A new scheme for real-time high-contrast imaging in lung cancer radiotherapy: a proof-of-concept study

Abstract

Visualization of anatomy in real time is of critical importance for motion management in lung cancer radiotherapy. To achieve real-time, and high-contrast in-treatment imaging, we propose a novel scheme based on the measurement of Compton scatter photons. In our method, a slit x-ray beam along the superior-inferior direction is directed to the patient, (intersecting the lung region at a 2D plane) containing most of the tumor motion trajectory. X-ray photons are scattered off this plane primarily due to the Compton interaction. An imager with a pinhole or a slat collimator is placed at one side of the plane to capture the scattered photons. The resulting image, after correcting for incoming fluence inhomogeneity, x-ray attenuation, scatter angle variation, and outgoing beam geometry, represents the linear attenuation coefficient of Compton scattering. This allows the visualization of the anatomy on this plane. We performed Monte Carlo simulation studies both on a phantom and a patient for proof-of-principle purposes. In the phantom case, a small tumor-like structure could be clearly visualized. The contrast-resolution calculated using tumor/lung as foreground/background for kV fluoroscopy, cone beam computed tomography (CBCT), and scattering image were 0.037, 0.70, and 0.54, respectively. In the patient case, tumor motion could be clearly observed in the scatter images. Imaging dose to the voxels directly exposed by the slit beam was ~0.4 times of that under a single CBCT projection. These studies demonstrated the potential feasibility of the proposed imaging scheme to capture the instantaneous anatomy of a patient on a 2D plane with a high image contrast. Clear visualization of the tumor motion may facilitate marker-less tumor tracking.

Figure 1

Compton scattering imaging. X-ray photons are scattered in the object and detected at the imager. A detector pixel detects scattered photons from all the illuminated voxels, e.g. P and P′.

Figure 2

a) The proposed tumor tracking scheme using Compton scattering imaging using b) pinhole collimation or c) slat collimation. d–e) An illustration of the system geometry viewed from the patient superior direction.

Figure 3

Phantoms used in our simulation studies. (a) A digital cylindrical phantom and (b) a phantom created with a patient CT. For each phantom, the left subfigure is a 3D rendering and the right one shows cross section images.

Figure 4

Top: raw images under the pinhole geometry with different diameters of 15 mm (a-1), 6mm (b-1) and 3mm(c-1). Bottom: raw images under the slat geometry with different heights of 20 mm (a-2), 50 mm (b-2) and 100 mm (c-2).

Figure 5

Image corrections. a-1) Illustration of illumination geometry. b-1)-e-1) Images of different correction factors. a-2)-e-2) Images during the stage of applying these correction factors. a-3)-e-3) and a-4)-e-4) are image intensity profiles along the dashed lines in a-2) during the image correction steps.

Figure 6

Comparison among different imaging modalities. Columns from left to right: CBCT, radiographic projection, and the proposed scattering imaging in two different setups. Each image is normalized to [0, 1] and the displaying window is [0.05, 1]. Rows from top to bottom: the reconstructed images, vertical and horizontal profiles.

Figure 7

MC simulated scattering imaging for tumor tracking in a lung patient case. a) The anatomy in coronal (left) and sagittal (right) planes to be imaged by the slit beam. b) 50% phase of the coronal and the sagittal view of the raw scattering images. c) zoom-in view of the scattering images at the ROIs shown for b) for all ten respiratory phases, as well as the corresponding 4DCT images.

Figure 8

Results of the MC imaging dose calculation for a lung cancer patient. a) CBCT projection with a large field of 26.7 × 20.0cm² at isocenter. b) Projection with a reduced field of 6.78 × 6.78 cm². c) The slit beam with 0.2 × 6.78 cm² corresponding to the setup shown in Figure 7a) right. d) Zoom-in regions inside the yellow boxes in c) displayed with a narrowed window of [0, 0.4]. The three rows represent transverse, sagittal and coronal views, respectively. Dash lines in a) indicate the location of other views.

Figure 9

Illustration of a possible setup on an existing linac for tumor tracking.

Figure 10

Illustration of geometry for the derivation of G(F) in the pinhole geometry.