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

Abstract

Purpose Lung stereotactic body radiation therapy (SBRT) requires delivering large radiation doses with millimeter accuracy, making image guidance essential. An approach to forming images of patient anatomy from Compton-scattered photons during lung SBRT is presented. Methods To investigate the potential of scatter imaging, a pinhole collimator and flat-panel detector are used for spatial localization and detection of photons scattered during external beam therapy using lung SBRT treatment conditions (6 MV FFF beam). MCNP Monte Carlo software is used to develop a model to simulate scatter images. This model is validated by comparing experimental and simulated phantom images. Patient scatter images are then simulated from 4DCT data. Results Experimental lung tumor phantom images have sufficient contrast-to-noise to visualize the tumor with as few as 10 MU (0.5 s temporal resolution). The relative signal intensity from objects of different composition as well as lung tumor contrast for simulated phantom images agree quantitatively with experimental images, thus validating the Monte Carlo model. Scatter images are shown to display high contrast between different materials (lung, water, bone). Simulated patient images show superior (~double) tumor contrast compared to MV transmission images. Conclusions Compton scatter imaging is a promising modality for directly imaging patient anatomy during treatment without additional radiation, and it has the potential to complement existing technologies and aid tumor tracking and lung SBRT image guidance.

Algorithms for joint activity–attenuation estimation from positron emission tomography scatter

“…The problem of estimating attenuation from scattered PET photons shares similarities with Compton scatter imaging, in which external Gamma sources are used to probe an object’s electron density for medical [27] or industrial [28] applications. While it is known from the latter that the nonlinearity of the problem favors thin, low-density objects, the impact of object size in scatter-based PET attenuation correction remains to be studied.…”

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

“…Given the high SNR achieved in this work, it may also be possible to apply CAI for markerless tracking of lung tumor motion. As discussed above in Section 1, the high density difference between dense tumors and sparse lung tissue may provide enough contrast to form useful images with high SNR 18 . Further study is required to verify this approach; however, initial simulations of this clinical scenario using the final CAI geometry from Section 3.E show promising results and are being investigated further.…”

“…As discussed above in Section 1, the high density difference between dense tumors and sparse lung tissue may provide enough contrast to form useful images with high SNR. 18 Further study is required to verify this approach; however, initial simulations of this clinical scenario using the final CAI geometry from Section 3.E show promising results and are being investigated further. Although counterintuitive based on common uses of CAI, the higher energy photon spectrums were identified as producing clearer images of the fiducial location.…”

“…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.…”