Split-filter dual energy computed tomography radiotherapy: From calibration to image guidance

Abstract

Background and purpose: Dual-energy computed tomography (DECT) is an emerging technology in radiotherapy (RT). Here, we investigate split-filter DECT throughout the RT treatment chain as compared to single-energy CT (SECT).

Materials and methods: DECT scans were acquired with a tin-gold split-filter at 140 kV resulting in a low- and high-energy CT reconstruction (recon). Ten cancer patients (four head-and-neck (HN)​, three rectum​, two anal/pelvis and one abdomen) were DECT scanned without and with iodine administered. A cylindrical and an anthropomorphic HN phantom were scanned with DECT and 120 kV SECT. The DECT images generated were: 120 kV SECT-equivalent (CT_mix), virtual monoenergetic images (VMIs), iodine map, virtual non-contrast (VNC), effective atomic number (Z_eff), and relative electron density (ρ_e,w). The clinical utility of these recons was investigated for calibration, delineation, dose calculation and image-guided RT (IGRT).

Results: A calibration curve for 75 keV VMI had a root-mean-square-error (RMSE) of 34 HU in closest agreement with the RSME of SECT calibration. This correlated with a phantom-based dosimetric agreement to SECT of γ_1%1mm > 98%. A 40 keV VMI recon was most promising to improve tumor delineation accuracy with an average evaluation score of 1.6 corresponding to "partial improvement". The dosimetric impact of iodine was in general < 2%. For this setup, VNC vs. non-contrast CT_mix based dose calculations are considered equivalent. SECT- and DECT-based IGRT was in agreement within the setup uncertainty.

Conclusions: DECT-based RT could be a feasible alternative to SECT providing additional recons to support the different steps of the RT workflow.

Keywords: Calibration; Delineation accuracy; Dose calculation; Dual-energy computed tomography; Image guidance.

Fig. 1

Contrast-enhanced DECT recons for HN patient 2. Top: CT_mix (left), VNC (α_w, middle) and rho (ρ_e,w, right). Bottom: VMI at 40 keV (left), iodine map (α_I, middle) and effective atomic number (Z_eff, right). For the top row and 40 keV, the window (W)/level (L) is 580/50 HU, for iodine W/L is 150/35 HU. Z_eff is shown in atomic numbers from 4 to 14. The contours shown are the GTV (red), spinal cord (dark green) and body outline (light green). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 2

Compensation strategy to reduce the impact of anatomical changes between the native and contrast-enhanced (CE) DECT of a rectum patient with GTV (red), clinical target volume (CTV, pink), PTV (cyan) and body (green) contours. Left: native CT_mix with air map (white). Middle: CE CT_mix with air map (dark blue). Right: CE CT_mix with native air map (white) assigned to air and non-overlapping air map volumes assigned to water (light blue) to reproduce native CT_mix anatomical conditions. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 3

Mass density vs. CT number in HU for different DECT recons as indicated in the figure key (left). The SECT-based calibration curve in the TPS is indicated by closed circles and solid line. The largest spread in CT numbers is seen beyond 1 g/cc. Square insert is a zoom of the soft tissue region (right).