Index of content:
Volume 35, Issue 6, June 2008
- Joint Imaging/Therapy Moderated Poster Session: Exhibit Hall E
- Moderated Poster — Area 3 (Joint): Advances in Image Guided Radiotherapy
35(2008); http://dx.doi.org/10.1118/1.2961386View Description Hide Description
Purpose: The ideal IGRT‐based delivery to manage intrafraction motion has two requirements: complete spatio‐temporal knowledge of the anatomy, and real‐time beam adaptation corresponding to motion‐induced anatomical changes. Toward this goal, we investigate an integrated strategy combining two powerful techniques — online image‐guidance using fast cine‐MR imaging (4D‐MRI) and real‐time, locally adaptive delivery using dynamic multileaf collimator (DMLC)‐based tracking. Method and Materials:ImageSNR and, therefore, field strength (Bo) requirements for an integrated MRI+Linac for the task of radiotherapy guidance were investigated. Using multisection, multiphase steady‐state free precession (SSFP) and spoiled gradient recall (SPGR) sequences, 3D volumes (1.4s/volume) and 2D coronal slices (0.5s/slice) of a volunteer's thoracic region were acquired with a 1.5T MRI. For each set, a region of interest encompassing the diaphragm was segmented and trajectories of superior‐inferior and left‐right motion were computed for voxels within. In order to simulate real‐time imaging with lower field strength MRI+Linacs, the SNR for each set was progressively degraded and corresponding motion trajectories recalculated. 4D locally‐adaptive IMRT was implemented using a DMLC tracking algorithm which adapts the beam aperture(s) in real‐time using 3D position information from an independent monitoring system. The aforementioned trajectories were programmed into a high‐resolution 3D‐programmable motion platform and geometric accuracy of DMLC tracking was measured. Results: The SSFP‐acquired 2D and 3D images yielded adequate SNR for registration, while the SPGR sequence yielded faster acquisition but poorer SNR. For SSFP images, even with a factor‐of‐six SNR reduction (corresponding to Bo ∼0.2T), no significant changes were observed in estimated motion trajectories. Finally, sub‐millimeter tracking accuracy was observed for these traces for simultaneous target motion in the S‐I and left‐right directions. Conclusion: These initial studies provide valuable insights into design requirements of integrated MRI+Linac systems and indicate that 4D‐MRI combined with DMLC tracking represents a highly promising approach for intrafraction motion management.
SU‐EE‐A3‐02: Image‐Guided Planning Margin Determination for Concomitant Boost Treatment of 3D‐Conformal, IMRT, and Stereotactic Lung Cancer Radiotherapy35(2008); http://dx.doi.org/10.1118/1.2961387View Description Hide Description
Purpose: To determine appropriate planning margins around the GTV for concomitant boost image‐guidedlungcancer treatments where the GTV is prescribed a higher dose than the CTV. Method and Materials: The dosimetric impact of the GTV planning margin (pGTV) margin was investigated with 3D isotropic expansions to the GTV, and simulations of interfractional target displacements relative to bone. Seven patients (IMRT and 3D‐conformal stereotactic) were included. GTVs and their respiratory motion envelopes were contoured from 4DCT images. All patients were aligned with daily projection imaging(IMRT) or CT(SBRT); perfect bony anatomy alignment was assumed. Average residual uncertainties (tumor versus bone) were previously determined from analysis of CBCTimages. A 3D displacement probability map was calculated (0.0+/−3.4mmAP, 0.8+/−3.4mmSI, and 0.5+/−2.7mmRL), assuming setup errors were normally distributed. To determine the worst‐case dose‐coverage, pGTVs were created by expanding the GTV by 6mm (95.5% probability). Cumulative dose distributions were calculated based on the 3D displacement probability map. The assumption of normally distributed displacements was not appropriate for hypo‐fractionated patients; therefore a worst‐case systematic displacement scenario was evaluated for one patient. Results: The 4mm pGTV demonstrated good dose coverage (> 90%). With the current planning techniques, the worst‐case analysis of GTV coverage showed that usually <=5% GTV would receive <95% Rx dose. Differences between the planned and cumulative dose distributions were minimal. For the hypofractionated Patient, the probabilities that the GTV will receive at least 98%, 96%, 94%, and 92% of the prescribed dose are 42%, 78%, 93%, and 95% respectively. Conclusion: Prescribing 90%–100% of the prescription dose to the GTV + 4mm planning margin should result in sufficient dosimetric coverage of the GTV for both IMRT and hypo‐fractionated stereotactic patients. When prescribing concomitant boosts a pGTV margin is required, but this need not be as great as the PTV margin around the CTV.
