kV On-board Imaging Research Articles

Migration of intraprostatic fiducial markers and its influence on the matching quality in external beam radiation therapy for prostate cancer

Purpose To assess the influence of fiducial marker (FM) migration on the matching quality in external beam radiation therapy (EBRT) for prostate cancer. Materials and methods The position of FMs were identified using on-board kV imaging (OBI) and their 3-D position established using an in-house reconstruction algorithm for 31 patients with prostate adenocarcinoma. To carry out the match, the positions were overlaid on the digitally reconstructed radiographs (DRR) generated from the planning CT. The distance between each FM was calculated for seven treatments throughout the EBRT course. Four radiotherapy technologists were asked to independently perform and rate the match from OBI to DRR which was then correlated to the extent of FM migration. Results All the matches were rated by at least three radiotherapy technologists as “very easy” (“easy” subgroup) for 24 patients (77%), while the other seven patients had their match rated less than “very easy” and considered the “not easy” subgroup. The average daily FM migration was 0.93 ± 0.34 mm for the “easy” subgroup vs. 1.82 ± 0.75 mm for the latter. An average migration >2 mm was seen in five/seven patients in the “not easy” subgroup as compared to none in the “easy” subgroup. There was a trend towards less FM migration and better matching if the planning CT was done later than the day of the FM implant ( p = 0.093). Conclusions FM migration >2 mm predicts for a more difficult matching process; PTV margins might have to be adjusted or the planning CT repeated.

Megavoltage versus kilovoltage image guidance for efficiency and accuracy in head and neck IMRT

AbstractAccurate patient positioning is vitally important in intensity modulated radiation therapy (IMRT) for head and neck (H&N) cancer. The introduction of kilovoltage (kV) on-board imaging (OBI) at our centre was anticipated to improve the accuracy and efficiency of H&N IMRT patient position verification over traditional megavoltage (MV) electronic portal imaging (EPI). This study compares these imaging systems with a phantom accuracy study and retrospective analysis of imaging workload in H&N IMRT at our centre. Six therapists performed online evaluation of phantom images, and residual positional errors for each system were recorded. The largest residual error was 1 mm for OBI and 3 mm for EPI. The estimated improvement in residual error in OBI over EPI was 0.57 mm (95% confidence interval 0.33–0.81 mm), suggesting treatment staff would be better able to detect set-up deviations with OBI. Electronic treatment records of 20 H&N IMRT patients (10 verified daily with MV EPI and 10 with kV OBI) were analysed. Mean imaging session duration was 9.51 min for EPI and 9.76 min for OBI. Analysis found no evidence of an effect on duration due to the imaging system used for this subset of patients (p = 0.664).

Assessment of Patient Setup Error in Prostate Radiation Therapy using Fiducial-based Image Guided Radiation Therapy with kV Onboard Imaging and Conebeam CT

Assessment of Patient Setup Error in Prostate Radiation Therapy using Fiducial-based Image Guided Radiation Therapy with kV Onboard Imaging and Conebeam CT

Tradeoffs of integrating real-time tracking into IGRT for prostate cancer treatment

This study investigated the integration of the Calypso real-time tracking system, based on implanted ferromagnetic transponders and a detector array, into the current process for image-guided radiation treatment (IGRT) of prostate cancer at our institution. The current IGRT process includes magnetic resonance imaging (MRI) for prostate delineation, CT simulation for treatment planning, daily on-board kV and CBCT imaging for target alignment, and MRI/MRS for post-treatment assessment. This study assesses (1) magnetic-field-induced displacement and radio-frequency (RF)-induced heating of transponders during MRI at 1.5 T and 3 T, and (2) image artifacts caused by transponders and the detector array in phantom and patient cases with the different imaging systems. A tissue-equivalent phantom mimicking prostate tissue stiffness was constructed and implanted with three operational transponders prior to phantom solidification. The measurements show that the Calypso system is safe with all the imaging systems. Transponder position displacements due to the MR field are minimal (<1.0 mm) for both 1.5 T and 3 T MRI scanners, and the temperature variation due to MRI RF heating is <0.2 °C. The visibility of transponders and bony anatomy was not affected on the OBI kV and CT images. Image quality degradation caused by the detector antenna array is observed in the CBCT image. Image artifacts are most significant with the gradient echo sequence in the MR images, producing null signals surrounding the transponders with radii ∼1.5 cm and length ∼4 cm. Thus, Calypso transponders can preclude the use of MRI/MRS in post-treatment assessment. Modifications of the clinical flow are required to accommodate and minimize the substantial MRI artifacts induced by the Calypso transponders.

