An evaluation of gating window size,delivery method,and composite field dosimetry of respiratory-gated IMRT

A respiratory gating system has been developed based on a commercial patient positioning system. The purpose of this study is to investigate the ability of the gating system to reproduce normal, nongated IMRT operation and to quantify the errors produced by delivering a nongated IMRT treatment onto a moving target. A moving phantom capable of simultaneous two-dimensional motion was built, and an analytical liver motion function was used to drive the phantom. Studies were performed to assess the effect of gating window size and choice of delivery method (segmented and dynamic multileaf collimation). Additionally, two multiple field IMRT cases were delivered to quantify the error in gated and nongated IMRT with motion. Dosimetric error between nonmoving and moving deliveries is related to gating window size. By reducing the window size, the error can be reduced. Delivery error can be reduced for both dynamic and segmented delivery with gating. For the implementation of dynamic IMRT delivery in this study, dynamic delivery was found to generate larger delivery errors than segmented delivery in most cases studied. For multiple field IMRT delivery, the largest errors were generated in regions where high field modulation was present parallel to the axis of motion. Gating was found to reduce these large errors to clinically acceptable levels.     

A phantom study on the positioning accuracy of the Novalis Body system

A phantom study was conducted to investigate inherent positioning accuracy of an image-guided patient positioning system-the Novalis Body system for three-dimensional (3-D) conformal radiotherapy. This positioning system consists of two infrared (IR) cameras and one video camera and two kV x-ray imaging devices. The initial patient setup was guided by the IR camera system and the target localization was accomplished using the kV x-ray imaging system. In this study, the IR marker shift and phantom rotation were simulated and their effects on the positioning accuracy were examined by a Rando phantom. The effects of CT slice thickness and treatment sites on the positioning accuracy were tested. In addition, the internal target shift was simulated and its effect on the positioning accuracy was examined by a water tank. With the application of the Novalis Body system, the positioning error of the planned isocenter was significantly reduced. The experimental results with the simulated IR marker shifts indicated that the positioning errors of the planned isocenter were 0.6 +/- 0.3, 0.5 +/- 0.2, and 0.7 +/- 0.2 mm along the lateral, longitudinal, and vertical axes, respectively. The experimental results with the simulated phantom rotations indicated that the positioning errors of the planned isocenter were 0.6 +/- 0.3, 0.7 +/- 0.2, and 0.5 +/- 0.2 mm along the three axes, respectively. The experimental results with the simulated target shifts indicated that the positioning errors of the planned isocenter were 0.6 +/- 0.3, 0.7 +/- 0.2, and 0.5 +/- 0.2 mm along the three axes, respectively. On average, the positioning accuracy of 1 mm for the planned isocenter was achieved using the Novalis Body system.     

Dosimetric effect of respiration-gated beam on IMRT delivery

Intensity modulated radiation therapy (IMRT) with a dynamic multileaf collimator (DMLC) requires synchronization of DMLC leaf motion with dose delivery. A delay in DMLC communication is known to cause leaf lag and lead to dosimetric errors. The errors may be exacerbated by gated operation. The purpose of this study was to investigate the effect of leaf lag on the accuracy of doses delivered in gated IMRT. We first determined the effective leaf delay time by measuring the dose in a stationary phantom delivered by wedge-shaped fields. The wedge fields were generated by a DMLC at various dose rates. The so determined delay varied from 88.3 to 90.5 ms. The dosimetric effect of this delay on gated IMRT was studied by delivering wedge-shaped and clinical IMRT fields to moving and stationary phantoms at dose rates ranging from 100 to 600 MU/min, with and without gating. Respiratory motion was simulated by a linear sinusoidal motion of the phantom. An ionization chamber and films were employed for absolute dose and 2-D dose distribution measurements. Discrepancies between gated and nongated delivery to the stationary phantom were observed in both absolute dose and 2-D dose distribution measurements. These discrepancies increased monotonically with dose rate and frequency of beam interruptions, and could reach 3.7% of the total dose delivered to a 0.6 cm3 ion chamber. Isodose lines could be shifted by as much as 3 mm. The results are consistent with the explanation that beam hold-offs in gated delivery allowed the lagging leaves to catch up with the delivered monitor units each time that the beam was interrupted. Low dose rates, slow leaf speeds and low frequencies of beam interruptions reduce the effect of this delay-and-catch-up cycle. For gated IMRT it is therefore important to find a good balance between the conflicting requirements of rapid dose delivery and delivery accuracy.     