35(2008); http://dx.doi.org/10.1118/1.2961388View Description Hide Description
Purpose: In prostate cancerradiotherapy, it is unknown how large the PTV margins must be to account for the isocenter correction tolerance and intrafraction motion. The risk of geographic miss can be minimized by the placement of fiducial markers in the prostate gland for daily pretreatment localization and adjustment of patient position if necessary. In this study, we assess the magnitude of interfraction and intrafraction isocenter displacement using implanted electromangnetic transponders, and validate the accuracy of interfraction localization using cone beam CT.Method and Materials: Fifteen supine prostate IMRT patients with three implanted transponders each were studied. Initial daily localization was based on three laser and skin marks. Daily localization error distribution was determined from offsets between the initial setup position and that determined by Calypso. Post setup with the Calypso system, isocenter localization was immediately independently verified by imaging the radio‐opaque transponders using an integrated cone beam CTimaging system. Both localization techniques produced lateral, longitudinal, and vertical target offsets from machine isocenter. Organ motion or patient movement during treatment was continuously monitored by the Calypso system at a 4‐mm threshold. Results: The mean interfraction displacement (± SD) in cm in the lateral, vertical, and longitudinal directions were −0.2 ± 0.6, 1.8 ± 1.3, and 0.3 ± 0.9, respectively. After any necessary isocenter corrections, the mean isocenter placement error relative to the cone beam CT (± SD) in cm in the lateral, vertical, and longitudinal directions were 0.0 ± 0.1, 0.1 ± 0.2, and 0.0 ± 0.1, respectively. Conclusion: Compared with use of skin marks, electromagnetic isocenter repositioning provides an increased degree of isocenter localization. Good agreement was observed between cone beam CT isocenter localization and electromagnetic repositioning. However, the electromagnetic technique, with real time continuous tracking, has the added advantage of threshold‐based intervention with no additional radiation dose.
35(2008); http://dx.doi.org/10.1118/1.2961389View Description Hide Description
Purpose: Daily image‐guided setup based on soft tissue target may improve target coverage. However, when significant anatomical changes occur, a simple isocenter shift may not be adequate. In this study, we propose an online IMRT replanning strategy for prostate cancer which uses the deformed dose distributions from the original treatment plan as its objective. Method and Materials: The replanning procedure used was as follows: (1) the dose distribution on the planning CT was deformed to the daily CT and used as the reference objective dose distribution for IMRT replanning; (2) the prescription isodose line on the reference dose distribution was auto‐segmented and used as the fictitious “target volume” to set the initial MLC leaf positions; (3) we developed and implemented a voxel‐by‐voxel dose‐based cost function. The IMRT treatment plan was optimized using the direct machine parameter optimization algorithm to achieve the following goals: (a) inside the region enclosed by the original prescription dose line, the replanned dose distribution was optimized to match with the reference objective; (b) outside this region, the objective function was chosen to lower the dose value and to penalize the dose value exceeding the reference dose for each voxel. The replanning process does not need re‐auto‐segmentation of patient's anatomy, although re‐segmented anatomic contours were used to evaluate the effectiveness of this approach. Results: We compared the dose distributions and the DVHs of the original plan and the daily plans using (1) image‐guided setup based on prostate alignment, (2) deformed dose distribution from the original plan, and (3) our re‐planning method. We found that our re‐planning strategy matched well with the original plan. Conclusion: The replanning strategy using the original dose distribution as the goal for optimization produces dose distributions similar to the original approved plan and is an effective approach for on‐line CT‐guided adaptive radiotherapy.
35(2008); http://dx.doi.org/10.1118/1.2961390View Description Hide Description
Purpose: To develop, evaluate and optimize a protocol for acquiring on‐board cone‐beam CT(CBCT)images for larger longitudinal coverage than the maximum 14 cm coverage allowed by the Varian OBI half‐fan single‐orbit mode. Extended coverage is needed to fully image many standard pelvic, thoracic, and head‐and‐neck treatment volumes and to support such tasks as deformable image registration and dosereconstruction on serial CBCTimages.Method and Materials: Multiple Varian single‐orbit data acquisitions separated by couch shifts were performed with small overlaps to provide the desired coverage. Simple approaches to aligning the two image volumes based on the nominal couch shift were validated against rigid image registration based upon orthogonal radiographic projections. An automatic algorithm was devised to decode the DICOM headers of the original slices and get corresponding position information, subsequently join the volumes, and finally rewrite the headers so that the resultant CBCTimage set will be correlated as a single DICOM image volume. Validation of the volume reconstruction was performed with AP/PA radiographs and CatPhan QA phantom to evaluate the combining accuracy and the image quality in the abutment region where data sampling violates Tuy's data sufficiency condition. Results: The above protocol successfully provides OBI CBCTimages with any desirable longitudinal coverage. An optimized protocol is implemented clinically, based upon the dose consequences and application simplicity. Conclusion: A practical method was developed to effectively extend the OBI longitudinal coverage, which improved the applicability of the OBI CBCTimages in image‐guided adaptive radiation therapy.
