Gated Radiation Therapy Research Articles

A Novel Tool for Image Guided Gated Radiation Therapy Delivery

A Novel Tool for Image Guided Gated Radiation Therapy Delivery

SU-GG-T-60: Verification of Gated Radiation Therapy: Dosimetric Shift and Target Coverage Due to Residual Motion

In gated radiation therapy, a window of beam delivery is selected, which typically incorporates residual motion. Consequently, beam delivery is to irradiate not only the true extent of disease, but also neighboring normal tissues. It is desired that the delivery cover the true target as a minimum, although inhibiting factors exists, that includes intermittent beam delivery and potential irreproducibility in breathing pattern. The intermittency can cause time delay as well as dose rate variation during delivery. The objectives of our study are therefore to validate if the intended dose is surely delivered to the true target and to understand the trend of dose delivery on the true target and the normal tissues around it while they move. To meet these objectives, an experimental study has been performed. For experiments, we have executed the entire process of radiation therapy by using a custom-made phantom with rectangle- and pyramid-shaped targets. On a moving platform, they were scanned for imaging. Various gating windows were selected and image integration was performed to generate targets for planning and delivery. These targets include the true target and the neighboring tissues. The planning was done conventionally for the rectangle target and IMRT optimization was done for the pyramid target. We have then performed evaluation of beam on a diode array after all beams were aligned at 180°. We have evaluated the dose profiles, and observed a systematic shift of them between treatment planning and delivery from gated beams for both targets. We have observed sufficient dose coverage on the true targets. The profile analysis also shows the amount of dose the neighboring normal tissues receive during gated delivery. This study proves that gated beam delivery is secure in spite of discontinous delivery and target motion. Computational modeling is underway, that explains the systematic shift mentioned above.

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-T-378: Development of Error Analysis Program for Phase-Based Respiratory Gating Radiation Therapy

Purpose: The RPM respiratory gating system (Varian, USA) establishes irradiation region using the amplitude-based or phase-based method. Although phase-based method is preferred because of the stability in the real treatment conditions, it has some limits to explain the exact correlation between the marker motion and organ motion. Even when the variation of amplitude which can introduce target motion considered as an error is produced, the phase-based method has the possibility to irradiate including the error positions. In this study, the error analysis program was developed for the verification of the tumor position's variation correlated with the variation of marker's amplitude which can be occurred during a phase-based respiratory gating treatment. Method and Materials: The analyzed program was created with the IDL5.5 (Research Systems Inc., USA) in order to evaluate the errors based on the log files which were recorded during a respiratory gated radiation therapy with The analysis program was verified with a virtual treatment record file and with a record file using moving phantom which were modified considering the irregular amplitude's variation simulating the real clinical situations. Results: In both cases, accurate discrimination of error points and error calculation were produced. When the treatment record files of a real patient were analyzed with the program, the accurate recognition and calculation of the error points were also verified. Conclusion: The analysis program developed in this study can be evaluated as a useful tool for the analysis of errors due to the irregular variation of patients' respiration during the phase-base respiratory gating radiation treatment.

Using patient data similarities to predict radiation pneumonitis via a self-organizing map

This work investigates the use of the self-organizing map (SOM) technique for predicting lung radiation pneumonitis (RP) risk. SOM is an effective method for projecting and visualizing high-dimensional data in a low-dimensional space (map). By projecting patients with similar data (dose and non-dose factors) onto the same region of the map, commonalities in their outcomes can be visualized and categorized. Once built, the SOM may be used to predict pneumonitis risk by identifying the region of the map that is most similar to a patient's characteristics. Two SOM models were developed from a database of 219 lung cancer patients treated with radiation therapy (34 clinically diagnosed with Grade 2+ pneumonitis). The models were: SOMall built from all dose and non-dose factors and, for comparison, SOMdose built from dose factors alone. Both models were tested using ten-fold cross validation and Receiver Operating Characteristics (ROC) analysis. Models SOMall and SOMdose yielded ten-fold cross-validated ROC areas of 0.73 (sensitivity/specificity = 71%/68%) and 0.67 (sensitivity/specificity = 63%/66%), respectively. The significant difference between the cross-validated ROC areas of these two models (p < 0.05) implies that non-dose features add important information toward predicting RP risk. Among the input features selected by model SOMall, the two with highest impact for increasing RP risk were: (a) higher mean lung dose and (b) chemotherapy prior to radiation therapy. The SOM model developed here may not be extrapolated to treatment techniques outside that used in our database, such as several-field lung intensity modulated radiation therapy or gated radiation therapy.

