coronary artery (LAD). Pre-treatment verification was carried out through the ExacTrac (BrainLAB AG, Germany) system and verified with electronic portal imaging (EPI). Moreover, daily real time EPIs in during modality (captured during the beam delivery) were taken in order to check the reproducibility.Results34 patients have been evaluated and 30 were eligible for vDIBH. Most patients showed small setup errors during the treatment course of below 5 mm in 94.9% of the recorded fields. Mean Displacement was less in cranio-caudal direction. Mean intra-fraction displacement was below 3 mm in all directions. vDIBH plans provided better cardiac dosimetry.ConclusionsvDIBH technique using ExacTrac (BrainLAB AG, Germany) monitoring system was applied with good reproducibility.

Monitoring frequency of intra-fraction patient motion using the ExacTrac system for LINAC-based SRS treatments.

PurposeThe aim of this study was to investigate the intra‐fractional patient motion using the ExacTrac system in LINAC‐based stereotactic radiosurgery (SRS).MethodA retrospective analysis of 104 SRS patients with kilovoltage image‐guided setup (Brainlab ExacTrac) data was performed. Each patient was imaged pre‐treatment, and at two time points during treatment (1st and 2nd mid‐treatment), and bony anatomy of the skull was used to establish setup error at each time point. The datasets included the translational and rotational setup error, as well as the time period between image acquisitions. After each image acquisition, the patient was repositioned using the calculated shift to correct the setup error. Only translational errors were corrected due to the absence of a 6D treatment table. Setup time and directional shift values were analyzed to determine correlation between shift magnitudes as well as time between acquisitions.ResultsThe average magnitude translation was 0.64 ± 0.59 mm, 0.79 ± 0.45 mm, and 0.65 ± 0.35 mm for the pre‐treatment, 1st mid‐treatment, and 2nd mid‐treatment imaging time points. The average time from pre‐treatment image acquisition to 1st mid‐treatment image acquisition was 7.98 ± 0.45 min, from 1st to 2nd mid‐treatment image was 4.87 ± 1.96 min. The greatest translation was 3.64 mm, occurring in the pre‐treatment image. No patient had a 1st or 2nd mid‐treatment image with greater than 2 mm magnitude shifts.ConclusionThere was no correlation between patient motion over time, in direction or magnitude, and duration of treatment. The imaging frequency could be reduced to decrease imaging dose and treatment time without significant changes in patient position.

Inherent uncertainty involved in six-dimensional shift determination in ExacTrac imaging system

AbstractObjectiveThis study was conducted for the assessment of in-built systematic and random errors in the ExacTrac imaging system due to the software of Brainlab, on that basis; recommending a new quality control programme for ExacTrac imaging system.MethodsA program was developed to compare the image dataset of real time anthropomorphic pelvic phantom using ExacTrac with the reference image dataset from computed tomography. Images were acquired 20 times in a day, on single sitting for 20 conjugative days. On the basic of these translational and rotational shifts, systematic and random errors were calculated that had arisen due to multiple time image acquisition and image registration between acquired and reference image dataset of the phantom.ResultsRandom errors were found as 0·006 cm in right-left (Rt-Lt) direction, 0·008 cm in superior-inferior (Sup-Inf) direction and 0·012 cm in anterior-posterior (Ant-Post) direction. On this basic, margins were calculated using Van Herk formula; it was found that there were 0·02 cm inherent shift in Rt-Lt direction, 0·03 cm in Sup-Inf direction and 0·03 cm in Ant-Post direction.ConclusionThis study concluded that there was inherent error in ExacTrac system which can be quantified and used as a quality assurance tool for the ExacTrac system.

A pilot study on geometrical uncertainties for intra ocular cancers in radiotherapy.

A system for stabilising and monitoring eye movements for linac-based stereotactic radiotherapy associated with the mobile eye, the Eye Tracker, was developed. Whilst the Eye Tracker design is based on a previously reported system, the purpose of this study was to confirm that the modified version can be used with clinically acceptable treatment margins. We report the estimates of the margin required to account for inter- and intra-fraction eye motion based on data from 12 consecutive patients treated with the Eye Tracker system in place. Patients were immobilised in a head and neck mask and were required to fixate on a light source. A camera system monitored eye movements relative to CT simulation baseline measurements. The Exactrac system (Brainlab, Feldkirchen, Germany) combined with the Varian TrueBeamSTx (Varian Medical Systems, Palo Alto, CA) confirmed pre- and intra-treatment setup of the head position. Displacement/rotation of the image of the pupil/iris was determined in the lateral and superior-inferior directions using a video display. A standard margin equation was applied to estimate the margin required to account for inter- and intra-fraction eye movement. The average displacement in both directions was 0.1-0.2mm (0.36mm SD). All patients maintained a position within 1mm of the intended position during treatment. Based on a Bayesian estimation of the systematic and treatment errors, accounting for displacements in two-planes and a standard deviation of the penumbral width of 1.3mm, the estimated margins to achieve coverage of the GTV with the 95% isodose in 90% of patients was found to be less than 1mm. Small random and systematic uncertainties due to inter- and intra-fraction movement of the eye were achieved with the Eye Tracker. Whilst the estimated margins are small (<1mm) they need to be considered in addition to contouring and treatment delivery uncertainties.