RPM System Research Articles

TU‐C‐AUD B‐07: Effect of Respiratory Gating On Dose Delilvery in a Moving Target

Purpose: To evaluate the effect of gating on dose delivery to a moving target. Method and Materials: A motorized platform with variable speed of 8RPM to 28RPM built to carry a 30×30cm solid water phantom and capable of delivering a 2cm range of motion in both in‐out and up‐down direction is used for the study. Retrospectively gated (4D) CT simulation is acquired on the Philips Gemini PET/CT imaging system. Acquisitions are made with phantom moving at 8, 15, 28 RPM with in‐out motion only and full motion. Waveforms are monitored by the Varian RPM system. Binning is for 10 phases equally spaced between waveform peaks. Based on the most stable range of the cycle phase range of 40%–60% for in‐out motion and 30%–50% for full motion was used for gating. Plans using single AP field, sliding window (SW) and step‐and‐shot (SS) IMRT fields are evaluated using ion chamber and film dosimetry. Results: For the in‐out motion only, the measured absolute dose at the center of the target was within 1% of the planed dose, gated or non‐gated. For full motion study, dose varies up to 2.5% for non‐gated plans and 1% for gated plans. In all motions, penumbra difference is within 2 mm for gated fields but pending on the phase range difference is significantly larger for non‐gated fields. With IMRT fields, good correlation is shown for all gated fields whereas for non‐gated cases, dose delivered deviates greatly from plan. Conclusion: With moving target, dose distribution differs greatly from plan if treatment is not gated. For IMRT fields and field with limited margins 4D treatment with optimized phase range is essential.

SU‐GG‐J‐183: The Use of Video Feedback for the Improvement of Respiratory Gating Radiation Treatments

Purpose: Breath‐hold respiratory gating is currently being used to improve the quality of radiotherapy, especially for sites subject to internal motion. One technical challenge that exists is patient regulation of respiration. Typically, the radiation therapist verbally coaches the patient during treatment. This may not be the most efficient or accurate practice, since the patient is receiving delayed commands based on secondary cues. As such, a video feedback system has been implemented. During treatment, a live plot of breathing amplitude versus time is displayed to the patient for first‐hand adjustments. Method and Materials: Varian's RPM system was used to track the patient's respiratory motion. i‐O Display Systems' i‐Glasses, a headset displaying video feed, was worn by the patient for visual feedback. The RPM software interface was displayed in this headset. Patients with treatment sites in the abdomen and thorax, such as lung, liver, and breast cancer, were of interest in this study. Those who were candidates for breath‐hold gating treatment were selected for either video feedback or verbal coaching respiratory management techniques, based on the patient's physical and mental condition. Results: The accuracy and efficiency of the video feedback system, using i‐Glasses, was compared to the more conventional system of audio commands given by the therapist. Accuracy was based on the magnitude of motion within the threshold, while efficiency was based on beam‐on time during a particular field of treatment. Conclusion: Video feedback proved better accuracy and efficiency of treatment compared to audio coaching. However, not all patients were candidates for the video feedback system. Some patients did not understand what the plot of amplitude versus time meant. Some patients could not resolve the plot, due to poor eye sight. Other patients found the headset to be uncomfortable during treatment. These problems are currently being taken into consideration for future studies.

SU‐GG‐T‐142: Organ Motional Effect On the Dose Distribution in Intensity Modulated Radiation Therapy

Purpose: The respiratory organ motional effect on the dosimetric accuracy in IMRT was analyzed and the function of RPM gating system in reducing the organ motional effect was evaluated with the respiration simulating phantom. Method and Materials: The phantom system which can simulate respiratory organ motion was manufactured. Total 4 IMRT plans for liver cancer were evaluated in the 3 cases of situation that were a static status, a no‐gated free‐breathing motion and a gated radiotherapy with RPM system. The motional range was set 1.6 cm considering the real movement of tumor in the fluoroscopic images and respiratory movement cycle was fixed to 5 sec. The dosimetric accuracy of all the fields of IMRT plan was analyzed with MapCHECK (SunNuclear, USA) device laid on the moving table of the phantom. The comparison analysis between a dose distribution in plan and a real one in treatment was done with a coronal dose distribution in the depth of diode detector arrays in the MapCHECK. The decision of pass in each measurement points was verified by a gamma evaluation in the criteria of 3 mm distance and 3% dose. Results: The average pass rates in each motional status are 98.4% at static case, 49.9% at motional case and 97.9% at gated IMRT case. These data shows that the RPM gated radiation therapy decreased a large amount of dosimetric error due to the organ motional effect in IMRT. Conclusion: The considerable dosimetric error due to organ motion was occurred even in the motional range (1.6 cm) of normal breathing condition. This organ motional error could be reduced with a RPM gating system and the gated radiation therapy with a proper gated phase should be performed for the IMRT of abdominal lesions which are moved by a respiration.

