Intrafractional Patient Movement Research Articles

Optimized workflow to minimise intra-fractional motion during stereotactic body radiotherapy of spinal metastases.

This study evaluated translational and rotational intra-fractional patient movement during spinal stereotactic body radiotherapy (SBRT) using 6D positioning based on 3D cone beam computerized tomography (CBCT) and stereoscopic kilovoltage imaging (ExacTrac). The aim was to determine whether additional intra-fractional image verification reduced intra-fractional motion without significantly prolonging treatment time, whilst maintaining acceptable imaging related dose. A retrospective analysis of 38 patients with 41 primary tumour volumes treated with SBRT between September 2018 and May 2021 was performed. Three different image-guided radiotherapy (IGRT) workflows were assessed. The translational and rotational positioning errors for the different imaging workflows, 3D translational vectors and estimates of imaging dose delivered for the different imaging workflows were evaluated. As the frequency of intra-fractional imaging increased from workflow 1 to 3, the mean intra-fraction 3D translational vector improved from 0.91mm (±0.52mm), to 0.64 (±0.34mm). 85%, 83% and 97% of images were within a tolerance of 1mm/1° for workflows 1, 2 and 3 respectively, based on post treatment CBCT images. The average treatment time for workflow 3 was 13min, as compared to 12min for workflows 1 and 2. The effective dose per treatment for IGRT workflows 1, 2 and 3 measured 0.6mSv, 0.95mSv and 1.8mSv respectively. The study demonstrated that the use of additional intra-fractional stereoscopic kilovoltage image-guidance during spinal SBRT, reduced the number of measurements deemed "out of tolerance" and treatment delivery could be optimized within a standard treatment timeslot without applying substantial additional radiation dose.

Clinical Implementation of a 6D Treatment Chair for Fixed Ion Beam Lines.

PurposeTo verify the practicality and safety of a treatment chair with six degrees of freedom (6DTC) through demonstrating the efficacy of the workflow in clinical settings and analyzing the obtained technical data, including intra-fraction patient movement during the use of the 6DTC.Materials and MethodsA clinical study was designed and conducted to test the clinical treatment workflow and the safety of the 6DTC. Based on the demonstrated dosimetric advantages, fifteen patients with head and neck tumors were selected and treated with the 6DTC. The positional error at the first beam position (PE-B1) and the second beam position (PE-B2) were analyzed and compared with the results from daily quality assurance (QA) procedures of the 6DTC and imaging system performed each day before clinical treatment. The intra-fraction patient movement was derived from the total patient alignment positional error and the QA data based on a Gaussian distribution formulism.ResultsThe QA results showed sub-millimeter mechanical accuracy of the 6DTC over the course of the clinical study. For 150 patient treatment fractions, the mean deviations between PE-B1 and PE-B2 were 0.13mm (SD 0.88mm), 0.25mm (SD 1.17mm), -0.57mm (SD 0.85mm), 0.02° (SD 0.35°), 0.00° (SD 0.37°), and -0.02° (SD 0.37°) in the x, y, z (translational), and u, v, w (rotational) directions, respectively. The calculated intra-fraction patient movement was -0.08mm (SD 0.56mm), 0.71mm (SD 1.12mm), -0.52mm (SD 0.84mm), 0.10° (SD 0.32°), 0.09° (SD 0.36°), and -0.04° (SD 0.36°) in the x, y, z, u, v, w directions, respectively.ConclusionsThe performance stability of the 6DTC was satisfactory. The position accuracy and intra-fraction patient movement in an upright posture with the 6DTC were verified and found adequate for clinical implementation.