Prediction of respiratory tumour motion for real-time image-guided radiotherapy

Image guidance in radiotherapy and extracranial radiosurgery offers the potential for precise radiation dose delivery to a moving tumour. Recent work has demonstrated how to locate and track the position of a tumour in real-time using diagnostic x-ray imaging to find implanted radio-opaque markers. However, the delivery of a treatment plan through gating or beam tracking requires adequate consideration of treatment system latencies, including image acquisition, image processing, communication delays, control system processing, inductance within the motor, mechanical damping, etc. Furthermore, the imaging dose given over long radiosurgery procedures or multiple radiotherapy fractions may not be insignificant, which means that we must reduce the sampling rate of the imaging system. This study evaluates various predictive models for reducing tumour localization errors when a real-time tumour-tracking system targets a moving tumour at a slow imaging rate and with large system latencies. We consider 14 lung tumour cases where the peak-to-peak motion is greater than 8 mm, and compare the localization error using linear prediction, neural network prediction and Kalman filtering, against a system which uses no prediction. To evaluate prediction accuracy for use in beam tracking, we compute the root mean squared error between predicted and actual 3D motion. We found that by using prediction, root mean squared error is improved for all latencies and all imaging rates evaluated. To evaluate prediction accuracy for use in gated treatment, we present a new metric that compares a gating control signal based on predicted motion against the best possible gating control signal. We found that using prediction improves gated treatment accuracy for systems that have latencies of 200 ms or greater, and for systems that have imaging rates of 10 Hz or slower.     

Dose verification for respiratory-gated volumetric modulated arc therapy

A novel commercial medical linac system (TrueBeam?, Varian Medical Systems, Palo Alto, CA) allows respiratory-gated volumetric modulated arc therapy (VMAT), a new modality for treating moving tumors with high precision and improved accuracy by allowing for regular motion associated with a patient's breathing during VMAT delivery. The purpose of this work is to adapt a previously-developed dose reconstruction technique to evaluate the fidelity of VMAT treatment during gated delivery under clinic-relevant periodic motion related to patient breathing. A Varian TrueBeam system was used in this study. VMAT plans were created for three patients with lung or pancreas tumors. Conventional 6 and 15 MV beams with flattening filter and high-dose-rate 10 MV beams with no flattening filter were used in these plans. Each patient plan was delivered to a phantom first without gating and then with gating for three simulated respiratory periods (3, 4.5 and 6 s). Using the adapted log-file-based dose reconstruction procedure supplemented with ion chamber array (Seven29?, PTW, Freiburg, Germany) measurements, the delivered dose was used to evaluate the fidelity of gated VMAT delivery. Comparison of Seven29 measurements with and without gating showed good agreement with gamma-index passing rates above 99% for 1%/1 mm dose accuracy/distance-to-agreement criteria. With original plans as reference, gamma-index passing rates were 100% for the reconstituted plans (1%/1 mm criteria) and 93.5-100% for gated Seven29 measurements (3%/3 mm criteria). In the presence of leaf error deliberately introduced into the gated delivery of a pancreas patient plan, both dose reconstruction and Seven29 measurement consistently indicated substantial dosimetric differences from the original plan. In summary, a dose reconstruction procedure was demonstrated for evaluating the accuracy of respiratory-gated VMAT delivery. This technique showed that under clinical operation, the TrueBeam system faithfully realized treatment plans with gated delivery. This methodology affords a useful tool for machine- and patient-specific quality assurance of the newly available respiratory-gated VMAT.     

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.     

Respiratory correlated cone-beam computed tomography on an isocentric C-arm

A methodology for 3D image reconstruction from retrospectively gated cone-beam CT projection data has been developed. A mobile x-ray cone-beam device consisting of an isocentric C-arm equipped with a flat panel detector was used to image a moving phantom. Frames for reconstruction were retrospectively selected from complete datasets based on the known rotation of the C-arm and a signal from a respiratory monitor. Different sizes of gating windows were tested. A numerical criterion for blur on the reconstructed image was suggested. The criterion is based on minimization of an Ising energy function, similar to approaches used in image segmentation or restoration. It is shown that this criterion can be used for the determination of the optimal gating window size. Images reconstructed from the retrospectively gated projection sequences using the optimal gating window data showed a significant improvement compared to images reconstructed from the complete projection datasets.     

