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MicroRT - Small animal conformal irradiator

A novel small animal conformal radiation therapy system has been designed and prototyped: MicroRT. The microRT system integrates multimodality imaging, radiation treatment planning, and conformal radiation therapy that utilizes a clinical 192Ir isotope high dose rate source as the radiation source (teletherapy). A multiparameter dose calculation algorithm based on Monte Carlo dose distribution simulations is used to efficiently and accurately calculate doses for treatment planning purposes. A series of precisely machined tungsten collimators mounted onto a cylindrical collimator assembly is used to provide the radiation beam portals. The current design allows a source-to-target distance range of 1-8 cm at four beam angles: 0 degrees (beam oriented down), 90 degrees, 180 degrees, and 270 degrees. The animal is anesthetized and placed in an immobilization device with built-in fiducial markers and scanned using a computed tomography, magnetic resonance, or positron emission tomography scanner prior to irradiation. Treatment plans using up to four beam orientations are created utilizing a custom treatment planning system-microRTP. A three-axis computer-controlled stage that supports and accurately positions the animals is programmed to place the animal relative to the radiation beams according to the microRTP plan. The microRT system positioning accuracy was found to be submillimeter. The radiation source is guided through one of four catheter channels and placed in line with the tungsten collimators to deliver the conformal radiation treatment. The microRT hardware specifications, the accuracy of the treatment planning and positioning systems, and some typical procedures for radiobiological experiments that can be performed with the microRT device are presented.

Development of the 4D Phantom for patient-specific end-to-end radiation therapy QA

In many patients respiratory motion causes motion artifacts in CT images, thereby inhibiting precise treatment planning and lowering the ability to target radiation to tumors. The 4D Phantom, which includes a 3D stage and a 1D stage that each are capable of arbitrary motion and timing, was developed to serve as an end-to-end radiation therapy QA device that could be used throughout CT imaging, radiation therapy treatment planning, and radiation therapy delivery. The dynamic accuracy of the system was measured with a camera system. The positional error was found to be equally likely to occur in the positive and negative directions for each axis, and the stage was within 0.1 mm of the desired position 85% of the time. In an experiment designed to use the 4D Phantoms encoders to measure trial-to-trial precision of the system, the 4D Phantom reproduced the motion during variable bag ventilation of a transponder that had been bronchoscopically implanted in a canine lung. In this case, the encoder readout indicated that the stage was within 10 microns of the sent position 94% of the time and that the RMS error was 7 microns. Motion artifacts were clearly visible in 3D and respiratory-correlated (4D) CT scans of phantoms reproducing tissue motion. In 4D CT scans, apparent volume was found to be directly correlated to instantaneous velocity. The system is capable of reproducing individual patient-specific tissue trajectories with a high degree of accuracy and precision and will be useful for end-to-end radiation therapy QA.

A convolution-adapted ratio-TAR algorithm for 3D photon beam treatment planning

A convolution-adapted ratio of tissue-air ratios (CARTAR) method of dose calculation has been developed at the Mallinckrodt Institute of Radiology. This photon pencil-beam algorithm has been developed and implemented specifically for three-dimensional treatment planning. In a standard ratio of tissue-air ratios (RTAR) algorithm, doses to points in irregular field geometries are not adequately modeled. This is inconsistent with the advent of conformal therapy, the goal of which is to conform the dose distribution to the target volume while sparing neighboring sensitive normal critical structures. This motivated us to develop an algorithm that can model the beam penumbra near irregular field edges, while retaining much of the speed for the original RTAR algorithm. The dose calculation algorithm uses two-dimensional (2D) convolutions, computed by 2D fast Fourier transform, of pencil-beam kernels with a beam transmission array to calculate 2D off-axis profiles at a series of depths. These profiles are used to replace the product of the transmission function and measured square-field boundary factors used in the standard RTAR calculation. The 2D pencil-beam kernels were derived from measured data for each modality using commonly available dosimetry equipment. The CARTAR algorithm is capable of modeling the penumbra near block edges as well as the loss of primary and scattered beam in partially blocked regions. This paper describes the dose calculation algorithm, implementation, and verification.

Compensation of breathing motion artifacts in thoracic PET images by wavelet-based deconvolution

In biological imaging of thoracic tumors using FDG-PET, blurring due to breathing motion often significantly degrades the quality of the observed image, which then obscures the tumor boundary. The effect could be detrimental in small lesions. We demonstrate a deconvolution technique that combines patient-specific motion estimates of tissue trajectories with wavelet decomposition to compensate for breathing-motion induced artifacts. The lung motion estimates were obtained using a breathing model that maps spatial trajectories in CT data as a function of tidal volume and airflow measured by spirometry. Initial results showed good improvement in the spatial resolution, especially in the direction of major lung motion (craniocaudal) on phantom data as well as on clinical data with large or small tumors

MO-D-I-611-06: Reduction of Motion Blurring Artifact Using Respiratory Gated CT: A Quantitative Evaluation