This work was supported by NIH P01 CA116602.
35(2008); http://dx.doi.org/10.1118/1.2961391View Description Hide Description
Purpose: Reporting preliminary evaluation results of an intensity weighted region of interest (IWROI) imaging technique that utilizes recent developments in cone‐beam CT(CBCT)reconstruction theory to reduce patient exposure and detected scattered radiation. Method and Materials: Patient dose can be reduced by decreasing the x‐ray source fluence, however this comes with the cost of a decreased signal‐to‐noise‐ratio in the resulting images. IWROI imaging introduces filters into the x‐ray beam such that the central ROI receives the full beam intensity, and thus maintains SNR level while the periphery of the field‐of‐view (FOV) is illuminated by a reduced intensity, filtered beam. Reconstruction is done with the recently developed chord‐based BPF algorithm, which has been shown to be robust against some forms of truncation and to have favorable noise propagation properties. This algorithm enables IWROI to be used more flexibly and yields higher image quality. Experimental studies were carried out by constructing aluminum and copper filters which could be attached directly to the CBCT source. Scans were taken with 125 kVp, 80 mA and 15 ms pulse length exposure setting. Dose measurements were made using LiF TLDs 10cm deep in a 20cm stack of 30cm×30cm solid water slabs. Results: For the 3mm copper filter case the measurements showed a 7% dose reduction for the central ROI and a 37% reduction for the low intensity region of the image. The coefficient of variation for the full intensity ROI after BPF reconstruction was 0.053 and 0.100 in the filtered region indicating relative noise levels. Conclusion: IWROI imaging can reduce radiation exposure to sensitive regions of the anatomy while still producing high image quality in the region of interest. The filtered region also contains enough information for comparison of the gross anatomy with the planning scan. Conflict of Interest: Work supported by Varian Medical Systems, Inc.
- Moderated Poster — Area 3 (Joint): Intrafraction Motion
35(2008); http://dx.doi.org/10.1118/1.2961362View Description Hide Description
Purpose: To assess the feasibility of on‐line kV image‐guidance and fluoroscopic monitoring during arc therapy. Method and Materials: A pelvis phantom with anthropomorphic prostate and seminal vesicles was implanted with 3 gold fiducial markers. CBCTimaging was performed before and during delivery of a prostate treatment plan using volumetric modulated arc therapy (VMAT). Both CBCTimages were assessed for image quality based on noise. The gold fiducials were segmented in the guidance CBCT volume and subsequently localized in each projection image taken during the VMAT delivery. Since the phantom is rigid and was not moved between images, no movement of fiducials in each projection image is expected. Any movement was defined as tracking error. A clinical action level of 3 mm was used to designate if the tracking error was significant. Results:CBCTimage noise (1 S.D.) increased from 10.4 HU to 20.6 HU due to increased scatter onto the kV detector as well as interference patterns induced by the MV beam pulses. The frame‐by‐frame tracking error below a 3mm clinical action level was 96.6% for the CBCT acquired for setup correction before the VMAT delivery. This value decreased to 88.7% for the CBCT volume captured concurrently with the VMAT delivery.Conclusion: Arc therapy promises to deliver highly conformal dose distributions quickly and efficiently. Simultaneous kV imaging allows for the capture of a residual image as well as an opportunity for fluoroscopic monitoring of the patient's position during treatment. CBCTimage quality is degraded during VMAT delivery but image quality is still suitable for assessment of residual setup error as well as intra‐fraction motion.
Research sponsored by Elekta Synergy Research Group.