TH-C-303A-03: Real-Time Motion Detection of Prostate Target During Volumetric Arc Therapy Using Onboard Imaging Devices

Purpose: To develop a real-time prostate position monitoring technique for modern arc radiation therapy through novel usage of cine-MV imaging together with as-needed kV imaging. Methods and Materials: We divide the task of monitoring intrafraction prostate motion into two separate but related parts: (1) to detect potential target motion beyond a pre-defined threshold and (2) to confirm whether it is beyond the threshold and how large the displacement is. By taking advantage of the gantry rotation during arc therapy, MV images from different viewing angles were used to stereoscopically evaluate the motion status of the prostate/marker and to allow us to detect potential over-threshold displacement. If a potential over-threshold event is detected by MV images, the onboard kV imager is turned on to confirm this through combined data from MV-kV imaging. The position information acquired from combined MV-kV data can be used as input for further motion management techniques. A Varian Trilogy linac with onboard kV imager was used to examine selected typical trajectories using a 4D motion phantom capable of moving in accordance with a pre-programmed trajectory. Performance analysis of the proposed method has been done using 536 patient-measured trajectories from 17 patients. Results: Prostate displacement beyond a set threshold (3mm) was detected for over 99.8% of the time with better than 1mm detection accuracy. Compared to other fluoroscopy-based tracking techniques, kV usage is significantly reduced to on average of ∼15 times per arc delivery. Conclusions: Cine MV imaging during arc therapy provides a valuable tool for detecting non-negligible intrafraction prostate motion with as-needed low kV usage. Our motion detection method is distinguished from current motion monitoring techniques which seek to continuously and accurately localize a moving target. The proposed technique can be readily implemented with linacs equipped with EPID and onboard kV imaging devices.

SU-FF-T-550: Evaluation of Setup Accuracy of a Stereotactic Radiotherapy Head Immobilization Mask Using KV On-Board Imaging

Purpose: To evaluate setup accuracy of the BrainLAB stereotactic radiotherapy head immobilization mask using orthogonal projections from on‐board imaging.Materials and Methods: Eight patients were simulated and treated with the BrainLAB stereotactic head immobilization masks and CT‐localizers for brain lesions using single and hypo‐fractions. Orthogonal pairs of kV projections were acquired using Varian Trilogy's on‐board imager. The kV projections were then registered with digitally‐reconstructed‐radiographs (DRR) obtained from treatment planning. Shifts between the kV images and reference DRR's were calculated: anterior‐posterior (A‐P), left‐right (L‐R) and superior‐inferior (S‐I). If the shifts were > 2 mm in any direction, the patient was reset within the immobilization mask until satisfying accuracy has been achieved. Results: Shifts as large as 5 mm in the A‐P, L‐R and S‐I directions between kV projections and DRR's were found. These shifts represent offsets between the treatment and simulation setup using immobilization mask. The mean offsets of 0.1 mm, 0.7 mm, and −1.6 mm represent systematic errors in the A‐P, L‐R and S‐I directions, respectively, using the BrainLAB localizer. The mean of the radial shifts is about 1.7 mm. The standard deviations of shifts 2.2 mm, 2.0 mm, and 2.6 mm represent patient setup accuracy with the BrainLAB mask in A‐P, L‐R and S‐I directions. Conclusions: The BrainLAB mask provides less invasive immobilization system than the head ring. Relying on this system for patient setup might be associated with errors as large as 5 mm. Image‐guidance with the kV on‐board imager provides shifts that are required to setup patient prior to or during patient treatment. The patient may relax or move during treatment, and thus uncontrolled and undetected setup errors may be produced with patients that are not well immobilized. The combination of stereotactic immobilization and image guidance will achieve more controlled and accurate patient setup within 2 mm.