Validation of target volume and position in respiratory gated CT planning and treatment

The capability of a commercial respiratory gating system based on video tracking of reflective markers to reduce motion-induced CT planning and treatment errors was evaluated. Spherical plastic shells (2.8-82 cm3), simulating the gross target volume (GTV), were placed in a water-filled body phantom that was moved sinusoidally along the longitudinal axis of the CT scanner and the accelerator for +/- 1 cm at 15-30 cycle/min. During gated CT imaging, the x-ray exposure was initiated by the gating system shortly before the end of expiration (so that the imaging time would be centered at the end of expiration); it was terminated by the scanner after completion of each slice. In nongated CT images, the target appeared distorted and often broken up. GTVs volume errors ranged 16%-110% in axial scans, and 7%-36% in spiral scans. In gated CT images, the spheres appeared 3 and 5 mm longer than their actual diameters (volume errors 2%-16%), at the respective respiration rates of 15 and 20 cycles/min. At 30 cycles/min the target appeared 1 cm longer, and volume error ranged 25%-53%. During treatment, gating kept the beam on for a duration equal to the CT acquisition time of 1 s/slice. The difference in positional errors between gated CT and portal films was 1 mm, regardless the size of residual motion errors. Because of the potential of suboptimal placement of the gating window between CT imaging and treatment, an extra 1.5-2.5 mm safety margin can be added regardless of the size of residual motion error. For respiratory rates > or = 30 cycles/min, the effectiveness of gating is limited by large residual motion in the 1 s CT acquisition time.     

Investigation of the reliability, accuracy, and efficiency of gated IMRT delivery with a commercial linear accelerator

This work reports an investigation on the reliability, accuracy, and efficiency of gated intensity modulated radiation therapy (IMRT) delivery with a commercial linear accelerator. The dosimetry measurements of segmented multileaf collimated IMRT (SMLC-IMRT) were performed by using radiographic films and a two-dimensional diode array. Testing involved a series of IMRT fields from actual patients combined with some manually generated fields. To examine the delivery time, dosimetry plans of standard beamlet IMRT, direct-aperture-optimized (DAO) IMRT, compensator IMRT, and three-dimensional conformal radiotherapy with wedges were delivered with and without gating. The results demonstrated that the gated SMLC-IMRT can be reliably and accurately delivered on this type of accelerators, as long as extremely high interruption frequencies and very low number of monitor units per segment are avoided. Beam flatness exceeded 5% and monitor linearity deviated more than 3% for the gated operation with 2.5 s breathing cycle and 20% duty cycle with segment sizes less than 10 MU. Gating does not change multi leaf collimator (MLC) positioning accuracy. The DAO IMRT is preferred for gated delivery because of its short delivery time.     

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.     

Dosimetric and radiobiological impact of dose fractionation on respiratory motion induced IMRT delivery errors: a volumetric dose measurement study

Respiratory motion can introduce substantial dose errors during IMRT delivery. These errors are difficult to predict because of the nonsynchronous interplay between radiation beams and tissues. The present study investigates the impact of dose fractionation on respiratory motion induced dosimetric errors during IMRT delivery and their radiobiological implications by using measured 3D dose. We focused on IMRT delivery with dynamic multileaf collimation (DMLC-IMRT). IMRT plans using several beam arrangements were optimized for and delivered to a polystyrene phantom containing a simulated target and critical organs. The phantom was set in linear sinusoidal motion at a frequency of 15 cycles/min (0.25 Hz). The amplitude of the motion was +/- 0.75 cm in the longitudinal direction and +/- 0.25 cm in the lateral direction. Absolute doses were measured with a 0.125 cc ionization chamber while dose distributions were measured with transverse films spaced 6 mm apart. Measurements were performed for varying number of fractions with motion, with respiratory-gated motion, and without motion. A tumor control probability (TCP) model for an inhomogeneously irradiated tumor was used to calculate and compare TCPs for the measurements and the treatment plans. Equivalent uniform doses (EUD) were also computed. For individual fields, point measurements using an ionization chamber showed substantial dose deviations (-11.7% to 47.8%) for the moving phantom as compared to the stationary phantom. However, much smaller deviations (-1.7% to 3.5%) were observed for the composite dose of all fields. The dose distributions and DVHs of stationary and gated deliveries were in good agreement with those of treatment plans, while those of the nongated moving phantom showed substantial differences. Compared to the stationary phantom, the largest differences observed for the minimum and maximum target doses were -18.8% and +19.7%, respectively. Due to their random nature, these dose errors tended to average out over fractionated treatments. The results of five-fraction measurements showed significantly improved agreement between the moving and stationary phantom. The changes in TCP were less than 4.3% for a single fraction, and less than 2.3% for two or more fractions. Variation of average EUD per fraction was small (< 3.1 cGy for a fraction size of 200 cGy), even when the DVHs were noticeably different from that of the stationary tumor. In conclusion, IMRT treatment of sites affected by respiratory motion can introduce significant dose errors in individual field doses; however, these errors tend to cancel out between fields and average out over dose fractionation. 3D dose distributions, DVHs, TCPs, and EUDs for stationary and moving cases showed good agreement after two or more fractions, suggesting that tumors affected by respiration motion may be treated using IMRT without significant dosimetric and biological consequences.     