Dosimetric Consequences of Electronic Delay on Gated Radiation Therapy (RT)

Dosimetric Consequences of Electronic Delay on Gated Radiation Therapy (RT)

Examination of geometric and dosimetric accuracies of gated step‐and‐shoot intensity modulated radiation therapy

Due to the complicated technical nature of gated radiation therapy, electronic and mechanical limitations may affect the precision of delivery. The purpose of this study is to investigate the geometric and dosimetric accuracies of gated step-and-shoot intensity modulated radiation treatments (SS-IMRT). Unique segmental MLC plans are designed, which allow quantitative testing of the gating process. Both ungated and gated deliveries are investigated for different dose sizes, dose rates, and gating window times using a commercial treatment system (Varian Trilogy) together with a respiratory gating system [Varian Real-Time Position Management system]. Radiographic film measurements are used to study the geometric accuracy, where it is found that with both ungated and gated SS-IMRT deliveries the MLC leaf divergence away from planned is less than or equal to the MLC specified leaf tolerance value for all leafs (leaf tolerance being settable from 0.5-5 mm). Nevertheless, due to the MLC controller design, failure to define a specific leaf tolerance value suitable to the SS-IMRT plan can lead to undesired geometric effects, such as leaf motion of up to the maximum 5 mm leaf tolerance value occurring after the beam is turned on. In this case, gating may be advantageous over the ungated case, as it allows more time for the MLC to reach the intended leaf configuration. The dosimetric precision of gated SS-IMRT is investigated using ionization chamber methods. Compared with the ungated case, it is found that gating generally leads to increased dosimetric errors due to the interruption of the "overshoot phenomena." With gating the average timing deviation for intermediate segments is found to be 27 ms, compared to 18 ms for the ungated case. For a plan delivered at 600 MU/min this would correspond to an average segment dose error of approximately 0.27 MU and approximately 0.18 MU for gated and ungated deliveries, respectively. The maximum dosimetric errors for individual intermediate segments are found to deviate by up to approximately 0.64 MU from their planned value when delivered at 600 MU/min using gating, this compares to only approximately 0.32 MU for the ungated case.

Internal‐external correlation investigations of respiratory induced motion of lung tumors

In gated radiation therapy procedures, the lung tumor position is used directly (by implanted radiopaque markers) or indirectly (by external surrogate methods) to decrease the volume of irradiated healthy tissue. Due to a risk of pneumothorax, many clinics do not implant fiducials, and the gated treatment is primarily based on a respiratory induced external signal. The external surrogate method relies upon the assumption that the internal tumor motion is well correlated with the external respiratory induced motion, and that this correlation is constant in time. Using a set of data that contains synchronous internal and external motion traces, we have developed a dynamic data analysis technique to study the internal-external correlation, and to quantitatively estimate its underlying time behavior. The work presented here quantifies the time dependent behavior of the correlation between external respiratory signals and lung implanted fiducial motion. The corresponding amplitude mismatch is also reported for the lung patients studied. The information obtained can be used to improve the accuracy of tumor tracking. For the ten patients in this study, the SI internal-external motion is well correlated, with small time shifts and corresponding amplitude mismatches. Although the AP internal-external motion reveals larger time shifts than along the SI direction, the corresponding amplitude mismatches are below 5 mm.

SU-FF-J-93: Respiration Induced Abdominal Organ Motions and Anatomic Changes: Indications for Gated Radiation Therapy