TU‐C‐303A‐02: Intrafraction Prostate Motion Monitoring with Cine‐MV and Minimal As‐Needed Onboard KV Imaging

Purpose: To examine the feasibility and performance of using treatment MV beam imaging with prior knowledge to estimate 3D prostate intrafraction motion and to instantaneously reposition based on information from minimal usage of on‐board kV imaging during IMRT. Methods and Materials: In contrast with current motion monitoring techniques which seek to accurately and continuously localize a moving target, we attempted for the first step only to detect potential motion beyond a pre‐defined threshold using MV images and in the second step through combined MV‐kV imaging (by turning on the kV imager) to confirm the over‐threshold event as well as obtaining accurate position information which could be used for instantaneously repositioning. EPID images were used to measure 2D prostate displacement in the imaging plane. By taking into account the strong correlation between prostate SI and AP motion, small displacement in the LR direction, and data from a previous IMRT session (different gantry angle), we estimated in‐line prostate motion as well which in turn allowed us to detect potential 3D over‐threshold motion. To minimize the influence from training data, no statistical motion probability distribution information was used. Simulation has been done using 536 patient‐measured trajectories from 17 patients. Experiments were performed on a Varian Trilogy linac using a motion phantom programmed for selected typical trajectories, and the results were compared with simulations. Results: Prostate displacement beyond a set threshold (3mm) was detected for over 99% of the time at the cost of negligible kV dose (&lt; 3 images/fraction on average). The position information required for repositioning was found to have sub‐millimeter accuracy using combined MV‐kV data. Conclusion: Significant reduction of adverse effects of intrafraction prostate motion is achievable. The technique can be readily implemented in clinics and incurs minimal imaging dose to the patient as compared with other stereoscopic imaging techniques.

ASSESSMENT OF A RADIOTHERAPY PATIENT CRANIAL IMMOBILIZATION DEVICE USING DAILY ON‐BOARD KILOVOLTAGE IMAGING

The purpose of this study was to utilize state-of-the-art on-board digital kilovoltage (kV) imaging to determine the systematic and random set-up errors of an immobilization device designed for canine and feline cranial radiotherapy treatments. The immobilization device is comprised of a custom made support bridge, bite block, vacuum-based foam mold and a modified thermoplastic mask attached to a commercially available head rest designed for human radiotherapy treatments. The immobilization device was indexed to a Varian exact couch-top designed for image guided radiation therapy (IGRT). Daily orthogonal kV images were compared to Eclipse treatment planning digitally reconstructed radiographs (DRRs). The orthogonal kV images and DRRs were directly compared online utilizing the Varian on-board imaging (OBI) system with set-up corrections mmediately and remotely transferred to the treatment couch prior to treatment delivery. Off-line review of 124 patient treatments indicates systematic errors consisting of +0.18 mm vertical, +0.39mm longitudinal and -0.08 mm lateral. The random errors corresponding to 2 standard deviations (95% CI) consist of 4.02 mm vertical, 2.97 mm longitudinal and 2.53 mm lateral and represent conservative CTV to PTV margins if kV OBI is not available. Use of daily kV OBI along with the cranial immobilization device permits reduction of the CTV to PTV margins to approximately 2.0 mm.

Use of MV and kV imager correlation for maintaining continuous real-time 3D internal marker tracking during beam interruptions