Assessment of Intra-fraction Motion during Frameless Stereotactic Radiosurgery

Frameless stereotactic radiosurgery (SRS) is facilitated by cone-beam CT (CBCT) imaging, online re-planning, and intra-fraction motion monitoring (IFMM). The purpose of this study was to evaluate the intra-fraction motion detected using frameless immobilization for SRS through the CBCT and IFMM systems. Patients treated with frameless SRS over a 2-month study period were included for analysis. For SRS, patients are immobilized in a personalized thermoplastic mask and headrest followed by attachment of a reflective marker on their nose. The patient’s treatment position is verified using a localization CBCT against the reference CBCT acquired at simulation, while the IFMM system continuously monitors their nose marker. Repeat localization CBCTs were acquired during treatment if IFMM thresholds (1.5mm) were exceeded or for voluntary patient breaks. Where feasible, a post-treatment CBCT was acquired after treatment delivery. Using bony anatomy for image registration, the difference between localization and post CBCTs, and localization and repeat localization CBCTs (in situations where IFMM thresholds were exceeded) were used to quantify intra-fraction motion. The IFMM data associated with the time points of the CBCTs were also extracted to assess intra-fraction motion. Thirty plans were reviewed from 26 patients (19 single fraction, 11 multi-fraction, 4 patients treated concurrently with single and multi-fractionation). Over 52 treatment fractions, 113 localization, repeat localization and post-treatment CBCTs were acquired. 28 sets of localization-post CBCTs and 25 sets of localization-repeat localization CBCTs were included for analysis. The average ± standard deviation intra-fraction motion quantified on CBCTs was -0.11±0.37mm, -0.13±0.20mm, 0.18±0.74mm in the left/right (L/R), anterior/posterior (A/P) and superior/inferior (S/I) axes. The average ± standard deviation rotational displacement measured on CBCTs was 0.07±0.56⁰, -0.14±0.64⁰, 0.02±0.54⁰ in pitch, yaw and roll. The average ± standard deviation intra-fraction motion quantified on IFMM was -0.09±0.68mm, -0.11±0.5mm, 0.25±0.76mm in the L/R, A/P and S/I axes. There was a positive correlation between the measured vector CBCT and IFMM displacements (r = 0.49). Preliminary analysis indicate the largest intra-fraction motion, as measured by both CBCT and IFMM systems for frameless GK-SRS, is in the S/I axis. Positional deviations noted by the IFMM system during SRS provides a good indication of intra-fraction patient movement.

Technical Note: Rotational positional error corrected intrafraction set-up margins in stereotactic radiotherapy: A spatial assessment for coplanar and noncoplanar geometry.

The aim of this study is to calculate setup margin based on six-dimensional (6D) corrected residual positional errors from kV cone beam computed tomography (CBCT) and from intrafraction projection kV imaging in coplanar and in noncoplanar couch positions in stereotactic radiotherapy. Six dimensional positional corrections were carried out before patient treatments, using a robotic couch and CBCT matching. A CBCT and stereoscopic ExacTrac image were acquired post-table position correction. Further, a series of intrafraction ExacTrac images were obtained for the variable couch position. Translational and rotational errors were identified as lateral (X), longitudinal (Y), vertical (Z); roll (Ɵ°), pitch (Φ°) and yaw (Ψ°). A total of 699 intrafraction image sets (361 coplanar and 338 noncoplanar) for 51 SRS/SRT patients were analysed. Rotational errors were corrected in terms of translational coordinates. Residual set-up margins were calculated from CBCT shifts. ExacTrac shifts give residual+intrafraction setup margins as a function of coplanar and noncoplanar couchpositions. The average residual positional error obtained from CBCT in X, Y, Z, Ɵ, Φ, Ψ were 0.1±0.4mm, 0.0±0.6mm,0.0±0.5mm, 0.2±0.8°, 0.1±0.6° and -0.1±0.7° respectively. For ExacTrac, the shits were -0.5±0.9mm, -0.0±1mm, -0.6±1.0mm, 0.4±0.9°, -0.2±0.6°, and -0.0±0.8°. CBCT calculated linear setup margins in X, Y, Z direction were 0.5, 1.2, and 1mm respectively. ExacTrac yielded coplanar and noncoplanar linear setup margins were 1.2, 1.3, 1.5, 1.4, 1.5, and 2.1mm respectively. CBCT-based gross residual set-up margin is equal to 1mm. ExacTrac calculated residual plus intrafraction setup margin falls within a 2mm range; attributed to intrafraction patient movement, table position inaccuracies, and poor image fusion in noncoplanar geometry. There could be variations in the required additional margin between centers and between machines, which require further studies.