Purpose: To quantify respiration induced organ motions and anatomic changes for organs at abdomen and to estimate benefits of gated radiation therapy in terms of dosimetry parameters and the normal tissue complication probability (NTCP). Method and Materials: The 4DCT datasets for ten patients with respiratory diaphragm motion larger than 1 cm were selected to analyze. The 4DCT datasets consist of CT sets of ten phases within each respiratory cycle. Various abdominal tumors and structures such as liver, pancreas, spleen and kidney were contoured on individual phase CT sets. Respiration induced motion and variations in organ volumes and centroid positions of these structures were studied by comparing their contours of different breathing phases. The study was performed for all respiratory phases (non‐gating) and for selected phases (gating). The magnitudes of the tumor respiratory motion under both gating and non‐gating conditions were used to determine margins for the planning target volumes (PTV). Representative 3D conformal radiotherapydosimetry plans were generated with or without gating. Dose volume histograms (DVHs) for these organs were calculated and compared. The normal liver DVHs were used to estimate the NTCP based on the Lyman model. Results: It is found that, unlike organs in thorax, the structures in abdomen show negligible changes (<3%) in their volumes during respiration, while their centroids vary during the breathing. The DVHs from the treatment plans of livertumor indicate that the gated delivery results in substantial sparing of normal tissues in abdomen. The gated delivery leads to significant reduction in NTCPs for the normal liver and other abdominal structures. Conclusion:Respiration induced volume changes for abdominal structures are negligible. The gated treatment reduces radiationdoses to normal tissues thus reduces NTCP.

WE‐E‐M100F‐06: Latency Measurements and Demonstration of a 4D Electromagnetic Localization System for LINAC Beam Gating

Purpose: The Calypso® 4D Localization System uses electromagnetic fields to localize and track Beacon® transponder implants. In this study, we used the Calypso® System to perform real‐time beam gating based on transponders mounted to a dynamic phantom. The system's latency was studied in conjunction with a Varian linac to determine feasibility of enabling gated radiation therapy delivery for respiratory applications. Method and Materials: An acrylic transponder holder with three embedded Beacon® transponders was placed 6 cm from the center of a rotating disk on a dynamic phantom. Film measurements were made with a 2 cm by 2 cm field delivered in a static, dynamic‐gated, and dynamic‐non‐gated fashion. A linear gating window with a 2 mm width was used for the film demonstration. Latency measurements compared an in‐volume / out‐of‐volume signal obtained directly from the dynamic phantom to the target‐current signal from the linac. The target signal represents the measured current in the MV electron beam, and is directly correlated to the presence (or absence) of the treatment beam. The investigational prototype gating system does not used predictive modeling; the linac beam is enabled when the transponder is within the gating window based solely on the last position estimate of the transponder. Results: Film measurements demonstrated that signals from a prototype electromagnetic gating system can be used to effectively trigger the beam on/off state of a linac. The combined, motion‐triggered latency of the localization system and linac was 65 msec to disable the beam and 75 msec to enable the beam. Conclusion: Use of wireless electromagnetic implanted transponders has the potential to enable real‐time linac beam gating with the latency and update rates required for respiratory applications without the use of predictive algorithms.

Intensity modulation with respiratory gating for radiotherapy of the pleural space

We wanted to describe a technique for the implementation of intensity-modulated radiotherapy (IMRT) with a real-time position monitor (RPM) respiratory gating system for the treatment of pleural space with intact lung. The technique is illustrated by a case of pediatric osteosarcoma, metastatic to the pleura of the right lung. The patient was simulated in the supine position where a breathing tracer and computed tomography (CT) scans synchronized at end expiration were acquired using the RPM system. The gated CT images were used to define target volumes and critical structures. Right pleural gated IMRT delivered at end expiration was prescribed to a dose of 44 Gy, with 55 Gy delivered to areas of higher risk via simultaneous integrated boost (SIB) technique. IMRT was necessary to avoid exceeding the tolerance of intact lung. Although very good coverage of the target volume was achieved with a shell-shaped dose distribution, dose over the targets was relatively inhomogeneous. Portions of target volumes necessarily intruded into the right lung, the liver, and right kidney, limiting the degree of normal tissue sparing that could be achieved. The radiation doses to critical structures were acceptable and well tolerated. With intact lung, delivering a relatively high dose to the pleura with acceptable doses to surrounding normal tissues using respiratory gated pleural IMRT is feasible. Treatment delivery during a limited part of the respiratory cycle allows for reduced CT target volume motion errors, with reduction in the portion of the planning margin that accounts for respiratory motion, and subsequent increase in the therapeutic ratio.