35(2008); http://dx.doi.org/10.1118/1.2961363View Description Hide Description
Purpose: In the presence of organ motion, geometric target uncertainty can hamper the benefits of highly conformal dose techniques such as IMRT. A critical step in dealing with intra‐fraction tumor motion is the real‐time monitoring of the tumor position. The aim of this study is the first time demonstration of a real‐time 3D internal fiducial tracking system based on onboard kV diagnostic imaging together with a MV electronic portal‐imaging device (EPID).Method and Materials: A Varian Trilogy radiotherapy system equipped with both kV and MV imaging systems was used in this work. A hardware frame grabber was used to capture both kV and MV video streams simultaneously at 30 fps. An in house built support vector machine (SVM) classifier tool using prior CT based knowledge was used to locate gold cylindrical markers in the kV/MV frames. Controlled kV beam switching, synced with the ‘step’ part of a step‐and‐shoot IMRT delivery, was investigated in allowing continuous 3D tracking in the presence of beam interruption. A correlation/prediction algorithm was used to buffer lost geometric marker information during kV/MV synchronization. The geometric tracking capabilities of the system were evaluated using a pelvic phantom with embedded fiducials placed on a 3D moveable stage. Results: The maximum 3D tracking speed of the kV‐MV system is approximately 9 Hz. The geometric accuracy of the system is found to be on the order of less than 1 mm in all three spatial dimensions. Synchronized kV/MV switching is found to reduce MV scatter interference on kV imaging and to reduce the overall kV diagnostic dose needed for continuous tracking. Conclusion: A real‐time 3D fiducial tracking system using combined kV and MV imaging has been successfully demonstrated for the first time. The technique is especially suitable for RT systems already equipped with on board kV and EPIDimaging devices.
SU‐DD‐A3‐03: Using Treatment Beam Imaging to Monitor Prostate Motion In Near Real‐Time On a Conventional LINAC35(2008); http://dx.doi.org/10.1118/1.2961364View Description Hide Description
Purpose: To directly use MV treatment beam imaging to monitor tumor motion represents an ideal realm for IGRT because it does not require additional hardware and delivers no extra radiation dose to patients. Here we report a method of obtaining the positions of implanted fiducials in near real‐time from cine EPIDimaging of prostate IMRT beams. Method and Materials: A Clinac 21EX is used for the study. During IMRT beam delivery, EPIDimaging of the treatment MV beam is acquired in cine mode. The framed images are captured and transferred to a PC, where a fully automated marker detection algorithm is developed to extract the coordinates of the implanted fiducials on the images of IMRT apertures that include the fiducials. The automated algorithm matches cross sections of cylindrical fiducials and evaluates multiple criteria to analyze the MV images. From an EPIDimage, only the coordinates of a fiducial on the plane perpendicular to the beam can be derived. The position of the marker in the direction parallel to the beam is then estimated from the pre‐treatment portal images and previous treatment beam(s) at different direction(s). This novel MV image guidance procedure is evaluated by using motion phantom experiments and five prostate patients with three implanted gold fiducials. Results: The phantom study suggests that the system is capable of detecting any motion greater than 1.2 mm. The variation of all fiducials from their planned positions was calculated on every image and the maximum variation was 4 mm with a standard deviation of 1.8 mm. The 3D positions of fiducials were calculated. The standard deviations of fiducial 3D position distributions were found to be 2.2 mm or less. Conclusion: This fully automated method provides prostate position during treatment without extra costs.
SU‐DD‐A3‐04: Analysis of Prostate Patient Setup Error and Organ Motion Error Using Calypso Setup Shift and Tracking Data35(2008); http://dx.doi.org/10.1118/1.2961365View Description Hide Description
Purpose: To evaluate the prostate patient setup and intra‐fraction organ motion error distributions by analyzing initial setup shift and target tracking data obtained from the Calypso system. Method and Materials: The spatial coordinates of the centroid of three implanted markers implanted in the prostates of 13 patients were monitored by our Calypso system with 10Hz sampling frequency. Patients were initially setup to tattoos and then shifted to the Calypso localized locations. Measurements were performed for average 32 fractions of average time about 8 minutes each. For each patient, the systematic setup correction and standard‐deviation (SD) over all fractions was obtained for the left‐right (LR), anterior‐posterior (AP) and superior‐inferior (SI) axes. Systematic setup error was obtained as the SD of the systematic setup errors of all patients. Similarly, the systematic and random components of prostate intra‐fractional motion were obtained from the tracking data. For all patients, correlation coefficients were calculated among the three axes of tracking data; the cumulative probability of total 3D displacement from initial setup was also calculated. Results: The systematic and random components of the initial setup errors for LR, AP and SI axes are 2.4mm, 2.6mm and 3.2mm, respectively and 3.6mm, 2.7mm and 3.3mm. The corresponding systematic and random components of prostate intra‐fractional motion errors are 0.3mm, 0.5mm and 0.6mm, and 0.6mm, 1.1mm and 1.0mm, respectively. The correlation between AP and SI motion was significant. The 3D displacement data indicates that 11 of 13 patients have 94.4% or higher probability of prostate motion within 5mm from the Calypso setup positions. However, one patient showed 28.7% probability of motion beyond 5mm. Conclusion: In our patient population, residual systematic and random error due to intra‐fraction prostate motion is small, once inter‐fraction setup‐organ motion errors have been eliminated by Calypso‐guided online setup.
