The system presented herein consists of a custom 200 kV electron tube with deflection magnets and stationary water-cooled targets for radiotherapy (RT) and imaging. The electron beam is deflected and dwelled along 41 discrete anode locations equally spaced by 1 cm in a line, at equivalent speeds of 1000 cm/s, to create a focused radiotherapy source. The treatment beam is collimated into a triangular polyhedron shape, producing a 4 cm focal spot at the isocenter and corresponding planning-treatment-volume (PTV). This beam shape would allow entry dose to be distributed over large areas for skin sparing. The source is mounted on a ring gantry that rotates at speeds up to 1.5 rpm. Preliminary dose rate measurements were collected in air at 140 kV beam energy, up to 80 mA beam current. Radiographic film was used to collect an image of the treatment beam at isocenter. Results are presented and can be extrapolated to a dose rate of 2 Gy/min for a 140 kV, 200 mA beam. The electron beam can be deflected within 1 ms from therapy path to an additional array of 19 imaging targets, which provides hardware capabilities for real-time tomosynthesis and image-guided RT. Onboard cone beam CT for patient positioning is also available. The utilization of 200 kV beam treatment energies compared to MV greatly reduces the required shielding (4-6 mm lead vs. 1-2 m concrete) and the cost of radiotherapy system installations. Systems can be mounted onto standard mobile trailers for use at remote locations.
Retrospective kV x-ray 4DCT treatment planning for lung cancer MV linac treatment is becoming a standard-of-care for this widely used procedure for the largest cancer cause-of-death in the US. It currently provides the best estimate of a fixed-in-time but undulating and closed 3D "shell" to which a minimum curative-intent radiation dose should be delivered to provide the best estimated patient survival and the least morbidity, usually characterized by quantitative dose-volume-histograms (DVHs). Unfortunately this closed shell volume or internal target volume (ITV) currently has to be increased enough to enclose the full range of respiratory lesion motion (plus set-up etc. uncertainties) which cannot yet be accurately determined in real time during treatment delivery. With accurate motion-tracking, the planning target volume (PTV) or outer “shell” may be reduced by up to 40%. However there is no single 2D plane that precisely follows the reduced-PTV-volume’s 3D respiratory motion, currently best estimated by the retrospective hand contouring by a trained and experienced MD radiation oncology MD using the full 3D-time information of 4DCT. Once available, 3D motion tracking in real time has the potential to substantially decrease DVH doses to surrounding organs-at-risk (OARs), while maintaining or raising the curative-intent dose to the lesion itself. The assertion argued here is that, the 3D volume-rendered imaging of lung cancer lesion-trajectories in real-time from TumoTrak digital x-ray tomosythesis, has the potential to provide more accurate 3D motion tracking and improved dose delivery at lower cost than the real time, 2D single slice imaging of MRI-guided radiotherapy.
The combinations of a 60 fps kV x-ray flat panel imager, a 19 focal spot kV x-ray tube enabled by a steered electron beam, plus SART or SIRT sliding reconstruction via GPUs, allow real time 6 fps 3D-rendered digital tomosynthesis tracking of the respiratory motion of lung cancer lesions. The tube consists of a “U” shaped vacuum chamber with 19 tungsten anodes, spread uniformly over 3 sides of a 30 cm x 30 cm square, each attached to a cylindrical copper heat sink cooled by flowing water. The beam from an electron gun was steered and focused onto each of the 19 anodes in a predetermined sequence by a series of dipole, quadrupole and solenoid magnets. The imager consists of 0.194 mm pixels laid out in 1576 rows by 2048 columns, binned 4x4 to achieve 60 fps projection image operation with 16 bits dynamic range. These are intended for application with free breathing patients during ordinary linac C-arm radiotherapy with modest modifications to typical system hardware or to standard clinical treatment delivery protocols. The sliding digital tomosynthesis reconstruction is completed after every 10 projection images acquired at 60 fps, but using the last 19 such projection images for each such reconstruction at less than 8 mAs exposure per 3D rendered frame. Comparisons, to “ground truth” optical imaging and to diagnostic 4D CT (10 phase) images, are being used to determine the accuracy and limitations of the various versions of this new “19 projection image x-ray tomosynthesis fluorooscopy” motion tracking technique.