SU‐GG‐J‐168: Respiratory‐Gated Radiation Therapy for the Liver: A 5‐Patient Study

Purpose: To investigate the feasibility and accuracy of using a combination of internal and external fiducials for respiratory‐gated image‐guided radiotherapy of liver tumors with commercially‐available systems. Method and Materials: Five patients were enrolled in the study. Patients were treated on either a 2100EX or Trilogy linear accelerator (Varian Medical Systems). Respiration was tracked and the beam was gated using the RPM system (Varian). Radio‐opaque fiducials implanted adjacent to the liver tumor were used for daily online set‐up using non‐simultaneous orthogonal electronic portal or kV images and ISOLOC software (MedTec). The threshold for couch shifts was 2 mm, while the respiratory gate was set at 1 mm. The accuracy of respiratory‐gated treatment using an external fiducial was verified offline using cine mode images acquired during treatment. Results: The range of tumor motion was between 8.0 mm and 44.3 mm in the supero‐inferior (SI) direction, which is the dominant direction of motion due to respiration. For all patients, interfractional variation, a measure of set‐up uncertainty, was 0.3 ± 2.3 mm (1σ) in the SI direction. The intrafractional variation, a measure of the precision of gating, was found to be ±1.0 mm (1σ). The average duty cycle varied between 18% and 28%. Conclusion: Respiratory gating with either EPID or kV imaging using readily available and affordable infrastructural elements can provide accurate, reproducible, and verifiable means to deliver precision radiotherapy to liver tumors. Daily analysis of the cine images is an effective way to verify that the treatment was delivered as planned and we strongly recommended it.

TU‐EE‐A3‐05: Simultaneous Tracking and Four‐Dimensional Radiotherapy Delivery: Accounting for Spatial and Morphological Tumor Changes

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 urn:x-wiley:00942405:media:mp2621:mp2621-math-0001 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 EPID images. The EPID images 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.

SU‐DD‐A3‐05: Experimental Investigation of a Monoscopic Real‐Time Tumor Tracking Method Combining Occasional X‐Ray Imaging and Continuous External Respiratory Monitoring

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 lung tumor 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.

Verifying 4D gated radiotherapy using time-integrated electronic portal imaging: a phantom and clinical study

BackgroundRespiration-gated radiotherapy (RGRT) can decrease treatment toxicity by allowing for smaller treatment volumes for mobile tumors. RGRT is commonly performed using external surrogates of tumor motion. We describe the use of time-integrated electronic portal imaging (TI-EPI) to verify the position of internal structures during RGRT deliveryMethodsTI-EPI portals were generated by continuously collecting exit dose data (aSi500 EPID, Portal vision, Varian Medical Systems) when a respiratory motion phantom was irradiated during expiration, inspiration and free breathing phases. RGRT was delivered using the Varian RPM system, and grey value profile plots over a fixed trajectory were used to study object positions. Time-related positional information was derived by subtracting grey values from TI-EPI portals sharing the pixel matrix. TI-EPI portals were also collected in 2 patients undergoing RPM-triggered RGRT for a lung and hepatic tumor (with fiducial markers), and corresponding planning 4-dimensional CT (4DCT) scans were analyzed for motion amplitude.ResultsIntegral grey values of phantom TI-EPI portals correlated well with mean object position in all respiratory phases. Cranio-caudal motion of internal structures ranged from 17.5–20.0 mm on planning 4DCT scans. TI-EPI of bronchial images reproduced with a mean value of 5.3 mm (1 SD 3.0 mm) located cranial to planned position. Mean hepatic fiducial markers reproduced with 3.2 mm (SD 2.2 mm) caudal to planned position. After bony alignment to exclude set-up errors, mean displacement in the two structures was 2.8 mm and 1.4 mm, respectively, and corresponding reproducibility in anatomy improved to 1.6 mm (1 SD).ConclusionTI-EPI appears to be a promising method for verifying delivery of RGRT. The RPM system was a good indirect surrogate of internal anatomy, but use of TI-EPI allowed for a direct link between anatomy and breathing patterns.