Supported in part by grant P01 CA 116162.
SU‐DD‐A3‐05: Experimental Investigation of a Monoscopic Real‐Time Tumor Tracking Method Combining Occasional X‐Ray Imaging and Continuous External Respiratory Monitoring35(2008); http://dx.doi.org/10.1118/1.2961366View Description Hide Description
Purpose: To demonstrate the feasibility of a monoscopic method for real‐time tumour tracking, combining occasional x‐ray imaging and continuous external respiratory monitoring, which incorporates two correlations; (1) the correlation between the two projected components of 3D tumour positions on the imager plane (xp , yp ) and the external respiratory signal (R), (2) the correlation between (xp , yp ) and the unresolved component (z ∥) along the monoscopic view direction using a prior 3D tumour trajectory that can be obtained by either MV/kV imaging or 4DCBCT. Method and Materials: A lungtumor trajectory acquired by a CyberKnife system was fed into 3D motion platform with a marker‐embedded phantom. The associated external respiratory signal was also fed into a separate 1D motion platform. The moving phantom was imaged by MV and kV imagers simultaneously. The 1D motion was also monitored by RPM system. The marker positions from images were extracted by Varian RPM‐Fluoro application. The performance of the proposed method was compared with stereoscopic estimation under various imaging frequencies. Results: The overall estimation error for continuous kV/MV imaging was 0.1±0.3 mm, which reflects the mechanical uncertainty of imaging systems and the marker extraction uncertainty. The error was 0.2±0.7 mm for stereoscopic estimation with 10‐s interval imaging and 0.6±0.7mm for monoscopic estimation with the same update interval. Conclusion: The proposed method can effectively estimate target position and thus be used for tumor tracking with gantry‐mounted single x‐ray imagers that major linac manufacturers offer. Conflict of Interest: Research supported by Varian Medical Systems.
35(2008); http://dx.doi.org/10.1118/1.2961367View Description Hide Description
Purpose: An integrated ultrasound and CT‐Sim system can be used to assist the daily setup of prostate IMRT patient. The purpose of this study is to investigate the influence of probe angle and the tissue elastic module on the displacement of prostate during ultrasound localization using Finite Element Method(FEM).Method and Materials: An ultrasound localization system (Resonant Medical System, Montreal, Canada) integrated with a CT‐Sim was used to obtain a full set of 3‐D ultrasound (US)‐CT images. The patients' anatomical structures, such bone, bladder, and prostate, were contoured on the CTimages by radiation oncologists. The ultrasound probe was positioned at 1cm inferior to bladder superior boundary. A 3D finite element model was generated for each of the patient. The corresponding displacement of prostate during ultrasound localization was calculated by FEM software (Ansys). Results: Under normal tissue elastic module (body (E=15kPa), bone (E=10GPa), bladder wall (E=300kPa), prostate (E=100kPa), and probe (E=3GPa)), when the angle of ultrasound probe increased from 10 to 60 degree to vertical plane with 2cm compression of ultrasound probe, the total displacement of prostate was 0.46∼0.6mm (0.43∼0.54mm inferior, 0.15∼0.25mm posterior). When the elastic module of bladder wall changed from 50kPa to 1MPa with the probe angle of 45 degree, the total displacement of prostate was increased from 0.26mm to 0.76mm (0.23 to 0.68mm inferior, 0.1 to 0.32mm posterior). There was almost no left and right displacement during compression. Conclusion: For the displacement of prostate, there was no significant dependent on the probe angle and limited dependent on the tissue elastic module. With proper controlled compression, the total prostate displacement can be limited within 2mm. This displacement can be corrected by FEM calculation.