Monte Carlo dose calculation of segmental IMRT delivery to a moving phantom using dynamic MLC and gating log files

Respiratory gating is emerging as a tool to limit the effect of motion for liver and lung tumors. In order to study the impact of target motion and gated intensity modulated radiation therapy (IMRT) delivery, a computer program was developed to simulate segmental IMRT delivery to a moving phantom. Two distinct plans were delivered to a rigid-motion phantom with a film insert in place under four conditions: static, sinusoidal motion, gated sinusoidal motion with a duty cycle of 25% and gated sinusoidal motion with duty cycle of 50% under motion conditions of a typical patient (A = 1 cm, T = 4 s). The MLC controller log files and gating log files were retained to perform a retrospective Monte Carlo dose calculation of the plans. Comparison of the 2D planar dose distributions between simulation and measurement demonstrated that our technique had at least 94% of the points passing gamma criteria of 3% for dose difference and 3 mm as the distance to agreement. This note demonstrates that the use of dynamic multi-leaf collimator and respiratory monitoring system log files together with a fast Monte Carlo dose calculation algorithm is an accurate and efficient way to study the dosimetric effect of motion for gated or non-gated IMRT delivery on a rigidly-moving body.     

Prospective respiratory-gated micro-CT of free breathing rodents

Microcomputed tomography (Micro-CT) has the potential to noninvasively image the structure of organs in rodent models with high spatial resolution and relatively short image acquisition times. However, motion artifacts associated with the normal respiratory motion of the animal may arise when imaging the abdomen or thorax. To reduce these artifacts and the accompanying loss of spatial resolution, we propose a prospective respiratory gating technique for use with anaesthetized, free-breathing rodents. A custom-made bed with an embedded pressure chamber was connected to a pressure transducer. Anaesthetized animals were placed in the prone position on the bed with their abdomens located over the chamber. During inspiration, the motion of the diaphragm caused an increase in the chamber pressure, which was converted into a voltage signal by the transducer. An output voltage was used to trigger image acquisition at any desired time point in the respiratory cycle. Digital radiographic images were acquired of anaesthetized, free-breathing rats with a digital radiographic system to correlate the respiratory wave form with respiration-induced organ motion. The respiratory wave form was monitored and recorded simultaneously with the x-ray radiation pulses, and an imaging window was defined, beginning at end expiration. Phantom experiments were performed to verify that the respiratory gating apparatus was triggering the micro-CT system. Attached to the distensible phantom were 100 microm diameter copper wires and the measured full width at half maximum was used to assess differences in image quality between respiratory-gated and ungated imaging protocols. This experiment allowed us to quantify the improvement in the spatial resolution, and the reduction of motion artifacts caused by moving structures, in the images resulting from respiratory-gated image acquisitions. The measured wire diameters were 0.135 mm for the stationary phantom image, 0.137 mm for the image gated at end deflation, 0.213 mm for the image gated at peak inflation, and 0.406 mm for the ungated image. Micro-CT images of anaesthetized, free-breathing rats were acquired with a General Electric Healthcare eXplore RS in vivo micro-CT system. Images of the thorax were acquired using the respiratory cycle-based trigger for the respiratory-gated mode. Respiratory gated-images were acquired at inspiration and end expiration, during a period of minimal respiration-induced organ motion. Gated images were acquired with a nominal isotropic voxel spacing of 44 microm in 20-25 min (80 kVp, 113 mAs, 300 ms imaging window per projection). The equivalent ungated acquisitions were 11 min in length. We observed improved definition of the diaphragm boundary and increased conspicuity of small structures within the lungs in the gated images, when compared to the ungated acquisitions. In this work, we have characterized the externally monitored respiratory wave form of free-breathing, anaesthetized rats and correlated the respiration-induced organ motion to the respiratory cycle. We have shown that the respiratory pressure wave form is an excellent surrogate for the radiographic organ motion. This information facilitates the definition of an imaging window at any phase of the breathing cycle. This approach for prospectively gated micro-CT can provide high quality images of anaesthetized free-breathing rodents.     