Respiratory gated beam delivery cannot facilitate margin reduction, unless combined with respiratory correlated image guidance

Purpose/objective In radiotherapy of targets moving with respiration, beam gating is offered as a means of reducing the target motion. The purpose of this study is to evaluate the safe magnitude of margin reduction for respiratory gated beam delivery. Materials/methods The study is based on data for 17 lung cancer patients in separate protocols at Rigshospitalet and Stanford Cancer Center. Respiratory curves for external optical markers and implanted fiducials were collected using equipment based on the RPM system (Varian Medical Systems). A total of 861 respiratory curves represented external measurements over 30 fraction treatment courses for 10 patients, and synchronous external/internal measurements in single sessions for seven patients. Variations in respiratory amplitude (simulated coaching) and external/internal phase shifts were simulated by perturbation with realistic values. Variations were described by medians and standard deviations (SDs) of position distributions of the markers. Gating windows (35% duty cycle) were retrospectively applied to the respiratory data for each session, mimicking the use of commercially available gating systems. Medians and SDs of gated data were compared to those of ungated data, to assess potential margin reductions. Results External respiratory data collected over entire treatment courses showed SDs from 1.6 to 8.1 mm, the major part arising from baseline variations. The gated data had SDs from 1.5 to 7.7 mm, with a mean reduction of 0.3 mm (6%). Gated distributions were more skewed than ungated, and in a few cases a marginal miss of gated respiration would be found even if no margin reduction was applied. Regularization of breathing amplitude to simulate coaching did not alter these results significantly. Simulation of varying phase shifts between internal and external respiratory signals showed that the SDs of gated distributions were the same as for the ungated or smaller, but the median values were markedly shifted. The gated distributions could generally not be covered by margins derived from ungated data, if the phase shift was not accounted for. Conclusions Margins can only be reduced for respiratory gated radiotherapy, if respiratory baseline shifts and variations in external/internal motion correlation are accounted for. Gated beam delivery alone cannot facilitate margin reduction. In the worst case, margins must be increased to accommodate inter-fraction variations in respiration.

Investigation of the dosimetric effect of respiratory motion using four-dimensional weighted radiotherapy

We have developed a four-dimensional weighted radiotherapy (4DW-RT) technique. This method involves designing the motion of the linear accelerator beam to coincide with the tumour motion determined from 4D-CT imaging while including a weighting factor to account for irregular motion and limitations of the delivery system. Experiments were conducted with a moving phantom to assess limitations of the delivery system when applying this method. Although the multi-leaf collimator motion remains within the tolerance of the linear accelerator, the extent of motion was less than 1 mm larger than the designed one, and there was a net system latency of approximately 0.2 s. The dose distributions were measured and simulated using different weighting factors and motion scenarios. The breathing characteristics (period, extent of motion, drift and standard deviations) of 32 patients were evaluated using the Varian RPM system. Breathing variability was assessed by plotting the average breathing motion as a function of the breathing phase. Simulations were carried out to determine the optimal weighting factor based on typical patient breathing characteristics. These results establish that the 4DW-RT method demonstrates potential for dose escalation without increasing exposure to healthy tissue.