The integration of onboard kV imaging together with a MV electronic portal imaging device (EPID) on linear accelerators (LINAC) can provide an easy to implement real-time 3D organ position monitoring solution for treatment delivery. Currently, real-time MV–kV tracking has only been demonstrated by simultaneous imagining by both MV and kV imaging devices. However, modalities such as step-and-shoot IMRT (SS-IMRT), which inherently contain MV beam interruptions, can lead to loss of target information necessary for 3D localization. Additionally, continuous kV imaging throughout the treatment delivery can lead to high levels of imaging dose to the patient. This work demonstrates for the first time how full 3D target tracking can be maintained even in the presence of such beam interruption, or MV/kV beam interleave, by use of a relatively simple correlation model together with MV–kV tracking. A moving correlation model was constructed using both present and prior positions of the marker in the available MV or kV image to compute the position of the marker on the interrupted imager. A commercially available radiotherapy system, equipped with both MV and kV imaging devices, was used to deliver typical SS-IMRT lung treatment plans to a 4D phantom containing internally embedded metallic markers. To simulate actual lung tumor motion, previous recorded 4D lung patient motion data were used. Lung tumor motion data of five separate patients were inputted into the 4D phantom, and typical SS-IMRT lung plans were delivered to simulate actual clinical deliveries. Application of the correlation model to SS-IMRT lung treatment deliveries was found to be an effective solution for maintaining continuous 3D tracking during ‘step’ beam interruptions. For deliveries involving five or more gantry angles with 50 or more fields per plan, the positional errors were found to have ⩽1 mm root mean squared error (RMSE) in all three spatial directions. In addition to increasing the robustness of MV–kV tracking against beam interruption, it was also found that use of correlation can be an effective way of lowering kV dose to the patient and for increasing kV image quality by reduction of MV scatter interference.

Real-time 3D internal marker tracking during arc radiotherapy by the use of combined MV–kV imaging

To minimize the adverse dosimetric effect caused by tumor motion, it is desirable to have real-time knowledge of the tumor position throughout the beam delivery process. A promising technique to realize the real-time image guided scheme in external beam radiation therapy is through the combined use of MV and onboard kV beam imaging. The success of this MV–kV triangulation approach for fixed-gantry radiation therapy has been demonstrated. With the increasing acceptance of modern arc radiotherapy in the clinics, a timely and clinically important question is whether the image guidance strategy can be extended to arc therapy to provide the urgently needed real-time tumor motion information. While conceptually feasible, there are a number of theoretical and practical issues specific to the arc delivery that need to be resolved before clinical implementation. The purpose of this work is to establish a robust procedure of system calibration for combined MV and kV imaging for internal marker tracking during arc delivery and to demonstrate the feasibility and accuracy of the technique. A commercially available LINAC equipped with an onboard kV imager and electronic portal imaging device (EPID) was used for the study. A custom built phantom with multiple ball bearings was used to calibrate the stereoscopic MV–kV imaging system to provide the transformation parameters from imaging pixels to 3D world coordinates. The accuracy of the fiducial tracking system was examined using a 4D motion phantom capable of moving in accordance with a pre-programmed trajectory. Overall, spatial accuracy of MV–kV fiducial tracking during the arc delivery process for normal adult breathing amplitude and period was found to be better than 1 mm. For fast motion, the results depended on the imaging frame rates. The RMS error ranged from ∼0.5 mm for the normal adult breathing pattern to ∼1.5 mm for more extreme cases with a low imaging frame rate of 3.4 Hz. In general, highly accurate real-time tracking of implanted markers using hybrid MV–kV imaging is achievable and the technique should be useful to improve the beam targeting accuracy of arc therapy.

Analysis of image quality for real‐time target tracking using simultaneous kV‐MV imaging

Real-time tracking can provide high accuracy localization for a moving target and minimize the effect of motion. Simultaneous kV-MV imaging has been proposed as a real-time tracking technique by utilizing the existing kV on-board imager (OBI) and the MV electronic portal device (EPID) mounted on the linear accelerator. The orthogonal pair of kV-MV images acquired simultaneously can provide 3-D localization in real-time. However, the kV and MV beams cross shooting the target interfere with each other with beam scattering, which affects the quality of images. The success of this modality heavily relies on the image quality, especially the visibility of the target, which was investigated in this study. The kV and MV images were acquired for a gold implant marker that was used as a surrogate of the target and placed in an IMRT thorax phantom, a dynamic phantom, and a pelvis phantom to test the image quality in different situations. Contrast-to-noise ration (CNR) was used to quantitatively describe the visibility of the target in the image. CNR can be obtained by statistical calculation from image processing and physics analysis with ion chamber measurement. The difference is described by contrast detection efficiency (CDE). By comparing the ratio (R) of CNR with and without the MV beam on, the MV beam scatter was found to have dramatically reduced the target visibility in the kV images (R=0.47), which was supported by an independent physics analysis that treats beam scatter as a noise. In contrast, the kV scatter effect on the MV images was minor (R=0.93). The effect of tumor motion was visible but tolerable for the target tracking purpose. CNR varied with different tumor sites and was lower for the pelvis than the thorax. Different kV imaging parameters such as kVp, mAs, and exposure time ms were tested for different cases. Considering a threshold of 1.0 CNR as a measure for the target visibility, a range of CNR from 1.3 to 4.2 was reached with appropriate tuning of those imaging parameters. This study has shown that CNR is a key parameter that can be used for assessing the visibility of the target in digital imaging and the quality of kV/MV images. It has also been shown that reasonable target visibility can be obtained using simultaneous kV-MV imaging for real-time target tracking.