Creating planning margins and imaging intervals for intracranial stereotactic radiosurgery treatment using patient log files

To account for intra-fractional patient movement during stereotactic radiosurgery, Cyberknife utilises two orthogonal ceiling mounted kV sources that can be used to track patient movement with a variable imaging interval of between 15 and 150 s according to the user’s discretion. Imaging intervals can be optimised to keep imaging dose to a minimum while retaining delivery accuracy. Log files recorded during the treatment delivery can be used to establish both optimum imaging interval and also a suitable treatment margin to account for the patient movement that cannot be accounted for by imaging. Log files for 56 intracranial patients (100 fractions, 5000 image pairs) were retrospectively assessed for patient movement. Movement between consecutive images was investigated, along with the movement of the patient over the duration of the treatment. It was found that from image to image, patient movement did not vary with respect to interval times. The standard deviation (SD) for the movement between images was found to be 0.32, 0.36 and 0.33 mm for superior/inferior (SI), left/right (LR) and anterior/posterior (AP) movement respectively. The rate of drift between the images was used to calculate that an interval of 28 s was required to restrict the translational movement of the patient to 0.1 mm for 95% of the population. As no relationship could be found between the interval times and movement, it cannot be assumed that the patient movement between images is linear. This movement cannot be accounted for by additional imaging but could be accounted for using additional planning margins. This work shows that log files that have been verified can be used to create treatment margins and imaging intervals. To ensure target coverage, a 0.8 mm margin (0.7 for stochastic and 0.1 for systematic) is recommended, and an imaging interval of 30 s.

EP-1382: Use of cone beam CT (CBCT) to evaluate the intrafraction patient movements during SBRT

EP-1382: Use of cone beam CT (CBCT) to evaluate the intrafraction patient movements during SBRT

Stereotactic radiosurgery for intradural spine tumors using cone-beam CT image guidance.

OBJECTIVE Cone-beam CT (CBCT) image guidance technology has been widely adopted for spine radiosurgery delivery. There is relatively little experience with spine radiosurgery for intradural tumors using CBCT image guidance. This study prospectively evaluated a series of intradural spine tumors treated with radiosurgery. Patient setup accuracy for spine radiosurgery delivery using CBCT image guidance for intradural spine tumors was determined. METHODS Eighty-two patients with intradural tumors were treated and prospectively evaluated. The positioning deviations of the spine radiosurgery treatments in patients were recorded. Radiosurgery was delivered using a linear accelerator with a beam modulator and CBCT image guidance combined with a robotic couch that allows positioning correction in 3 translational and 3 rotational directions. To measure patient movement, 3 quality assurance CBCTs were performed and recorded in 30 patients: before, halfway, and after the radiosurgery treatment. The positioning data and fused images of planning CT and CBCT from the treatments were analyzed to determine intrafraction patient movements. From each of 3 CBCTs, 3 translational and 3 rotational coordinates were obtained. RESULTS The radiosurgery procedure was successfully completed for all patients. Lesion locations included cervical (22), thoracic (17), lumbar (38), and sacral (5). Tumor histologies included schwannoma (27), neurofibromas (18), meningioma (16), hemangioblastoma (8), and ependymoma (5). The mean prescription dose was 17 Gy (range 12-27 Gy) delivered in 1-3 fractions. At the halfway point of the radiation, the translational variations and standard deviations were 0.4 ± 0.5, 0.5 ± 0.8, and 0.4 ± 0.5 mm in the lateral (x), longitudinal (y), and anteroposterior (z) directions, respectively. Similarly, the variations immediately after treatment were 0.5 ± 0.4, 0.5 ± 0.6, and 0.6 ± 0.5 mm along x, y, and z directions, respectively. The mean rotational angles were 0.3° ± 0.4°, 0.3° ± 0.4°, and 0.3° ± 0.4° along yaw, roll, and pitch, respectively, at the halfway point and 0.5° ± 0.5°, 0.4° ± 0.5°, and 0.2° ± 0.3° immediately after treatment. CONCLUSIONS Radiosurgery offers an alternative treatment option for intradural spine tumors in patients who may not be optimal candidates for open surgery. CBCT image guidance for patient setup for spine radiosurgery is accurate and successful in patients with intradural tumors.

EP-1809: Intrafractional patient movement during an online adaptive replanning procedure for cranial SRS

EP-1809: Intrafractional patient movement during an online adaptive replanning procedure for cranial SRS

EP-1757: Intra-fraction patient movements during SBRT: CBCT vs Surface Optical Markers

EP-1757: Intra-fraction patient movements during SBRT: CBCT vs Surface Optical Markers

EP-1769: Evaluation of the intra-fraction patient movement for SBRT treatments in our Institution

EP-1769: Evaluation of the intra-fraction patient movement for SBRT treatments in our Institution

A.119 - Evaluation of planning target volume margins by analyzing intra-fraction patient movements during stereotactic body radiation therapy (SBRT) of lung lesions

Introduction: To evaluate the adequacy of the adopted planning margins in SBRT estimating the intra-fraction movements obtained with Cone Beam CT (CBCT) and a non invasive infrared Optical Tracking System (OTS).