- Moderated Poster — Area 3 (Joint): Motion Management
TU‐EE‐A3‐01: Breathing‐Motion Induced Tomotherapy Dose Delivery Errors in the Presence of Beam Modulation35(2008); http://dx.doi.org/10.1118/1.2962617View Description Hide Description
Purpose: To determine breathing‐motion induced Tomotherapy dose delivery errors in the presence of beam modulation. Method and Materials: Previous studies have shown that dose delivery errors are possible due to breathing motion. While these studies have systematically investigated the roles of field size, couch velocity, and breathing motion magnitude on the dose errors, they have neglected the role of intensity modulation. This study used measured breathing patterns and clinically realistic delivery parameters to simulate the breathing‐motion induced errors. Modulation was included by varying the simulated delivered dose on two timescales; the short timescale simulating the 51 angular subsets that subdivide the delivered fluence patterns. A step function was used that varied the leaves open and close time every 0.4 seconds, or every 7.0°, simulating a modulation factor of 2.0. Modulation was also conducted on a longer timescale corresponding to four beam intensity directions (two of high dose and two of low dose). The earlier studies using 52 patients' breathing patterns were repeated with the addition of fluence modulation. Results: The impact of breathing motion on Tomotherapy delivery results in delivery errors of greater than 10%, even for relatively small breathing motions. This is due to the subtle variation in the breathing patterns, including changes in breathing waveform and drifting. The addition of fluence modulation varied the delivered dose patterns slightly, but the magnitude of dose delivery errors was unchanged. Conclusion: These results indicate that previous simulations indicating the challenges of using Tomotherapy dose delivery due to breathing motion are valid in the presence of beam modulation. Treatment planners should take care when planning treatments for mobile tumors.
This work supported in part by Tomotherapy.
35(2008); http://dx.doi.org/10.1118/1.2962618View Description Hide Description
Purpose: The purpose of this study was to develop a technique to dynamically predict tumor position and uncertainty in three dimensions in real time. Method and Measurements: A novel multi‐dimensional time delay kernel regression (MD‐TDKR) technique was developed to predict motion up to 1.5 seconds into the future. This technique uses historical data and the correlations between the multiple dimensions to predict target position, and allows for continuous memory updates to dynamically retrain the model. This technique could be used in a variety of motion prediction applications. Results: In one example, the algorithm inaccurately predicted tumor position, but was able to identify the corresponding uncertainty increase. When the model was unable to accurately predict position, the 95 percent uncertainty interval becomes quite large. Likewise, the predictions are accurate where the uncertainty is small. This information can be used in determining when to temporarily trigger the beam on‐and‐off. The average percent error was computed at different latencies. For predictions 1.5 seconds into the future the error was 7.83%, for 1 second it was 6.55%, and for 0.5 seconds it was only 5.27%. The average uncertainty for the predictions at 0.5, 1, and 1.5 seconds into the future was 4.99%, 6.18%, and 7.12%, respectively. Conclusion: This study shows that MD‐TDKR can learn complex relationships in multidimensional respiration data. Because of its ease of building and updating the memory matrix, the model can adjust to new operating conditions, such as changes in a patients breathing pattern. It was also able to determine the uncertainty for a given query vector, allowing for the ability to hold the beam off when the prediction is unsure. These factors indicate that MD‐TDKR has great potential for use in radiation oncology. It is also anticipated that the MD‐TDKR paradigm could be easily implemented into commercial respiratory gating systems.
TU‐EE‐A3‐03: On‐Board Four‐Dimensional Digital Tomosynthesis (4D‐DTS): Optimization of Respiratory Motion Dependent Acquisition and Reconstruction Parameters35(2008); http://dx.doi.org/10.1118/1.2962619View Description Hide Description
Purpose: Amplitude and period of respiratory motion vary among patients. For four‐dimensional digital tomosynthesis (4D‐DTS), the frequency of projection acquisition and the phase window must be optimized based on respiratory motion. The purpose of this study was to demonstrate optimization of these parameters. Method and Materials: Experiments were performed to demonstrate optimization of projection acquisition frequency and phase window based on respiratory motion characteristics. Projection images of a CIRS Dynamic Thorax Phantom were acquired using an on‐board imager (OBI) mounted on a clinical accelerator. The trajectory of a radiopaque marker attached to the phantom was monitored in projection space and used to assign phases to and sort projections for 4D‐DTS reconstructions. 4D‐DTS images were reconstructed for motion profiles ranging in superior‐inferior amplitude from 10–40‐mm and ranging in period from 3.5–7‐sec. DTS images for each profile were reconstructed from various sets of projections, simulating different projection acquisition frequencies and phase windows. Results: For a desired phase window, the frequency of projection acquisition must be optimized based on the respiratory period. If projections are acquired at too high of a frequency, many of the projections will not be used in the reconstructions, resulting in unnecessary imaging dose. If projections are acquired at too low of a frequency, 4D‐DTS images can be reconstructed with missing projections or with larger phase windows. The effect of reconstructing with missing projections is minor if the fraction of missing projections is small. As the number of missing projections increases, vertical streaking artifacts appear in the images.Conclusion: This work is part of a feasibility analysis for 4D‐DTS imaging. It establishes relationships between optimal projection acquisition frequency, phase window and respiratory motion characteristics. Projection acquisition frequency based on respiratory period was derived and demonstrated. Conflict of Interest: Research sponsored by Varian.
