Characterization of Compton-scatter imaging with an analytical simulation method

Abstract

By collimating the photons scattered when a megavoltage therapy beam interacts with the patient, a Compton-scatter image may be formed without the delivery of an extra dose. To characterize and assess the potential of the technique, an analytical model for simulating scatter images was developed and validated against Monte Carlo (MC). For three phantoms, the scatter images collected during irradiation with a 6 MV flattening-filter-free therapy beam were simulated. Images, profiles, and spectra were compared for different phantoms and different irradiation angles. The proposed analytical method simulates accurate scatter images up to 1000 times faster than MC. Minor differences between MC and analytical simulated images are attributed to limitations in the isotropic superposition/convolution algorithm used to analytically model multiple-order scattering. For a detector placed at 90° relative to the treatment beam, the simulated scattered photon energy spectrum peaks at 140–220 keV, and 40–50% of the photons are the result of multiple scattering. The high energy photons originate at the beam entrance. Increasing the angle between source and detector increases the average energy of the collected photons and decreases the relative contribution of multiple scattered photons. Multiple scattered photons cause blurring in the image. For an ideal 5 mm diameter pinhole collimator placed 18.5 cm from the isocenter, 10 cGy of deposited dose (2 Hz imaging rate for 1200 MU min−1 treatment delivery) is expected to generate an average 1000 photons per mm2 at the detector. For the considered lung tumor CT phantom, the contrast is high enough to clearly identify the lung tumor in the scatter image. Increasing the treatment beam size perpendicular to the detector plane decreases the contrast, although the scatter subject contrast is expected to be greater than the megavoltage transmission image contrast. With the analytical method, real-time tumor tracking may be possible through comparison of simulated and acquired patient images.

Compton scatter imaging: A promising modality for image guidance in lung stereotactic body radiation therapy

“…Specifically, this equation is meant to emphasize that: (a) scatter image intensity will be affected by both entrance and exit attenuation; (b) the probability of Compton scattering is proportional to electron density (which is roughly proportional to physical density), and therefore, that scatter imaging contrast will originate in these density differences; and (c) scatter image intensity will be dependent on the angular orientation between source, object, and detector apparatus. A more rigorous characterization of the scatter imaging discussed in this work, using an analytical simulation method, is the subject of a separate work …”

MR‐linac is the best modality for lung SBRT

“…35 In addition to these mature technologies, the feasibility of other real-time lung-tumor monitoring has also been demonstrated with radiation-dose-free Compton-scatter imaging using the therapy beam 37 and with PET emission guidance. However, for lungs, such monitoring capability is not unique to MRI due to high x-ray imaging contrast and minimally invasive fiducial implantation procedures.…”

“…Meanwhile, exposure-free radiofrequency tracking has been FDA-approved for lung and can be integrated with real-time MLC-tracking. 35 In addition to these mature technologies, the feasibility of other real-time lung-tumor monitoring has also been demonstrated with radiation-dose-free Compton-scatter imaging using the therapy beam 37 and with PET emission guidance. 38 In summary, MR-linacs should be better used where they are needed.…”

Optimizing Coded Aperture Imaging techniques to allow for online tracking of fiducial markers with high‐energy scattered radiation from treatment beam

“…Current studies are investigating the imaging of lung tumors using both keV and MeV energy ranges, with different methods of collimating the scatter to localize the source (such as a pinhole camera, or antiscatter grids), and then form an image with high SNR which can accurately visualize and track a moving lung tumor. [17][18][19] This required density contrast can be introduced in other organs/tumors of interest by placing high Z (gold) fiducial markers in the low Z tissue. These small seeds will appear as a brighter source of scattered radiation compared to the surrounding background.…”

“…However, one of the areas that does provide a great deal of contrast is in the lungs, where the density of a tumor is higher (~3–4×) than the surrounding tissue and will produce a large scatter signal in a lower background. Current studies are investigating the imaging of lung tumors using both keV and MeV energy ranges, with different methods of collimating the scatter to localize the source (such as a pinhole camera, or antiscatter grids), and then form an image with high SNR which can accurately visualize and track a moving lung tumor 17–19 . This required density contrast can be introduced in other organs/tumors of interest by placing high Z (gold) fiducial markers in the low Z tissue.…”