Initial evaluation of intrafraction motion using frameless CyberKnife VSI system

AimTo analyze intrafraction movement in patients undergoing frameless robotic radiosurgery and evaluate the influence of image acquisition frequency on global accuracy. BackgroundStereotactic radiosurgery requires high spatial accuracy in dose delivery. In conventional radiosurgery, a rigid frame is used to guarantee a correct target alignment and no subsequent movement. Frameless radiosurgery with thermoplastic mask for immobilization cannot completely eliminate intrafraction patient movement. In such cases, it is necessary to evaluate its influence on global treatment accuracy. Materials and methodsWe analyzed the intrafraction motion of the first 15 patients undergoing intracranial radiosurgery (39 fractions) with the CyberKnife VSI system at our institution. Patient position was measured at a 15–90-s interval and was used to estimate intrafraction patient movement. ResultsWith our acquisition image protocol and immobilization device, the 99% displacement error was lower than 0.85mm. The systematic movement components were lower than 0.05mm and the random component was lower than 0.3mm in the 3 translational axes. Clear linear time dependence was found in the random component. ConclusionsSelection of the X-ray image acquisition time is necessary to meet the accuracy required for radiosurgery procedures with the CyberKnife VSI system. We verified that our image acquisition protocol met the 1-mm criterion.

Is helical tomotherapy accurate and safe enough for spine stereotactic body radiotherapy?

We assessed the accuracy and safety of spine stereotactic body radiation therapy (SBRT) using helical tomotherapy (HT) via evaluating intrafractional patient movement. From July 2009 to April 2011, 22 patients with spine lesions received SBRT using HT, with a total of 61 fractions. To evaluate intrafractional movement, we compared post-treatment megavoltage CT scans with planning CT images and obtained translational [lateral (X), craniocaudal (Y), anterioposterior (Z)] offsets and total displacements (R). We analyzed the correlation of intrafractional motion with patient and treatment characteristics. We also analyzed dosimetric change to the target and spinal cord, resulting from intrafractional movement, in the three patients that showed the greatest R values. Intrafractional movements were 0.7 ± 0.6 mm (X), 1.1 ± 0.7 mm (Y), 0.9 ± 0.6 mm (Z), and 1.8 ± 0.6 mm (R). This movement did not correlate with age, pain score, treatment time, or treatment site. Only patients with lower BMIs have a tendency to move more during treatment. Patient immobilization using wrapping form (thermoplastic mask and BodyFIX(®) system) resulted in less lateral movement and total displacement than others (0.498 ± 0.409 vs. 1.138 ± 0.637 mm, P < 0.001 for X; and 1.638 ± 0.691 vs. 1.976 ± 0.495 mm, P = 0.032 for R). However, this intrafractional motion did not affect the dose delivery to the target and spinal cord. SBRT using HT can be a safe treatment modality for spine metastasis with enhanced targeting accuracy.

Is the in vivo dosimetry with the OneDosePlusTM system able to detect intra-fraction motion? A retrospective analysis of in vivo data from breast and prostate patients

BackgroundThe OneDosePlusTM system, based on MOSFET solid-state radiation detectors and a handheld dosimetry reader, has been used to evaluate intra-fraction movements of patients with breast and prostate cancer.MethodsAn Action Threshold (AT), defined as the maximum acceptable discrepancy between measured dose and dose calculated with the Treatment Planning System (TPS) (for each field) has been determined from phantom data. To investigate the sensitivity of the system to direction of the patient movements, fixed displacements have been simulated in phantom. The AT has been used as an indicator to establish if patients move during a treatment session, after having verified the set-up with 2D and/or 3D images. Phantom tests have been performed matching different linear accelerators and two TPSs (TPS1 and TPS2).ResultsThe ATs have been found to be very similar (5.0% for TPS1 and 4.5% for TPS2). From statistical data analysis, the system has been found not sensitive enough to reveal displacements smaller than 1 cm (within two standard deviations). The ATs applied to in vivo treatments showed that among the twenty five patients treated for breast cancer, only four of them moved during each measurement session. Splitting data into medial and lateral field, two patients have been found to move during all these sessions; the others, instead, moved only in the second part of the treatment. Patients with prostate cancer have behaved better than patients with breast cancer. Only two out of twenty five moved in each measurement session.ConclusionsThe method described in the paper, easily implemented in the clinical practice, combines all the advantages of in vivo procedures using the OneDosePlusTM system with the possibility of detecting intra-fraction patient movements.