SU-FF-J-125: Fractionated Stereotactic Body Radiation Therapy (SBRT) of Lung Tumors - 4DCT Simulation with An AcQSim CT and Varian's RPM System

Purpose: To demonstrate that 4DCT can be acquired for lung patients using a single slice CT simulator and Varian's RPM system. The 4DCT can be used for the determination of ITV. Fractionated SBRT was accomplished using Varian's Ex21 linac equipped with OBI/CBCT system. Materials and methods: Varian's RPM system was connected to a single slice AcQSim CT for 4D simulation of lung patients. Before scans, the couch height was set such that the side horizontal laser was approximately at the middle level of the target. A spiral scan was acquired first for conventional simulation and the patient was tattooed without raising or lowering the couch. After that, the RPM system was turned on and four sets of gated axial scans were acquired at four different phases: 0, 25, 50 and 75%. The gated scans covered a 12cm scan range, 6cm from each side of the central axis. Results: Six patients were simulated with this technique. Because the couch height was kept the same for the spiral and gated CT scans,image fusion between spiral and each gated set can be made using the shared DICOM coordinates and therefore very accurate. GTV in the spiral as well as in each gated CT was contoured and merged together to obtain the ITV. PTV was ITV plus 3mm margins expansion. Treatment planning was performed in Eclipse (4 fractions of 12Gy/fraction). Generally 5–6 beams were placed (every ∼30 degree) around the target for an IMRT plan and good dose conformality was achieved from the plan. Before each treatment, CBCT was acquired and 3D image fusion based on the local ITV/PTV volumes was made. Conclusion: A technique for the determination of ITV using gated CT sets at multiple phases was developed for fractionated SBRT for lungtumors using Ex21 linac equipped with the OBI/CBCT system.

WE‐D‐AUD‐07: Temporal, Dosimetric, and Geometric Precision of Gated Step‐And‐Shoot Intensity Modulated Radiation Therapy

Purpose: Due to the complicated technical nature of gated RT, electronic and mechanical limitations may affect the precision of delivery. The purpose of this study is to investigate the temporal, dosimetric, and geometric accuracies of gated RT. Method and Materials: A Varian Trilogy, equipped with a RPM system, is used to examine the accuracy of both un‐gated and gated deliveries. Data recorded using radiographic films and EPIDs allow extraction of geometric and system response accuracy. The dosimetric precision of each segment making up a step‐and‐shoot IMRT treatment plan is individually recorded and analyzed using ionization chamber methods. Results: The time required from when the target object enters the specified phase to when the radiation beam is actually turned on is approximately ∼ 0.15 s. This time error is systematic and is the same for both normal and SS‐IMRT gated deliveries. Dosimetric data from 50+ repetitions of a fixed segment dose for both un‐gated and gated deliveries are obtained. At higher dose rates, the dosimetric errors are seen to become greater in the gated case. A geometric comparison of the un‐gated to gated RT delivery show clear positional inconsistencies in the thicker outer MLC leafs in the non‐gated case. These leaf positional discrepancies are attributed to motion of the thicker outer leaf pairs while the radiation beam is on. Conclusion: A system response time of ∼150 ms is obtained using a simple moving phantom procedure. Dosimetrically we have found that gating negatively effects dose accuracy by interruption of the “overshoot” effect. This interruption could lead to large dosimetric errors, especially for plans consisting of a large number of small MU segments that are delivered at high dose rates. Geometrically very little error for both un‐gated and gated deliveries are seen, provided that a well‐defined leaf tolerance is used.

SU‐FF‐T‐400: The Application of a Simplified Framework for the Treatment of Left Breast Cancer Using Deep Inspiration Breath‐Hold (DIBH) for Reduced Heart Dose

Purpose: A simplified framework for deep inspiration breath‐hold (DIBH) was developed and used for patient simulation. This report presents the details of using the framework for left breast patient treatment. Materials and methods: For each patient, two plans were made, one with the free breathing CT and the other with the DIBH CT. Organ structures (left breast, heart and left lung) were contoured separately on each set of CT. Dose volume histograms (DVH) and dose statistics were used for comparison. Before treatment, the framework was set to patient in the same way as for simulation. Patient setup was made at DIBH such that P point's SSD (anterior) matches that from the simulation DIBH CT. Portal images were taken at DIBH and compared to DRRs generated from the DIBH CT for setup. Because the RPM inside the treatment room and that inside the simulation room had different setups, in case the DIBH amplitudes were different, the two gating threshold lines can be shifted and overlaid to the captured DIBH signal before treatment. The RPM system can guide the linac to turn on or off the radiation automatically. Results: Comparing the two plans based on two CT sets, it was found that the left breast dose coverage was similar. While the absolute DVH for left lung was higher with DIBH, the relative DVH was similar or even lower because of the much enlarged lung volume. The heart volume did not change much but the dose was appreciably reduced. One breath hold was generally enough for the treatment of each beam. We did find that the DIBH portal images matched better to DRRs generated from DIBH CT than with free breathing CT. Conclusion: The framework was successfully applied for the treatment of three left breast patients with reduced heart dose.