TU‐EE‐A3‐04: Registration of On‐Board Digital Tomosynthesis and Planning CT for Partial Breast Irraadiation Patient Setup Verification with Surgical Clips35(2008); http://dx.doi.org/10.1118/1.2962620View Description Hide Description
Purpose: This study examines the accuracy of On‐Board Digital Tomosynthesis (OB‐DTS) imaging by the localization of individual surgical clips and registration to planning CT instead of a reference‐DTS (RDTS). Using planning CT instead of a RDTS for registration might decrease the uncertainty of the registration resulting from the added edge blurring of DTS and simplify the setup verification process. Method and Materials: A solid water breast phantom containing four surgical clips was imaged with CT and OB‐DTS modalities. The OB‐DTS was acquired for a 40° lateral angle (kV source: 250°–290°). An open‐source GPU based FDK filtered backprojection algorithm was used for reconstruction of DTS image sets. The centroids of all the clips were localized in the DTS image sets and locations were compared with the clip locations in CTimage sets. Using a filtered cross‐correlation algorithm, the OB‐DTS was registered with the CT and RDTS image sets and the registration accuracy of OB‐DTS was calculated. Results: Clip centroids were found to be within 1 mm in DTS and CTimage sets. The OB‐DTS was registered with the CT within 1 mm ± 2 mm along the x‐axis (direction of projection) and less than 1 mm ± 1 mm along the two orthogonal (y and z) axes with a maximum translation of ±1.4 cm in the orthogonal axes and ±4.7 cm along the x‐axis. The OB‐DTS was registered with the RDTS within 4 mm ± 11.5 mm along the x‐axis, 1.4 mm ± 2.8 mm along the y‐axis and 0.5 mm ± 0.7 mm along the z‐axis, with a maximum translation of ±1.4 cm in the orthogonal axes and ±2.8 cm along the axis of propagation. Conclusion: OB‐DTS can be registered to CT with equal or better accuracy than when registered with RDTS and with greater translations along the propagation axis.
TU‐EE‐A3‐05: Simultaneous Tracking and Four‐Dimensional Radiotherapy Delivery: Accounting for Spatial and Morphological Tumor Changes35(2008); http://dx.doi.org/10.1118/1.2962621View Description Hide Description
Purpose: The aim of this work was to develop the formalism for, and experimentally verify, radiotherapy delivery to tumors undergoing spatial and morphological changes induced by respiration.Method and Materials: To determine the leaf sequence to be delivered at a given time based on a 4D plan, LPlan (M,θ), in which the leaf sequence varies not only with monitor units, M, but also respiratory phase θ can be summarized by where T is the 3D target position and L are leaf positions. A 4D treatment plan of a translating, rotating, deforming tumor exhibiting hysteresis with values well above those typically observed clinically was created. The theory derived was coded into a prototype 4D MLC controller. The treatment plan was loaded onto the controller, and delivered on a linear accelerator. The motion was detected by the monitoring of the marker block by the RPM system. The RPM signal was output in real time to the MLC controller that reshaped the beam according to the position and phase of the incoming signal. An EPID, operating in cine mode was used as the detector.Results: The treatment plan from phase T0–T9 matched the real‐time EPIDimages. The EPIDimages demonstrate the ability of the MLC leaves, driven based on the theory derived above, to conform to spatial and morphological changes. Conclusion: A theory has been developed to deliver 4D plans in which the leaf sequences can vary as a function of phase to account for the spatial and morphological tumor and normal tissue changes with respiration. This theory has been integrated into a MLC controller. A 4D radiotherapy treatment plan that includes translation, rotation, hysteresis and deformation was delivered. The method allows for variable respiratory patterns during treatment delivery. Conflict of Interest: Research supported by Varian Medical Systems.