Delineation of moving targets with slow MVCT scans: implications for adaptive non-gated lung tomotherapy

Accurate imaging is a prerequisite for adaptive radiation therapy of mobile tumours. We present an evaluation of the performance of slow computed tomography (CT) for mapping and delineating the excursion boundary of a moving object using a tumour phantom scanned with the helical MVCT scanner of a tomotherapy unit. A spherical test object driven by sinusoidal motion in both the lateral and cranial-caudal directions was used to determine how well MVCT images depict the true envelope of the motion. Such information is useful in interpreting the CT images relative to the static object case when radiotherapy gating is to be used or in determining the internal target volume (ITV) when beam gating is not possible. A computer simulation of the CT imaging process was developed which incorporates the third generation fan beam geometry and helical acquisition technique of the tomotherapy MVCT system. Motion artefacts are mainly characterized by the parameter α = Tgantry/Trespiration which is interpreted as the period of the gantry rotation (Tgantry) in units of the respiratory period (Trespiration). Experimental tests were performed using a fixed gantry period of 10 s per full rotation and respiratory period ranging from 4.0 (α = 2.5) to 1.0 (α = 10) s. These cases represent typical clinical imaging conditions on the tomotherapy unit, as well as an extreme test case where the gantry period is intentionally set to be much greater than the respiratory period (termed an ‘ultra-slow’ scan). The accuracy of target (ITV) delineation is evaluated by comparing volumes generated using iso-density contours on the MVCT images to the true motion envelope, known a priori in this phantom study. As expected, motion artefacts are present in clinical MVCT images and they are not averaged over the slow gantry period of rotation. Furthermore, artefacts are not significantly affected by scanning with different helical pitch values. Greater distortions from the true density distribution are observed for lateral motion compared to cranial-caudal motion. Volumes generated by iso-density contours yield better agreement with the motion envelope for scans performed under ultra-slow conditions (α = 10) compared to typical clinical imaging conditions (α = 2.5). If the MVCT gantry cannot be rotated very quickly due to engineering constraints in order to achieve ultra-fast CT, we suggest an opposite approach as an interim measure for mapping the ITV. Adjusting MVCT scan conditions to a very slow acquisition (α = 10) may be a good compromise for determining the ITV for non-gated adaptive tomotherapy of moving lung tumours.

Assessing Residual Motion for Gated Proton-Beam Radiotherapy

Gated radiation therapy is a promising method for improving the dose conformality of treatments to moving targets and reducing the total volume of irradiated tissue. Target motion is of particular concern in proton beam radiotherapy, due to the finite range of proton dose deposition in tissue. Gating allows one to reduce the extent of variation, due to respiration, of the radiological depth to target during treatment delivery. However, respiratory surrogates typically used for gating do not always accurately reflect the position of the internal target. For instance, a phase delay often exists between the internal motion and the motion of the surrogate. Another phenomenon, baseline drifting refers to a gradual change in the exhale position over time, which generally affects the external and internal markers differently. This study examines the influence of these two physiological phenomena on gated radiotherapy using an external surrogate.

Strategy of Respiratory Gated Radiation Therapy for Lung Cancer Patient in Stereotactic Radiosurgery: Phantom Study

Purpose: We propose the measurement method of tumor movement for respiratory gated therapy in lung cancer patient for stereotactic radiosurgery, contouring method of tumor for radiation treatment planning using measured tumor movement. And through phantom study, we ascertain that the tumor movement is properly reflected in determination of PTV, and the tumor is properly and safely treated in full respiration phases and respiratory gated therapy. Materials and Methods: Lung cancer phantom and 1-dimensional moving phantom were made to evaluate respiratory gated radiation therapy for lung SRS. 4D CT scan was performed using these phantoms and 10 sets of CT images and post-processed MIP (Maximum Intensity Projection) images were used to measure the tumor movement. The measured tumor movement in 4D CT images and MIP images were compared. Also, during radiation exposure in full respiration phases and respiratory gated phases, tumor movement included in radiation exposure was measured using EPID image and compared with measured data in 4D CT images and MIP images. Results: The tumor movement measured in full respiration phases was 28.8mm and 29.1 mm in 4D CT images and MIP images respectively, and in respiratory gated phases, 30∼70% phases, was 12 mm and 12.2 mm respectively. The tumor contoured in each phase images and MIP images was well agreed in full respiration phases and respiratory gated phases. The tumor movement included in radiation exposure was 29.3 mm and 8.4 mm in full respiration phases and respiratory gated phases respectively. Conclusion: The tumor movement measured in 4D CT images and MIP images was well agreed, so we propose to use of MIP image for contouring of tumor in full respiration phases and respiratory gated phases. In full respiration radiation treatment and respiratory gated radiation therapy, the tumor movement included in radiation exposure was well agreed with measured tumor movement in 4D CT images or MIP images, so we ascertain though this phantom study we can exactly treat the tumor including tumor movement. In respiratory gated radiation therapy, the tumor movement included in radiation exposure was about 30% smaller than measured tumor movement in 4D CT images or MIP images, so we ascertain that we can safely treat the tumor including tumor movement in current provided technique. (J Lung Cancer 2007;6(1):29 �� 34)

