MO‐AB‐BRA‐02: A Novel Scatter Imaging Modality for Real‐Time Image Guidance During Lung SBRT

Abstract

Purpose: A novel scatter imaging modality is developed and its feasibility for image‐guided radiation therapy (IGRT) during stereotactic body radiation therapy (SBRT) for lung cancer patients is assessed using analytic and Monte Carlo models as well as experimental testing. Methods: During treatment, incident radiation interacts and scatters from within the patient. The presented methodology forms an image of patient anatomy from the scattered radiation for real‐time localization of the treatment target. A radiographic flat panel‐based pinhole camera provides spatial information regarding the origin of detected scattered radiation. An analytical model is developed, which provides a mathematical formalism for describing the scatter imaging system. Experimental scatter images are acquired by irradiating an object using a Varian TrueBeam accelerator. The differentiation between tissue types is investigated by imaging simple objects of known compositions (water, lung, and cortical bone equivalent). A lung tumor phantom, simulating materials and geometry encountered during lung SBRT treatments, is fabricated and imaged to investigate image quality for various quantities of delivered radiation. Monte Carlo N‐Particle (MCNP) code is used for validation and testing by simulating scatter image formation using the experimental pinhole camera setup. Results: Analytical calculations, MCNP simulations, and experimental results when imaging the water, lung, and cortical bone equivalent objects show close agreement, thus validating the proposed models and demonstrating that scatter imaging differentiates these materials well. Lung tumor phantom images have sufficient contrast‐to‐noise ratio (CNR) to clearly distinguish tumor from surrounding lung tissue. CNR=4.1 and CNR=29.1 for 10MU and 5000MU images (equivalent to 0.5 and 250 second images), respectively. Conclusion: Lung SBRT provides favorable treatment outcomes, but depends on accurate target localization. A comprehensive approach, employing multiple simulation techniques and experiments, is taken to demonstrate the feasibility of a novel scatter imaging modality for the necessary real‐time image guidance.

Characterization of Compton-scatter imaging with an analytical simulation method

“…The energy-response of the detector will cause a low-energy weighting of the image, which may help to maintain image quality even if high energy photons leak through the collimator. The full manifestation of these realistic effects is unknown, but preliminary experimental images suggest that quality images are achievable with low dose deposition (Redler et al 2015). Due to the similarities in expected photon energies, many of the extensive technologies developed for medium energy (<400 keV) nuclear imaging may be adapted for scatter imaging.…”

“…By analyzing the simple 3C scatter image, intensity proportionality to electron density may be assessed. The 3C and LT phantoms have been used previously to show agreement between experimental and MC simulated scatter images (Redler et al 2015). Therefore, analytically simulated 3C and LT images may be transitively compared to experimental images via MC validation.…”

“…Imaging of the Compton-scattered, therapy-beam photons is a promising alternative method for tumor tracking because images may be acquired from many angles without delivery of additional dose (Redler et al 2015), but further characterization of the technique is required.…”

“…An image may be formed by identifying the origin of the scattered photons through proper collimation (Lale 1959, Farmer and Collins 1971, Clarke et al 1976, Harding and Tischler 1986), by mapping energies to spatial coordinates (Norton 1994; Lenti 2008), with a Compton camera (Mundy and Herman 2010), or through coded-aperture techniques (MacCabe et al 2013). Of these, pinhole collimation (Redler et al 2015) is a conceptually simple technique that is amenable to experiment and simulation.…”

“…More recently, scatter imaging has been proposed as a possible technique for real-time tracking of tumor motion during radiotherapy treatment (Redler et al 2015, Yan et al 2016). Because photons are scattered in all directions, scatter imagers can be placed at many points surrounding a patient to provide imaging at different viewing angles.…”