Four Clinical Issues of Tracking Multiple Moving Fiducials in Real-time based On Simultaneous Onboard kV and Treatment MV Imaging

Four Clinical Issues of Tracking Multiple Moving Fiducials in Real-time based On Simultaneous Onboard kV and Treatment MV Imaging

TH‐C‐AUD C‐06: Nearly Real‐Time Tumor‐Position Monitoring During Arc Therapy with Combined MV and KV Imaging

Purpose: To examine the feasibility and accuracy of using treatment MV beam and on‐board kV imaging for monitoring of the positions of implanted fiducials during arc therapy. Method and Materials: A Varian Trilogy LINAC with onboard kV imager was used for the study. A phantom with 13 ball bearings at known locations was used to calibrate the hybrid MV/kV imaging system to determine the spatial transformation matrix from the pixel coordinates to the radiation‐source‐centered coordinates. The feasibility and accuracy of the fiducial tracking system was examined using a 4D motion phantom capable of moving in accordance with a pre‐programmed trajectory. During an arc delivery, MV images acquired by the EPID and cine kV images were obtained simultaneously using a two‐channel frame‐grabber for real‐time analysis. A fast fiducial detection algorithm was developed to extract fiducial coordinates. Tracking results are compared with the preset trajectories. The accuracy of the tracking system was evaluated for a number of fiducial motion trajectories and for a variety of kV beam sampling rates ranging 15fps to 0.5fps. Results: The studies showed that it is feasible to use treatment MV beam and the orthogonal kV beam to monitor the fiducial motion in nearly real‐time during arc therapy. A time delay of ∼150ms was observed, which was caused by the imaging electronics and data analysis. This delay does not pose any significant error in fiducial tracking for the motion speed commonly seen in the clinics and can be compensated by a motion prediction algorithm when needed. Overall, the spatial accuracy was found to be better than 1mm. Conclusion: Nearly real‐time monitoring of implanted markers using hybrid MV/kV imaging during arc treatments is achievable. The system requires no hardware modification and delivers much less dose to the patient as compared with the conventional stereoscopic imaging technique.

WE-C-351-06: Tracking Multiple Moving Fiducials During Treatment Based On Simultaneous Onboard KV and Treatment MV Imaging

Purpose: Intra‐fractional organ motion can limit the advantage of highly conformal dose techniques as IMRT due to target position uncertainty. A new strategy has been proposed and investigated to track implanted fiducial markers during treatment on a LINAC with an onboard kV imaging system by using simultaneous kV and treatment MV beam imaging.Method and Materials: A pelvic phantom implanted with three gold cylindrical seeds was tested on a moving platform on a Varian Trilogy. Two phantom verification plans for 3DCRT and IMRTtreatments from real patients were delivered on this phantom while the platform was moving at different speeds. A new pattern matching algorithm has been developed to locate multiple cylindrical fiducials on MV and kV images. Their 3D positions were then calculated from simultaneous orthogonal MV and kV imaging results. Results: Four issues were studied towards the clinical application of this method. 1) Multiple methods were adopted to predict marker positions and reduce search regions to achieve an analysis speed of 10 frame/second. 2) The kV and MV images were acquired while the phantom was moving at various speeds and fiducials could be successfully detected at a linear speed of 1.6 cm/s or less. 3) Varying MV field size and kV source to imager distance indicated that the detection of fiducials on kV images was not affected by the scattering from simultaneous treatment MV beams. 4) Prediction based on relative positions of fiducials was used to closely locate positions of fiducials blocked by a multileaf collimator.Conclusion: These four studies pave the way to automatically track nearly real‐time 3D positions of multiple moving fiducial markers during treatment based on simultaneous kV and MV imaging. It does not require extra hardware such as stereo kV imaging systems and utilizes the treatment beam to reduce dose.