Prospective evaluation of a dedicated spine radiosurgery program using the Elekta Synergy S system

Cone beam CT (CBCT) image guidance has recently been adopted for the delivery of spine radiosurgery. In 2007, the authors' institution began a dedicated spine radiosurgery program using the Elekta Synergy S system, which incorporates CBCT technology. In this study, the authors prospectively evaluated the Synergy S platform as a dedicated spine radiosurgery delivery system, including an evaluation of the accuracy of patient positioning using this technology, as part of a quality assurance program. One hundred sixty-six spine and paraspinal lesions were treated using the Elekta Synergy S 6-MV LINAC with a beam modulator and CBCT image guidance combined with a HexaPOD couch that allows correction of patient positioning in 3 translational and 3 rotational directions. Stratifying the lesion by location, there were 28 cervical, 69 thoracic, 48 lumbar, and 21 sacral lesions. The most common histological types for the metastatic lesions (136 cases total) were breast, lung, sarcomas, and renal cells. The most common benign tumors (30 cases total) included 10 schwannomas, 5 neurofibromas, and 5 meningiomas. Twenty-eight lesions (17%) were intradural. To measure intratreatment patient movement, 3 quality assurance CBCTs were performed and recorded at separate times: immediately before treatment started; at the first third of the procedure; and at the second third of the procedure. The positioning data and fused images of the planning CT and CBCT were analyzed to determine intrafraction patient movements. From each of 3 quality assurance CBCT images, 3 translational and 3 rotational coordinates were obtained. The prescribed dose to the gross tumor volume, delivered in a single fraction, ranged from 12 to 20 Gy (mean 16 Gy) in this cohort. This dose was delivered by between 7 and 14 coplanar intensity-modulated radiation therapy beams (mean 9 beams). The gross tumor volumes ranged from 1.2 to 491.7 cm(3) (mean 39.2 cm(3)). Mean treatment time including setup was 64 minutes. At the first third of the treatment, the magnitude of the 3D translational vector (X, Y, Z) was 1.1 ± 0.7 mm. Similarly, the 3D translational vector at the second third of the treatment was 1.0 ± 0.6 mm. The means ± SDs of the rotational angles were 0.2° ± 0.4°, 0.4° ± 0.5°, and 0.3° ± 0.5° along yaw, roll, and pitch, respectively, at the first third of the treatment, and 0.2° ± 0.3°, 0.4° ± 0.5°, and 0.4° ± 0.5°, respectively, at the second third of the treatment. Single-fraction spine radiosurgery performed using the Synergy S platform and incorporating CBCT image guidance was determined to be feasible, accurate, and safe. This technique provides an overall translational position accuracy of < 2.0 mm.

Volumetric Imaging by Megavoltage Computed Tomography for Assessment of Internal Organ Motion During Radiotherapy for Cervical Cancer

To assess the internal organ motion of the cervix and uterus by megavoltage computed tomography (MVCT) during intensity-modulated radiotherapy (IMRT). Ten patients with Stage IIB-IVA cervical cancer underwent daily MVCT imaging. Internal organ motion was evaluated on 150 pretreatment MVCT images by measuring shifts in their boundaries between the MVCT and kilovoltage (kV) planning CT scan in the anterior, posterior, left and right lateral, and superior and inferior direction. Additional intrafractional patient movement was evaluated on 50 posttreatment MVCT images. Measured cervical motion (mean +/- SD) was 0.4 +/- 10.1 mm in the anterior, -3.0 +/- 6.9 mm in the posterior direction, -3.5 +/- 4.9 mm in the left and 0.2 +/- 4.5 mm in the right lateral direction, 2.2 +/- 8.0 mm in the superior and 0.5 +/- 5.0 mm in the inferior direction. Compared to the cervix, larger uterine motion was observed. Patient movement during treatment was limited to 1.1 +/- 1.3 mm, -0.3 +/- 1.6 mm, and 0.2 +/- 2.3 mm in anteroposterior, laterolateral and superoinferior direction respectively. MVCT imaging can be used to study patient setup accuracy and cervical and uterine motion during IMRT. This data may be used to refine treatment margins.

