Characterization of Markerless Tumor Tracking Using the On-Board Imager of a Commercial Linear Accelerator Equipped With Fast-kV Switching Dual-Energy Imaging

Abstract

Purpose: To describe and characterize fast-kV switching, dual-energy (DE) imaging implemented within the on-board imager of a commercial linear accelerator for markerless tumor tracking (MTT).

Methods and materials: Fast-kV switching, DE imaging provides for rapid switching between programmed tube voltages (ie, 60 and 120 kVp) from one image frame to the next. To characterize this system, the weighting factor used for logarithmic subtraction and signal difference-to-noise ratio were analyzed as a function of time and frame rate. MTT was evaluated using a thorax motion phantom and fast kV, DE imaging was compared versus single energy (SE) imaging over 360 degrees of rotation. A template-based matching algorithm was used to track target motion on both DE and SE sequences. Receiver operating characteristics were used to compare tracking results for both modalities.

Results: The weighting factor was inversely related to frame rate and stable over time. After applying the frame rate-dependent weighting factor, the signal difference-to-noise ratio was consistent across all frame rates considered for simulated tumors ranging from 5 to 25 mm in diameter. An analysis of receiver operating characteristics curves showed improved tracking with DE versus SE imaging. The area under the curve for the 10-mm target ranged from 0.821 to 0.858 for SE imaging versus 0.968 to 0.974 for DE imaging. Moreover, the residual tracking errors for the same target size ranged from 2.02 to 2.18 mm versus 0.79 to 1.07 mm for SE and DE imaging, respectively.

Conclusions: Fast-kV switching, DE imaging was implemented on the on-board imager of a commercial linear accelerator. DE imaging resulted in improved MTT accuracy over SE imaging. Such an approach may have application for MTT of patients with lung cancer receiving stereotactic body radiation therapy, particularly for small tumors where MTT with SE imaging may fail.

Figure 1

Single energy (120 kVp—left) and dual energy image (right) for the static phantom. Dual energy subtraction removes the simulated bones and highlights the individual targets ranging in size from 25 mm (superior) to 5 mm (inferior).

Figure 2

Single energy (left) and dual energy subtraction images (right) of the CIRS motion phantom obtained using a right lateral imaging angle.

Figure 3

The average weighting factor obtained over 30 seconds plotted as a function of frame rate (left) and the weighting factor versus time acquired at 15 frames/s (right).

Figure 4

Average signal-difference-to-noise ratio plotted as a function of frame rate for each of the spherical targets in the static phantom.

Figure 5

Receiver-operating characteristics for single and dual energy imaging for A) 5 mm; B) 10 mm; C) 15 mm; D) 20 mm; and E) 25 mm targets in the CIRS motion phantom.