35(2008); http://dx.doi.org/10.1118/1.2962622View Description Hide Description
Purpose: To assess the impact of respiratory velocity on target volume using four‐dimensional computed tomography (4DCT). Method and Materials: A 20 mm diameter object in a QUASAR™ phantom sinusoidally moved with 10 mm amplitude along the longitudinal axis of the CT couch. The motion period was set in the range of 2–12 sec at 2 sec intervals. 4DCT data were acquired on a General Electric 4‐slice Lightspeed RT CT scanner in an axial cine mode. Respiratory motion was recorded by a Varian Real‐time Positioning Management system. A CT slice thickness and image acquisition time were 1.25 mm and 0.5 sec, respectively. The cine duration was set to the motion period plus 2 sec. The number of 10 images per each couch position was reconstructed.Measurement repeated 3 times for each pattern. The object was automatically segmented using threshold on CTimages. Volumetric analysis was performed to evaluate variations in the object size by different periods. Results: The maximum volume of the object was 6.35 ml at a maximum instantaneous velocity (V max) of 30.11 mm/sec, which was larger by 51.2% than true volume. While the probability that a difference between imaged volume and true volume was more than 5%was 37.3% at the velocity of ⩽ 10.68 mm/sec corresponding to the V max with the period of 5.87 sec, it increased to 96.3% at the velocity of >10.68 mm/sec. A significant difference was seen between the mean volume with the period of ⩽ 10.68 mm/sec and >10.68 mm/sec (P<0.01). Conclusion: Severe motion artifacts are more pronounced at higher respiratory velocity. Even if the respiratory period is slow, motion artifacts remain as long as the object moves during CTdata acquisition.
- Moderated Poster — Area 3 (Joint): Non‐Tomographic Localization
TU‐FF‐A3‐01: Analysis of Dose to Patient, Spouse/caretaker, and Staff, From An Implanted Trackable Radioactive Fiducial for Use in the Radiation Treatment of Prostate Cancer35(2008); http://dx.doi.org/10.1118/1.2962650View Description Hide Description
Purpose: A fiducial tracking system based on a novel radioactive tracking technology is being developed for realtime target tracking in external beamradiation therapy. In this study we calculate the radiationdose to the patient, the spouse/caretaker, and the medical staff that would result from a 50uCi Ir192 radioactive fiducial marker permanently implanted in the prostate of a radiation therapy patient. Method and Materials: The local dose to the surrounding tissue was calculated using a Monte Carlo simulation. The equivalent whole body dose to the patient was calculated by summing the equivalent doses to the sensitive organs using standard organ weighting factors. The exposure of the spouse/caretaker was calculated according to the NRC guidelines. The exposure of the medical staff was calculated based on estimates of proximity to the patient and time spent in the vicinity of the patient. Results: The lifetime local dose to the surrounding tissue is below 40Gy at 4mm and below 10Gy at 6mm. The lifetime whole body equivalent dose to the patient is 32mSv. The lifetime dose to the spouse/caretaker is 0.2mSv, and the annual exposure of the medical staff is 0.1mSv for a doctor performing implantations and 0.17mSv for a radiation therapist positioning patients for therapy. Conclusion: The local dose to the tissue surrounding the implant, which is irradiated during therapy, is not expected to have any clinically significant effect. The equivalent lifetime whole body dose to the patient is insignificant in comparison to the whole body dose received from the therapy itself. The radiation exposure of the spouse/caretaker and medical staff is well below the recommended limits. We conclude that with respect to radiation exposure, there is no contraindication to applying this novel system in the radiation treatment of prostate cancer.Conflict of Interest: Research sponsored by Navotek Medical Ltd.
35(2008); http://dx.doi.org/10.1118/1.2962651View Description Hide Description
Purpose: To explore the use of real‐time surfaceimaging technologies for interactive non‐invasive head setup in craniospinal irradiation. Method and Materials: A real‐time surfaceimaging system was installed in a treatment room at our proton therapy center. The system provides surface motion monitoring up to 10 frames per second. Real‐time deltas of patient misalignments can be displayed at the same frame rate. Phantom and patient studies were performed to investigate the feasibility of surfaceimaging based interactive setup, relying on the real‐time deltas graphic interface. Interactive setup on a head phantom was carried out by multiple users, with offline evaluation of residual shifts/rotations. Patient studies were focused on checking surfacevisibility in two adult subjects. Results: As for translational misalignments, all users were able to reposition the head phantom within tolerance. Rotations turned out to be more critical, with residual values slightly above the threshold. Pre‐alignment with lasers is recommended to achieve better performance. Translational misalignments can be handled more efficiently when relying on the treatment couch console to apply the displayed deltas. Patient studies revealed that, when the viewing angle is not obstructed, visibility in the neck and lower head region is satisfactory: blank areas in the 3D surfaceimage occur where thick hair is in place. Average surface reproducibility, compared to the treatment planningCT scan, was found to be within 3 mm. Breathing motion in one patient was quantified in less than 1.5 mm. Conclusion: Interactive head setup relying on real‐time surfaceimaging is feasible. Residual alignment errors can be kept within tolerance even by naive users by relying on a real‐time display of measured misalignments. The application in medulloblastoma patient setup can potentially reduce imaging dose and the time needed for patient alignment, providing valuable tools for intra‐session monitoring of patient positioning and breathing during treatment.