SU‐FF‐T‐43: A Simplified Framework for CT Simulation of Left Breast Patients Using Deep Inspiration Breath‐Hold (DIBH)

Purpose: We presented a simplified framework for deep inspiration breath hold (DIBH) in AAPM 2006. This study presents the implementation of the framework for patient simulation using AcQSim CT simulator. Materials and methods: An aqua‐plastic mask band of 1.0–1.5in was made before simulation while the patient was instructed in DIBH for 30s or longer. The mask band was set near the umbilicus and the RPM marker box was set ∼1cm superior or inferior to the mask. The DIBH signal was also displayed on a monitor set close to patient, which was a duplicate display of patient's signal in the RPM computer. The patient can physically achieve DIBH by the mask band. She can also visually guide herself for stable breathing amplitude. The CT images were acquired in spiral rather than in gated axial mode such that much faster scanning can be made. The RPM system was simply used to monitor the breathing and in case patients cannot hold her breath, the spiral scanning can be manually terminated and then resumed for the next breath hold. For each patient, two sets of CT were acquired, one in DIBH and the other in free breathing. Patient tattoo was made while the patient was in DIBH. Results: The framework was successfully applied for the simulation of three left breast patients. During the simulation, two patients can hold their breath for ∼60s or longer with very stable amplitudes, such that CT scanning can be made in spiral rather than in the gated axial mode in just one breath hold. The DIBH CT images have less motion artifacts compared to the free breathing CT images. Conclusion: The system can be reliably applied for the simulation of left breast patients with DIBH. A modern multislice CT simulator can be more efficient to make the DIBH scanning.

TU-FF-A3-05: Real-Time Lung Tumor Motion Prediction Using Neural Network Based Models Constructed From Unsorted Cine CT Images

Purpose: To develop neural network based models for predicting respiratory induced tumor motion from gating signals and unsorted cine CT images. The ability of the models to predict target motion given real-time surrogate marker motion was verified using a dynamic phantom with a target moving according to motion profiles derived from patient respiratory data. Methods & Materials: Tumor motion profiles were derived using patient data and imported into CIRS Dynamic Thorax Phantom. The phantom was imaged with a 4-slice CT scanner in cine mode. Surrogate marker motion from the phantom exterior was recorded simultaneously using Varian's RPM system. Neural networks were constructed to model relationships between target presence or absence at each slice location and external marker motion. The combined networks for each of the slice locations were used to predict target motion. To verify the ability of the models to infer target position from surrogate signal, additional scans were performed. Models were used to predict target motion from external marker signals and compared to the motion patterns imported into the phantom. Results: Five preliminary experiments were performed with motion patterns from different patients. The models performed well for two cases, predicting motion with average errors on the order of image resolution, 1.26 mm and 1.39 mm root mean squared errors (RMSEs). For motion patterns with large peak-to-peak displacements or highly irregular frequencies / amplitudes, the predictions were less accurate. Conclusion: A neural network based method has been developed for modeling internal target motion using external surrogate signals and unsorted cine CT images. Preliminary results indicate that the approach may be useful for some cases. Adaptive networks utilizing real-time image information during the prediction process may be used to improve prediction accuracy for high frequency / amplitude motion patterns. Conflict of Interest: Partially supported by Varian research grant.

95: Integrated Analysis of Pancreatic Tumor Motion Using Multiple Image-guided Modalities

To provide a comprehensive analysis of the inter- and intrafraction variation of pancreatic tumor position through integration of data from multiple image-guided modalities (Varian Trilogy system, Cyberknife stereotactic body radiotherapy [SBRT] with the Synchrony respiratory tracking system, and 4D-CT planning scans with the Varian RPM system).