Residual motion of lung tumors in end‐of‐inhale respiratory gated radiotherapy based on external surrogates

It has been noted that some lung tumors exhibit large periodic motion due to respiration. To limit the amount of dose to healthy lung tissues, many clinics have begun gating radiotherapy treatment using externally placed surrogates. It has been observed by several institutions that the end-of-exhale (EOE) tumor position is more reproducible than other phases of the breathing cycle, so the gating window is often set there. From a treatment planning perspective, end-of-inhale (EOI) phase might be preferred for gating because the expanded lungs will further decrease the healthy tissue within the treatment field. We simulate gated treatment at the EOI phase, using a set of recently measured internal/external anatomy patient data. This paper attempts to answer three questions: (1) How much is the tumor residual motion when we use an external surrogate gating window at EOI? (2) How could we reduce the residual motion in the EOI gating window? (3) Is there a preference for amplitude- versus phase-based gating at EOI? We found that under free breathing conditions the residual motion of the tumors is much larger for EOI phase than for EOE phase. The mean values of residual motion at EOI were found to be 2.2 and 2.7 mm for amplitude- and phase-based gating, respectively, and, at EOE, 1.0 and 1.2 mm for amplitude- and phase-based gating, respectively. However, we note that the residual motion in the EOI gating window is correlated well with the reproducibility of the external surface position in the EOI phase. Using the results of a published breath-coaching study, we deduce that the residual motion of a lung tumor at EOI would approach that at EOE, with the same duty cycle (30%), under breath-coaching conditions. Additionally, we found that under these same conditions, phase-based gating approaches the same residual motion as amplitude-based gating, going from a 28% difference to 11%, for the patient with the largest difference between the two gating modalities. We conclude that it is feasible to achieve the same reproducibility of tumor location at EOI as at EOE if breath coaching is implemented, enabling us to reap the benefits of the dosimetric advantage of EOI gating.

Radiotherapy of Mobile Tumors

In this overview, we discuss some major issues related to the management of mobile tumors and gating in radiotherapy. For most types of organ motion, there are both interfraction and intrafraction components. For respiratory motion, the magnitudes of these 2 components can be comparable and therefore both should be handled carefully. The motion artifacts in computed tomography (CT) simulation are discussed and the 4-dimensional CT scan technique is recommended for treatment simulation of patients with mobile tumors. There are various methods for handling organ motion in treatment delivery. Caution should be exercised when using patient-specific motion information for treatment planning because motion characteristics may vary from the treatment simulation time to the treatment delivery sessions. Respiratory gating is potentially accurate, easy to implement, and may be widely adopted in clinical practice in the near future, if existing technical problems can be resolved.

Evaluation of an infrared camera and X-ray system using implanted fiducials in patients with lung tumors for gated radiation therapy

To report on the initial clinical use of a commercially available system to deliver gated treatment using implanted fiducials, in-room kV X-rays, and an infrared camera tracking system. ExacTrac Adaptive Gating from BrainLab is a localization system using infrared cameras and X-rays. Gating signals are the patient's breathing pattern obtained from infrared reflectors on the patient. kV X-rays of an implanted fiducial are synchronized to the breathing pattern. After localization and shift of the patient to isocenter, the breathing pattern is used to gate the radiation. Feasibility tests included localization accuracy, radiation output constancy, and dose distributions with gating. Clinical experience is reported on treatment of patients with small lung lesions. Localization accuracy of a moving target with gating was 1.7 mm. Dose constancy measurements showed insignificant change in output with gating. Improvements of dose distributions on moving targets improved with gating. Eleven patients with lung lesions were implanted with 20 mmx0.7 mm gold coil (Visicoil). The implanted fiducial was used to localize and treat the patients with gating. Treatment planning and repeat computed tomographic scans showed that the change in center of gross target volume (GTV) to implanted marker averaged 2.47 mm due in part to asymmetric tumor shrinkage. ExacTrac Adaptive Gating has been used to treat lung lesions. Initial system evaluation verified its accuracy and usability. Implanted fiducials are visible in X-rays and did not migrate.