Time delay measurement for linac based treatment delivery in synchronized respiratory gating radiotherapy

A time delay in a respiratory gating system could cause an unexpected phase mismatch for synchronized gating radiotherapy. This study presents a method of identifying and measuring the time delay in a gating system. Various port films were taken for a motion phantom at different gating window levels with a very narrow window size. The time delay for the gating system was determined by comparing the motion curve (the position of a moving object versus the gating time) measured in the port films to the motion curve determined by the video cameras. The measured time delay for a linac-based gating system was 0.17+/-0.03 s. This time delay could induce target missing if it was not properly taken into account for the synchronized gating radiotherapy. Measurement/verification of the time delay should be considered as an important part of the accepting/commissioning test before the clinical use of the gating system.     

Technical and dosimetric aspects of respiratory gating using a pressure-sensor motion monitoring system

This work introduces a gating technique that uses 4DCT to determine gating parameters and to plan gated treatment, and employs a Siemens linear accelerator to deliver the gated treatment. Because of technology incompatibility, the 4DCT scanner (LightSpeed, GE) and the Siemens accelerator require two different motion-monitoring systems. The motion monitoring system (AZ-773V, Anzai Med.) used for the gated delivery utilizes a pressure sensor to detect the external respiratory motion (pressure change) in real time. Another system (RPM, Varian) used for the 4DCT scanner (LightSpeed, GE) is based on an infrared camera to detect motion of external markers. These two motion monitoring systems (RPM and Anzai systems) were found to correlate well with each other. The depth doses and profile measured for gated delivery (with a duty cycle of 25% or 50%) were found to agree within 1.0% with those measured for ungated delivery, indicating that gating did not significantly alter beam characteristics. The measurement verified also that the MU linearity and beam output remained unchanged (within 0.3%). A practical method of using 4DCT to plan a gated treatment was developed. The duty cycle for either phase or amplitude gating can be determined based on 4DCT with consideration of set-up error and delivery efficiency. The close-loop measurement involving the entire gating process (imaging, planning, and delivery) showed that the measured isodose distributions agreed with those intended, validating the accuracy and reliability of the gating technique. Based these observations, we conclude that the gating technique introduced in this work, integrating Siemens linear accelerator and Anzai pressure sensor device with GE/Varian RPM 4DCT, is reliable and effective, and it can be used clinically to account for respiratory motion during radiation therapy.     

呼吸门控对呼吸运动引起组织位移造成靶区精度复位差的作用

背景：在常规高强度聚焦超声治疗中,如何避免呼吸运动引起的脏器组织位移从而影响辐照靶区定位精度及治疗的安全性是目前急需解决的问题。 目的：探讨在高强度聚焦超声治疗过程中,利用呼吸门控技术提高高强度聚焦超声治疗靶点定位精度的可行性。 方法：在相同声强、辐照时间情况下,利用压阻式微型呼吸传感器,基于呼吸门控技术控制高强度聚焦超声治疗设备对活体兔肝脏组织进行定点辐照,与用常规高强度聚焦超声治疗方法的定点辐照对比,并检测凝固性坏死的情况。 结果与结论：辐照后将动物解剖,取出其肝脏,找到辐照靶区,测得利用呼吸门控技术定点辐照的最大剖面焦区大小为 4.5 mm×2.0 mm,用常规的高强度聚焦超声治疗方法定点辐照的最大剖面焦区大小为9.0 mm×2.5 mm。提示呼吸门控技术能够有效消除呼吸运动引起组织移位造成的靶区定位精度的影响。     