Integrating respiratory gating into a megavoltage cone‐beam CT system

We have previously described a low-dose megavoltage cone beam computed tomography (MV CBCT) system capable of producing projection image using one beam pulse. In this study, we report on its integration with respiratory gating for gated radiotherapy. The respiratory gating system tracks a reflective marker on the patient's abdomen midway between the xiphoid and umbilicus, and disables radiation delivery when the marker position is outside predefined thresholds. We investigate two strategies for acquiring gated scans. In the continuous rotation-gated acquisition, the linear accelerator (LINAC) is set to the fixed x-ray mode and the gantry makes a 5 min, 360 degree continuous rotation, during which the gating system turns the radiation beam on and off, resulting in projection images with an uneven distribution of projection angles (e.g., in 70 arcs each covering 2 degrees). In the gated rotation-continuous acquisition, the LINAC is set to the dynamic arc mode, which suspends the gantry rotation when the gating system inhibits the beam, leading to a slightly longer (6-7 min) scan time, but yielding projection images with more evenly distributed projection angles (e.g., approximately 0.8 degrees between two consecutive projection angles). We have tested both data acquisition schemes on stationary (a contrast detail and a thoracic) phantoms and protocol lung patients. For stationary phantoms, a separate motion phantom not visible in the images is used to trigger the RPM system. Frame rate is adjusted so that approximately 450 images (13 MU) are acquired for each scan and three-dimensional tomographic images reconstructed using a Feldkamp filtered backprojection algorithm. The gated rotation-continuous acquisition yield reconstructions free of breathing artifacts. The tumor in parenchymal lung and normal tissues are easily discernible and the boundary between the diaphragm and the lung sharply defined. Contrast-to-noise ratio (CNR) is not degraded relative to nongated scans of stationary phantoms. The continuous rotation-gated acquisition scan also yields tomographic images with discernible anatomic features; however, streak artifacts are observed and CNR is reduced by approximately a factor of 4. In conclusion, we have successfully developed a gated MV CBCT system to verify the patient positioning for gated radiotherapy.

MO‐E‐224C‐02: Quantification of Respiration‐Induced Dosimetric Impact On Liver for Abdominal Radiotherapy

Purpose: To quantify the dosimetric impact of respiratory motion on liver dose for IMRT abdominal radiotherapy based on 4D CT imaging Method and Materials: 4D‐CT images of a patient with klatskin tumor were acquired on a GE CT scanner during free‐breathing. Respiratory information was recorded using the Varian RPM system. The images were sorted retrospectively into ten respiratory phases. For each phase‐binned 4D data set, manual contours for liver were delineated. The Corvus inverse planning system (NOMOS Inc.) was used to generate optimized treatment plans. Two kinds of image registration methods, feature‐based iterative closest point (ICP) rigid image registration and normalized mutual information (NMI) non‐rigid image registration method, were implemented to transform each of the ten phases to a reference phase. The reference phase was the end of expiration (50%phase). A composite 4D‐DVH, which accounts for respiratory induced voxel motion, was subsequently calculated using equal weighting for each respiratory phase. A dose of 45Gy to the target was prescribed. The treatment plan was optimized to provide a minimum of 95% isodose coverage for PTV. Results: Variation of liver dose throughout each of the 10 calculated phases was within 5%. 4D effective DVH showed reduced volume coverage up to 18% for the same dose. In the high dose region of the liver DVH, the dose was less for the NMI algorithm as compared to the ICP algorithm. Conclusions: Two kinds of image registration methods have been implemented to derive 4D effective DVHs for an abdominal patient. In this study we found that respiratory‐induced motion does not produce a significant alteration of the liver DVH between the 10 phases. However, the 4D effective DVH method shows that the liver received less dose.