Respiratory Gating for Radiotherapy

The discipline of radiation oncology has incorporated significant technological advances into its practice during recent years. Examples of these advances are the implementation of intensity-modulated radiotherapy, image-guided radiotherapy, and respiratory-gated radiotherapy. The technology and integration of these advances into clinical practice continue to evolve. These techniques have the common goal of the better conformation of a radiation dose distribution to the intended target volume, thereby potentially decreasing the toxicity of the treatment and concurrently permitting dose escalation and improved tumor control.

Respiration correlated cone-beam computed tomography and 4DCT for evaluating target motion in Stereotactic Lung Radiation Therapy

An image-guidance process for using cone-beam computed tomography (CBCT) for stereotactic body radiation therapy (SBRT) of peripheral lung lesions is presented. Respiration correlated CBCT on the treatment unit and four dimensional computed tomography (4DCT) from planning are evaluated for assessing respiration-induced target motion during planning and treatment fractions. Image-guided SBRT was performed for 12 patients (13 lesions) with inoperable early stage non-small cell lung carcinoma. Kilovoltage (kV) projections were acquired over a 360 degree gantry rotation and sorted based on the pixel value of an image-based aperture located at the air-tissue interface of the diaphragm. The sorted projections were reconstructed to provide volumetric respiration correlated CBCT image datasets at different phases of the respiratory cycle. The 4D volumetric datasets were directly compared with 4DCT datasets acquired at the time of planning. For ten of 12 patients treated, the lung tumour motion, as measured by respiration correlated CBCT on the treatment unit, was consistent with the tumour motion measured by 4DCT at the time of planning. However, in two patients, maximum discrepancies observed were 6 and 10 mm in the anterior-posterior and superior-inferior directions, respectively. Respiration correlated CBCT acquired on the treatment unit allows target motion to be assessed for each treatment fraction, allows target localization based on different phases on the breathing cycle, and provides the facility for adaptive margin design in radiation therapy of lung malignancies. The current study has shown that the relative motion and position of the tumour at the time of treatment may not match that of the planning 4DCT scan. Therefore, application of breathing motion data acquired at simulation for tracking or gating radiation therapy may not be suitable for all patients — even those receiving short course treatment techniques such as SBRT.

SU‐FF‐J‐70: Respiratory Gated Radiation Therapy Directly Using Patient's Fluoroscopic Images Instead of External Or Internal Markers

Purpose: To develop a respiratory gated radiation therapy (RGRT) technique by directly matching the patient's anatomical structures acquired during the imaging cycles to that in a reference image. Method and Materials: Studies showed that the external markers and the internal treatment volume are not always well correlated during the RGRT. Here we propose to measure the correlations of a reference image to the fluoroscopic images acquired during the breathing cycles. Such correlation measurement can be the surrogate for the external markers. A threshold of the correlation can be used to trigger the linear accelerator (Linac) for RGRT. With the capability of acquiring fluoroscopic images from the on‐board imaging (OBI) device developed by Varian, this approach becomes available. In this study, we measured the correlation coefficient (cc) and the root mean square (rms) error of the reference image to the on‐line fluoroscopic images. Results: In order to do efficient gating, the calculation of the correlation of one image to the reference should be 1–10ms and this can be easily achieved in our preliminary study. Also, the correlation can be better calculated by sparsely sampling the calculation pixels around the treatment volume rather than using a local ROI inside the volume. In the former, we found that both the cc and the rms demonstrated good periodicity. As we expected, the external marker locations recorded by the RPM software not always correlate well with the cc and rms. Conclusion: We demonstrated that RGRT by matching the patient's anatomical structures is highly feasible. The reference image can be the one from a gated fluoroscopic image or DRR from a gated CT scan. This technique will allow both gating and tracking and can be significant for high dose high‐precision radiation therapy with reduced margins for moving targets.