SU‐GG‐I‐22: Measurement and Correction of Sagging Shifts in KV Cone‐Beam CT From An On‐Board Imaging System

Purpose: To evaluate the effects of sagging on mechanical isocenter accuracy and image quality of kV cone‐beam CT (CBCT) and to develop a pre‐processing algorithm for removing these effects from on‐board‐imaging (OBI). Method and Materials: OBI sagging shifts were measured by scanning a cubical phantom with a 2 mm metal marker. The metal marker was aligned with radiation isocenter and its position was tracked in cone‐beam projections using normalized cross‐correlation image registration algorithm of kV CBCT system. The position of the marker was determined by 650 projections of a CBCT scan over an angular range from 0°–360°. To remove sagging shifts, projections were corrected to reconstruct CBCT images using Feldkamp back projection. The marker center of the cubical phantom in the corrected CBCT reconstructed images was used to determine OBI isocenter. The CATPHAN from phantom lab was used to compare image quality parameters such as blurring, uniformity and linearity for CBCT images before and after OBI sagging shift corrections. Results: We found that sagging affects localization accuracy of the OBI isocenter and image quality in CBCT. The marker position shifts about ± 2 mm over 360° range due to sagging and this produces about 1.4 mm isocenter shift in CBCT reconstructed from projections before correction for sagging. CBCT numbers vary by about 1% in full‐fan scans and up to 3.5% in half‐fan scans. Blurring and spatial distortion artifacts are produced in CBCT before correction of OBI sagging shifts. Conclusion: Sagging of the kV OBI system produces a systematic isocenter shift of CBCT. Further, sagging shifts lead to image artifacts such as spatial distortion, blurring, degradation of soft tissue contrast, position resolution, variation in CBCT number linearity and uniformity. Correction of OBI sagging in cone‐beam projections prior to CBCT reconstruction provides more accurate isocenter localization and improves image quality.

Fast internal marker tracking algorithm for onboard MV and kV imaging systems.

Intrafraction organ motion can limit the advantage of highly conformal dose techniques such as intensity modulated radiation therapy (IMRT) due to target position uncertainty. To ensure high accuracy in beam targeting, real-time knowledge of the target location is highly desired throughout the beam delivery process. This knowledge can be gained through imaging of internally implanted radio-opaque markers with fluoroscopic or electronic portal imaging devices (EPID). In the case of MV based images, marker detection can be problematic due to the significantly lower contrast between different materials in comparison to their kV-based counterparts. This work presents a fully automated algorithm capable of detecting implanted metallic markers in both kV and MV images with high consistency. Using prior CT information, the algorithm predefines the volumetric search space without manual region-of-interest (ROI) selection by the user. Depending on the template selected, both spherical and cylindrical markers can be detected. Multiple markers can be simultaneously tracked without indexing confusion. Phantom studies show detection success rates of 100% for both kV and MV image data. In addition, application of the algorithm to real patient image data results in successful detection of all implanted markers for MV images. Near real-time operational speeds of approximately 10 frames/sec for the detection of five markers in a 1024 x 768 image are accomplished using an ordinary PC workstation.