SU‐FF‐T‐114: Breath Coaching with Visual Feedback for End‐Expiratory Gated Radiotherapy

Purpose: To determine if breath coaching to maintain a consistent breathing amplitude at expiration is feasible in a clinical setting. Method and Materials: This technique consists of two phases of observation and active breathing to restrict the end‐expiratory phase so as to never exceed the lower limit of the gate. The first phase consists of 2 minutes of observation followed by 3 minutes of active breathing using visual feedback, while the second phase consists of 2 minutes of observation followed by 8 minutes of active breathing again using visual feedback. The purpose of the 2‐minute observation periods is to set the end‐expiratory gate at a comfortable level for the patient. Following the first phase the patient assesses whether the lower limit of the gate is appropriate and if it can be maintained in a reproducible manner for an extended period without exhaling below the gate. We have studied ten healthy volunteers of varying ages. The RPM system (Varian) was used to monitor the respiratory cycles of the volunteers. The volunteers self‐monitored their respiration via goggles that were turned on following each 2‐minute observation period. Results: The visual feedback was the key factor to reproducibly maintain the end‐expiratory gate. We were able to determine that all subjects were successful in sustaining their breathing pattern such that they did not exceed the lower limit of the gate and they were all able to do so comfortably. Only one volunteer would have needed to have the gate shifted upwards for actual treatment delivery. Only one volunteer would have needed to have the gate shifted upwards for actual treatment delivery. Conclusion: Breath coaching to maintain a consistent expiratory amplitude is feasible. It should not place a heavy burden on the patient during respiratory‐gated radiotherapy.

SU‐FF‐T‐386: Respiratory Gating in the Treatment of Liver Tumors

Purpose: To determine if respiratory gated radiotherapy is feasible for the treatment of liver tumors. Method and Materials: The patient was a 70 year‐old woman with metastatic liver cancer. We used the RPM system (Varian Medical Systems) to track the respiratory cycle of the patient and gate the beam at the end of the respiratory cycle. Three implanted gold fiducials were used for daily patient positioning with AP and Lateral electronic portal images. The fiducials were also used to verify the location of the tumor during treatment. Prior to the initial treatment, the patient underwent a mock treatment in which a series of portal images were taken to verify the respiratory cycle correlated with the position of the tumor at the end of expiration. Results: The patient's mock treatment was successful, with nearly all fiducial locations lying within a sphere of 2.5 mm. Analysis of the data obtained over the course of treatment (∼600 data points) showed that 95% of the fiducial locations stayed within a sphere of about 7 mm. The uncertainty due to set‐up was ±4.6 mm, while the precision in the gating window was ±2.2 mm. Conclusion: The location of the liver tumor was well reproduced throughout the course of treatment. Our results suggest that we can use respiration to successfully track the motion of a liver tumor and gate the beam at the end of the respiratory cycle.

SU‐FF‐I‐33: A Graphical and Quantitative Procedure for Amplitude‐Gated Treatment Planned Using Phase‐Based 4DCT Using the Varian RPM System

Purpose: A graphical and quantitative procedure for amplitude‐gated treatment planned using phase based 4DCT of free breathing patients using the Varian RPM gating system has been developed as a part of the study of phase and amplitude based imaging and treatment. There are several phase‐based CT scanning systems that produce phase‐based images for gated treatments, but for free breathing phase may not be a reliable predictor of tumor location. Good correlation has been shown between abdomen height and diaphragm position, making it a promising treatment modality. Method and Materials: A phase‐based retrospective CT scan from a Philips large bore 16 slice CT scanner is obtained and the phase markers are adjusted to delineate actual breaths. A gating window that is a subset of the whole CT scan is set and a visual representation of gated motion called a MIP(maximum intensity projection) is created to visualize any anatomical excursions. The Varian RPM data is then compared with the scan data to quantify the actual marker block motion(or chest height) during the gating window as well as for an entire phase. The scan data is then presented not only as a percentage of an average breath, but as a chest height range within the total. This information is checked on treatment days and adjusted if necessary. The gating signal and fluoroscopy are compared to a 4D DRR generated from the MIP to check for geographic as well as systematic misses. Before treatment a gated port film (gated over several breaths and dose limited) can be captured to verify gating of the treatment beam. Results: For phantom based studies of regular breathing the method is robust. The model converts phase to amplitude of abdomen height. Conclusion: This method can validate a gated treatment using different imaging modalities.