Purpose: To develop a technique for reducing respiratory motion blurring artifacts using respiratory-gated CT, and to quantitatively evaluate the artifact reduction. Method and Materials: Similar to electrocardiogram (ECG) gated imaging for the heart, a synthetic sinogram was built from multiple scans intercepting a respiration gated window. A gated CT image was then reconstructed by the filtered back-projection algorithm. CT images of wedge phantoms moving at different speeds, and 13 patients were taken with synchronized respiratory motion measurement. The scanner was operated in cine mode with 100 and 15 scans (0.5 s rotation) acquired consecutively at each couch position for phantoms and patients, respectively. Two error functions were fit to the CT profile across the air-phantom or lung-diaphragm boundaries for a quantitative evaluation of the blurring artifact. Results: The blurring artifact was reduced significantly at the air-phantom boundaries in the gated image. The gated image of phantoms with a motion of 20 mm/s showed similar blurring artifacts as the non-gated image of phantoms with a motion of 10 mm/s. The blurring artifact had a linear relationship with both the speed and the tangent of the wedge angles. The blurring artifacts were also reduced at the lung-diaphragm boundaries for patients. Centers of the two fitted error functions provided a reliable measure of large blurring, and were found equivalent to 25% and 75% locations of the CT profile. Conclusion: The respiratory gated CT imaging reduced the blurring artifacts for both moving phantoms and patients. This technique may be applied for other tomographic imaging modalities that require long imaging times with significant motion blurring artifacts, such as PET.

TU-D-J-6C-01: A Comparison Between Amplitude Sorting and Phase Sorting Using External Respiratory Measurements for 4D CT

Purpose: To compare amplitude sorting and phase sorting techniques using external respiratory measurements for 4D‐CT for patients undergoing quiet breathing. Method and Materials: We have developed a 4D CT technique for mapping respiratory motion in radiotherapytreatment planning. A 16‐slice CT scanner was operated in cine mode to acquire 25 scans consecutively at each couch position. The scans were sorted into 12 respiratory‐windows based on the amplitude and direction (inhalation or exhalation), and on the phase (0–360°) of a synchronized external respiratory measurement. An air content measure (the amount of air in a 16‐slice CT segment, used as a surrogate for internal motion) was correlated to the respiratory amplitude and phase throughout the lung.Imagesreconstructed based on the two sorting techniques were displayed for a qualitative comparison. Also, the variations in the amplitude of the respiratory measurement during the entire scan session were compared using 8, 12, 24, and 48 respiratory windows. Results: The air content showed a higher correlation with the respiratory amplitude than with the respiratory phase for most cases. Imagesreconstructed based on the amplitude sorting technique displayed fewer artifacts, especially at the lung‐diaphragm boundaries, than imagesreconstructed based on the phase sorting technique. The variations in the respiratory amplitude were much smaller with amplitude sorting than those with phase sorting. These variations decreased significantly with finer amplitude respiratory windows while showed insignificant changes with finer phase respiratory windows. Conclusion: The amplitude sorting was generally better than phase sorting, especially for patients whose breathing was less reproducible. The use of finer respiratory windows did not improve the consistency for phase sorting. Keywords: 4‐D CT, respiratory sorting, motion, radiotherapy.

TU‐FF‐A4‐06: Dynamic Accuracy of An Implanted Wireless AC Electromagnetic Sensor for Guided Radiation Therapy; Implications for Real‐Time Tumor Position Tracking

Purpose: A wireless tumor localization and tracking system using three implanted ACelectromagnetic transponders is in clinical trials for use in prostate cancer (Calypso® Medical). Phantom‐based studies have shown sub‐millimeter spatial localization accuracy in static tests. Accuracy has not been evaluated for dynamic motion found in lung tumors. This study was designed to determine the feasibility of using this patient positioning system for real‐time tumor‐tracking. Materials and Methods: A 4‐dimensional (4D) stage capable of arbitrary multidimensional motion with speeds up to 10 cm/sec was constructed. Two elliptical trajectory paths were created with peak‐to‐peak motion of 1cm × 2cm × 1cm and 2cm × 4cm × 2cm in the x, y and z directions. Each trajectory was operated with periods of 15 – 20 cycles per minute. The Calypso System was operated using one and two transponders with radiofrequency signal integration times of 33 ms 100 ms. The transponders were mounted on the 4D‐stage, with the ellipse centroids positioned 14 cm from the array. The effects of ellipse size, speed, number of transponders and signal integration time on transponder localization accuracy were evaluated by comparing the intended and measured trajectories. Results: The root mean square (RMS) position difference was less than 1 mm for all tested combinations. While small, the RMS error was largest for the large ellipse at 20 cycles per minute compared with the small ellipse at 15 cycles per minute. The single‐transponder system with 67 ms integration time had the smallest overall error, with a maximum single‐point error of 1.3mm. Conclusions Use of a wireless electromagnetic implanted transponder system for real‐time tumor‐tracking is feasible, with RMS errors less than 1mm for high‐speed multidimensional ellipses. This compares favorably with continuous fluoroscopic tracking methods without an ionizing radiation burden. This work is currently being expanded to patient‐derived tumor trajectories.