Fiducial-Based Translational Localization Accuracy of Electromagnetic Tracking System and On-Board Kilovoltage Imaging System

The Calypso medical four-dimensional localization system uses AC electromagnetics, which do not require ionizing radiation, for accurate, real-time tumor tracking. This investigation compared the static and dynamic tracking accuracy of this system to that of an on-board imaging kilovoltage X-ray system for concurrent use of the two systems. The localization accuracies of a kilovoltage imaging system and a continuous electromagnetic tracking system were compared. Using an in-house developed four-dimensional stage, quality-assurance fixture containing three radiofrequency transponders was positioned at a series of static locations and then moved through the ellipsoidal and nonuniform continuous paths. The transponder positions were tracked concurrently by the Calypso system. For static localization, the transponders were localized using portal images and digitally reconstructed radiographs by commercial matching software. For dynamic localization, the transponders were fluoroscopically imaged, and their positions were determined retrospectively using custom-written image processing programs. The localization data sets were synchronized with and compared to the known quality assurance fixture positions. The experiment was repeated to retrospectively track three transponders implanted in a canine lung. The root mean square error of the on-board imaging and Calypso systems was 0.1 cm and 0.0 cm, respectively, for static localization, 0.22 mm and 0.33 mm for dynamic phantom positioning, and 0.42 mm for the canine study. The results showed that both localization systems provide submillimeter accuracy. The Calypso and on-board imaging tracking systems offer distinct sets of advantages and, given their compatibility, patients could benefit from the complementary nature of the two systems when used concurrently.

Effect of MV Scatter on kV Image Quality During Simultaneous kV-MV Imaging

Interactive tracking the tumor motion during treatments for accurate localization and dynamic targeting is one of the goals of imaging-guided radiation therapy (IGRT), but difficult to achieve. Simultaneous imaging with the kV on-board imaging and MV portal imager has the potential to achieve this goal. This work attempts to study the effect of MV scatter on kV image quality during simultaneous kV-MV imaging.

Daily Bone Alignment With Limited Repeat CT Correction Rivals Daily Ultrasound Alignment for Prostate Radiotherapy

Daily ultrasound (US) alignment is commonly used to correct for internal prostate displacements. However, the accuracy of US is user-dependent. In a previous study, we found that daily CT-guided alignment, via on-board kV imaging or in-room CT, is a more accurate option but could be time-consuming. In this study, we investigated a third alignment solution: daily bone alignment plus a correction factor for the systematic internal prostate displacement (CFID) as described by de Boer et al. [de Boer 2005]. Ten prostate patients were CT-scanned 3 times per week prior to radiotherapy using an integrated CT-LINAC system. They were aligned using US for their daily treatments. An IMRT plan was designed on the planning CT image with current clinical margins (8 mm anterior and lateral, 7 mm superior/inferior, 5 mm posterior). The treatment plan was copied onto the repeat CT images and aligned using several daily alignment schemes: (1) bone alignment plus CFID after 3 offline CT scans (bone+3CT), (2) bone alignment plus CFID after 6 offline CT scans (bone+6CT), (3) bone alignment, (4) US alignment, and (5) CT alignment. The accuracy of repeat US and CT measurements to determine the CFID was compared. The dosimetric impact on target coverage for each scenario was reported. Statistical significance was calculated with the Wilcoxon Matched Pairs Signed Ranks test. The CFID for internal systematic prostate displacements was more accurately measured with repeat CT than with repeat US (residual error: 0.0 ± 0.7 mm and 2.0 ± 3.2 mm respectively); therefore the CFID from repeat CT scans were used. Average prostate minimum doses (0.1 cm3) from the bone+3CT and bone+6CT alignments were equivalent to each other and to the prostate minimum doses from daily US alignment (Table). The alignment precision (1StDev in daily prostate minimum dose) was better with bone+3CT and bone+6CT than with daily US alignment (Table). Daily bone alignment plus CFID for internal systematic prostate displacements provided better daily dose precision and equivalent prostate dose coverage when compared to daily US alignment. The CFID should be based on at least 3 repeat CT scans, which could be collected before the start of treatment or during the first three treatment days. Daily bone alignment plus CFID provides another option for accurate prostate patient positioning.

Comparing Optical Localization to kV On-Board Imaging During Image-Guided Stereotaxy

Our intent is to determine the accuracy of on-board kV imaging (kVi) by comparing its suggested patient positioning to that obtained using an infrared optical camera (SOC), having an established accuracy of <0.6 mm and 0.1°.