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.
Field-emission x-ray source arrays have been studied for both tomosynthesis and CT applications, however these arrays tend to have limited output. We propose the use of multi-source x-ray arrays using thermionic cathodes, contained within a single vacuum housing. A prototype 3-source x-ray array has been fabricated and tested, and the utility of multi-x-ray-source arrays has been demonstrated using physical simulations in both tomosynthesis and in cone beam CT. The prototype x-ray tube made use of a cylindrical molybdenum anode, machined to have 3 specific focal tracks. Grid-controlled cathode assemblies were fabricated and aligned to each focal tract, and the individual x-ray focal spots were evaluated with a star pattern at 35 kV and 40 mA. The 3-source assembly was used to physically simulate tomosynthesis imaging geometry, and tomosynthesis images of a lemon were obtained. Physical simulations using a cone beam breast CT scanner were also performed, by vertically moving the single x-ray source into 5 different locations – simulating 5 different source positions. A new geometry for cone beam CT imaging is proposed, where each source of a multi-x-ray source array is individually collimated to eliminate rays involving large cone angles. This geometry also allows three sources to be simultaneously pulsed onto a single flat panel detector, achieving better duty cycle and view sampling in cone beam CT. A reconstruction algorithm was written to accommodate the different source positions, and phantoms designed to demonstrate cone beam artifacts were imaged. The tomosynthesis images illustrate appropriate depth resolution in the test object. Analysis of the CT data demonstrate marked improvement compared to one source. We conclude that multi-source x-ray arrays using thermionic cathodes will have important applications in medical imaging, especially breast tomosynthesis and cone beam computed tomography.
This study examines the potential of a multisource x-ray system to reduce cone beam artifacts in a dedicated breast CT acquisition geometry. A breast CT scanner (Doheny), built at our institution, was used to demonstrate the potential of multiple x-ray sources in a single x-ray tube housing. Both 3 focal spot and 5 focal spot thermionic systems were physically simulated in this study. The x-ray tube is mounted on a vertical actuator on the breast CT system gantry, allowing the single x-ray source to be positioned at different vertical locations in the field of view. Five acquisition geometries were used to acquire raw cone beam CT data with the x-ray source locations placed at 2 cm intervals. Data was collected using a 15-cm tall Defrise phantom. The individual acquisitions of raw CT data were reconstructed using filtered back projection, aligned and summed. The reconstructed CT volume data set using three sources and five sources were compared to that produced from a single source. Both multi-source datasets demonstrated less visible cone beam artifact, and the contrast clearly improved. The resolvable field of view in the vertical direction was extended by 50% when comparing the one source to the three source geometry and extended by 120% when comparing the one source to the five source geometry. This physical simulation of a multisource x-ray CT system successfully demonstrated that a reduction in cone beam CT artifacts could be achieved using a multi-source x-ray tube on a breast CT scanner.
Two simulated sets of digital tomosynthesis images of the lungs, each acquired at a 90 degree angle from the other,
with 19 projection images used for each set and SART iterative reconstructed, gives dual tomosynthesis slice image
quality approaching that of spiral CT, and with a data acquisition time that is 3% of that of cone beam CT. This fast
kV acquisition, should allow near real time tracking of lung tumors in patients receiving SBRT, based on a novel
TumoTrakTM multi-source X-ray tube design. Until this TumoTrakTM prototype is completed over the next year, its
projected performance was simulated from the DRR images created from a spiral CT data set from a lung cancer
patient. The resulting dual digital tomosynthesis reconstructed images of the lung tumor were exceptional and
approached that of the gold standard Feldkamp CT reconstruction of breath hold, diagnostic, spiral, multirow, CT
data. The relative dose at 46 mAs was less than 10% of what it would have been if the digital tomosynthesis had been
done at the 472 mAs of the CT data set. This is for a 0.77 fps imaging rate sufficient to resolve respiratory motion in
many free breathing patients during SBRT. Such image guidance could decrease the magnitudes of targeting error
margins by as much as 20 mm or more in the craniocaudal direction for lower lobe lesions while markedly reducing
dose to normal lung, heart and other critical structures. These initial results suggest a wide range of topics for future
A Monte Carlo-based tungsten anode spectral model, conceptually similar to the previously-developed TASMIP model,
was developed. This new model provides essentially unfiltered x-ray spectra with better energy resolution and
significantly extends the range of tube potentials for available spectra. MCNPX was used to simulate x-ray spectra as a
function of tube potential for a conventional x-ray tube configuration with several anode compositions. Thirty five x-ray
spectra were simulated and used as the basis of interpolating a complete set of tungsten x-ray spectra (at 1 kV intervals)
from 20 to 640 kV. Additionally, Rh and Mo anode x-ray spectra were simulated from 20 to 60 kV. Cubic splines were
used to construct piecewise polynomials that interpolate the photon fluence per energy bin as a function of tube potential
for each anode material. The tungsten anode spectral model using interpolating cubic splines (TASMICS) generates
minimally-filtered (0.8 mm Be) x-ray spectra from 20 to 640 kV with 1 keV energy bins. The rhodium and molybdenum
anode spectral models (RASMICS and MASMICS, respectively) generate minimally-filtered x-ray spectra from 20 to 60
kV with 1 keV energy bins. TASMICS spectra showed no statistically significant differences when compared with the
empirical TASMIP model, the semi-empirical Birch and Marshall model, and a Monte Carlo spectrum reported in AAPM
TG 195. The RASMICS and MASMICS spectra showed no statistically significant differences when compared with
their counterpart RASMIP and MASMIP models. Spectra from the TASMICS, MASMICS, and RASMICS models are
available in spreadsheet format for interested users.