Intrafraction Geometric Uncertainties in Frameless Image-Guided Radiosurgery

Frameless radiosurgery allows the possibility of intrafraction patient movement. Because radiosurgery delivers the entire radiation dose during one or a few fractions, intrafraction misalignment can result in significant underdosage of the treatment site. This article compares alignment errors and their effect on target coverage for treatments that rely only on initial (fixed) alignment vs. those that make intrafraction corrections (dynamic alignment). This study analyzed 577 records of intrafraction patient movement observed during frameless cranial and spinal radiosurgery. For each fraction, the average misalignment per fraction was calculated. Then each fraction was divided into n minifractions and margin formulae developed for hyperfractionated radiotherapy were used to estimate the planning margin that would be necessary to preserve target coverage for the observed intrafraction movement. Dynamic alignment reduced the number of fractions with a mean misalignment greater than 2 mm from approximately 20% to nearly zero. For fixed alignment, the estimated margins for optimal target coverage were 3.6-4.5 mm for the various treatment sites. For dynamic alignment, the optimal margins were 1.2-1.6 mm. The estimated margins show the large influence of systematic intrafraction shifts and the capacity of dynamic alignment to correct for them. For dynamic alignment, the margin approximately equates with the traditional precision tolerances for radiosurgery, whereas for fixed alignment, the margin is three times greater. Although these margins may not be directly applicable to radiosurgery planning, they expose the effects of intrafraction motion on target coverage.

Evaluation of Intra-Fraction Patient Movements During Image Guided LINAC-based Spinal Radiosurgery

Evaluation of Intra-Fraction Patient Movements During Image Guided LINAC-based Spinal Radiosurgery

Evaluation of Intra-Fractional Patient Movement During Image Guided Stereotactic Body Radiation Therapy of Spine and Lung Patients

Evaluation of Intra-Fractional Patient Movement During Image Guided Stereotactic Body Radiation Therapy of Spine and Lung Patients

1028: Use of On-Board Imaging to Evaluate Residual Errors for Target Localization in SBRT: A Feedback Analysis

Accurate target localization for stereotactic body radiotherapy (SBRT) requires identification of 3D anatomical information. On-board imaging provides both 2D radiographs and cone beam CT (CBCT) for target localization and is being introduced for clinical applications. However, the effective use of this system is still under development. This study is aimed to develop a new scheme of using on-board imaging as a feedback system to evaluate residual localization errors for SBRT due to both inter-fraction variation between simulation and treatment and intra-fraction patient movement associated longer treatment time. The process for on-board target localization for SBRT includes: 1) align patient within immobilization device; 2) obtain orthogonal 2D kV/MV radiographs to compare to reference DRRs and document shifts accordingly; 3) obtain 3D CBCT for comparison to the planning CT; 4) shift patient to align target between CBCT and planning CT; 5) obtain orthogonal 2D kV/MV radiographs to document shifts; 6) treatment; 7) obtain orthogonal 2D kV/MV radiographs to assess intra-fraction motion. The shifts as identified by steps 2 and 3 indicate the improvement of target localization accuracy from inter-fraction deviation by incorporating 3D anatomical information. Steps 5 and 7 reflect the degree of intra-fraction variation which provides feedback information about the treatment accuracy. We analyzed localization data for 17 treatments from 10 patients treated with SBRT during last 6 months. The localization accuracy as identified in step 3 is illustrated in the figure, where CBCT target is overlaid by the planning target contour. Its accuracy is visually judged within 1 mm. The figure also shows the residual errors of target localization based on CBCT after 2D radiographic corrections for each treatment. The intra-fraction movement assessed via 2D radiographs prior to and after treatment ranged from 0 to 4 mm with an average of 1 mm, indicating further improvement of patient immobilization may be needed. On the other hand, 3D imaging takes at least 3 times more working efforts compared to 2D imaging. The on-board imaging system provides a powerful tool to assess the overall accuracy of treatment. While CBCT is favorable for the identification of inter-fraction deviation, the intra-fraction errors could be effectively identified using 2D imaging method. The residual positioning errors identified by on-board imaging provide useful feedback information for improving patient treatment accuracy.