TH‐C‐204B‐10: Implementation of a Small Animal Image Guided Microirradiator: The MicroIGRT

Purpose: Implementation of a conformal small animal image guided microirradiation therapy instrument (microIGRT) consisting of a cone beam microCT subsystem for submillimeter low dose structural imagingimage guided radiotherapy and orthovoltage conformal microirradiation with high dose rate and high throughput. Method and Materials: The microCT subsystem is based on an 80kVp micro‐focus x‐ray source with 75×75 μm2 focal spot and a flat panel amorphous silicondetector with 1024×1024 pixels. The irradiator consists of a high power commercially available 320 kVp orthovoltage source with a 0.4×0.4 mm2 focal spot that can be operated at a nominal power of 800W. The beam characteristics are controlled with two variable jaws used to pre‐collimate the radiation beam along each orthogonal direction. An aperture exchange mechanism is used to conform the beam cross section by using computer generated apertures. The microCT radiationdose the orthovoltage source spectral output and dose rate are under evaluation using a mouse digital phantom and a pencil beam algorithm. Results:CTimaging with micrometric resolution is achievable using 128 projections and a maximum radiationdose of 2cGy. Automatic animal positioning and handling is performed within sub‐millimeter precision. The treatment beam can be aimed at different latitude and longitude angles and translated with 500 μm steps. The source was tested to deliver a radiationdose rate of 20 Gy/min when is filtered to a half‐value layer of 4.6 mm Cu. Conclusion: We present our progress and initial tests of a highly conformal image guided small animal microirradiator.

A fast inverse consistent deformable image registration method based on symmetric optical flow computation

Deformable image registration is widely used in various radiation therapy applications including 4D-CT and treatment planning adaptation. In this work, a simple and efficient inverse consistency deformable registration method is proposed with aims of higher registration accuracy and faster convergence speed. Instead of registering image I to the second image J, two images are symmetrically deformed toward one another in multiple passes, until both deformed images are registered. In every pass, a delta motion field is computed by minimizing a symmetric optical flow system cost function using the modified optical flow algorithms. The images are then further deformed with the delta motion field in positive and negative directions, respectively, and then used for the next pass. The magnitude of the delta motion field is forced to be less than 0.4 voxel for every pass in order to guarantee the smoothness and invertibility of the two overall motion fields which are accumulating the delta motion fields in positive and negative directions, respectively. The final motion fields to register the original images I and J, in either direction, are calculated by inverting one overall motion field and composing the inversion result with the other overall motion field. The final motion fields are inversely consistent and this is ensured by the symmetric way that registration is carried out. Results suggest that the method is able to improve the overall accuracy by 30% or more, reduce the inverse consistency error, and increase the convergence rate. The computation speed may slightly decrease, or increase in some cases because the new method converges faster. Comparing to previously published inverse consistency algorithms, the proposed method is simpler in theory, easier to implement, and faster.

SU-GG-T-380: Design and Shielding Considerations for the World's First Compact Synchrocyclotron Proton Therapy Unit

Purpose: Our institution will soon take delivery of the Monarch 250 Compact Proton Therapy Unit (Still River Systems). We performed facility shielding calculations that accounted for the produced neutron flux. Method and Materials: Workload assumptions (80Gy/day) yielded the number of protons produced per day. The vendor did not provide extraction efficiency, neutron yields from the treatment field shaping system, or neutron yield estimates from the synchrocyclotron. To provide a conservative neutron yield we assumed; the largest scatteredfield size and complete portal blocking, conservatively low cyclotron production efficiency (25%), and a high quality factor (20) for neutron dose equivalent calculations. Neutrons were assumed to be produced at the isocenter location, by stopping the SOBP protons in a Cu target. The room design includes a sub‐floor to allow rotation of the accelerator below the sub‐floor level. Classical analytical methods were used to calculate required barrier thicknesses based on neutron yields, accounting for angular dependences, and based on concrete (standard or high density (HD)), earth, and steel. Maze calculations used analytical approaches, heavily influenced by the number of bounce paths, and maze area. Conservative use and occupancy factors were used. Results: The shielding calculations resulted in barrier thicknesses similar to existing proton facilities; from 4ft to 6.25ft of HD concrete. Some design features included; reducing the maze ceiling height to reduce the source area of scatteredneutrons, adding an additional leg at the door entrance to allow for an unshielded door, and adding steel to surround unavoidable barrier voids. To reduce cost, the barriers that did not shield occupy‐able space, used standard concrete. Conclusion: The neutron yield from the Monarch 250 is unknown, so conservative values were used in the shielding calculations. Vault design strategies were developed to reduce overall construction cost. We allow for contingencies if we underestimated any barrier thickness.