Motion adaptive x-ray therapy: a feasibility study

Intrafraction motion caused by breathing requires increased treatment margins for chest and abdominal radiotherapy and may lead to 'motion artefacts' in dose distributions during intensity modulated radiotherapy (IMRT). Technologies such as gated radiotherapy may significantly increase the treatment time, while breath-hold techniques may be poorly tolerated by pulmonarily compromised patients. A solution that allows reduced margins and dose distribution artefacts, without compromising delivery time, is to synchronously follow the target motion by adapting the x-ray beam using a dynamic multileaf collimator (MLC), i.e. motion adaptive x-ray therapy, or MAX-T for short. Though the target is moving with time, in the MAX-T beam view the target is static. The MAX-T method superimposes the target motion due to respiration onto the beam originally planned for delivery. Thus during beam delivery the beam is dynamically changing position with respect to the isocentre using a dynamic MLC, the leaf positions of which are dependent upon the target position. Synchronization of the MLC motion and target motion occurs using respiration gated radiotherapy equipment. The concept and feasibility of MAX-T and the capability of the treatment machine to deliver such a treatment were investigated by performing measurements for uniform and IMRT fields using a mechanical sinusoidal oscillator to simulate target motion. Target dose measurements obtained using MAX-T for a moving target were found to be equivalent to those delivered to a static target by a static beam.     

Individualized gating windows based on four-dimensional CT information for respiration-gated radiotherapy

The purpose of this work is to relate the gating window and displacement of a moving tumor target and develop a systematic method to individualize the gating window for respiration-gated radiation therapy (RT). As the relationship between patient anatomy and respiration phase is contained in 4D images, we aim to quantify this information and utilize the data to guide gated treatment planning. After 4D image acquisition, the target and organs at risk were delineated manually on the selected gating phase. The contours were propagated automatically onto every phase-specific image set using a control volume-based contour mapping technique. The mean and maximum distances between the contours in the gating phase and each of other phases were evaluated in three dimensions. The gating window was determined in such a way that the residual movement of the target within the window is smaller or equal to the patient's setup error. The proposed method was applied to plan the gated treatments of 12 lung cancer patients. As a result of this work, a method to calculate patient-specific gating windows has been developed. The general reference drawn from this study is that, with the aide of 4D images and automated 4D contour propagation, it is feasible to individualize the gating widow selection. As compared with the current practice, the proposed technique has a potential to eliminate the guesswork involved in choosing a gating window and avoid dosimetric error in planning gated RT. In conclusion, individualization of gating windows reduces the subjectivity in respiration-gated RT and improves the treatment of moving targets.     

Commissioning and quality assurance of an optically guided three-dimensional ultrasound target localization system for radiotherapy

Recently, there has been proliferation of image-guided positioning systems for high-precision radiation therapy, with little attention given to quality assurance procedures for such systems. To ensure accurate treatment delivery, errors in the imaging, localization, and treatment delivery processes must be systematically analyzed. This paper details acceptance tests for an optically guided three-dimensional (3D) ultrasound system used for patient localization. While all tests were performed using the same commercial system, the general philosophy and procedures are applicable to all systems utilizing image guidance. Determination of absolute localization accuracy requires a consistent stereotactic, or three-dimensional, coordinate system in the treatment planning system and the treatment vault. We established such a coordinate system using optical guidance. The accuracy of this system for localization of spherical targets imbedded in a phantom at depths ranging from 3 to 13 cm was determined to be (average +/- standard deviation) AP = 0.2 +/- 0.7 mm, Lat = 0.9 +/- 0.6 mm, Ax = 0.6 +/- 1.0 mm. In order to test the ability of the optically guided 3D ultrasound localization system to determine the magnitude of an internal organ shift with respect to the treatment isocenter, a phantom that closely mimics the typical human male pelvic anatomy was used. A CT scan of the phantom was acquired, and the regions of interest were contoured. With the phantom on the treatment couch, optical guidance was used to determine the positions of each organ to within imaging uncertainty, and to align the phantom so the plan and treatment machine coordinates coincided. To simulate a clinical misalignment of the treatment target, the phantom was then shifted by different precise offsets, and an experimenter blind to the offsets used ultrasound guidance to determine the magnitude of the shifts. On average, the magnitude of the shifts could be determined to within 1.0 mm along each axis.