This document summarizes a review article on the current and future applications of dual energy CT (DECT) in radiation therapy. It describes how DECT can be used to estimate electron density, decompose tissue into effective atomic numbers, and quantify contrast material for improved dose calculations, tissue characterization, and treatment planning. Several clinical applications are discussed, including more accurate dose calculations for brachytherapy and proton therapy, metal artifact reduction, and normal tissue assessment. The document concludes that DECT has the potential to improve accuracy at various stages of the radiotherapy workflow and will likely be used more in the future to provide additional diagnostic information over single energy CT.

1. Physics Journal Club
Dual energy CT in radiotherapy: Current applications and future
outlook
Wouter van Elmpt, Guillaume Landry, Marco Das, Frank Verhaegen
Department of Radiation Oncology (MAASTRO), GROW – School for Oncology and Developmental Biology,
Maastricht University Medical Centre, The Netherlands; Department of Medical Physics, Faculty of Physics, Ludwig-
Maximilians-Universität München, Garching b. München, Germany; Department of Radiology, GROW – School for
Oncology and Developmental Biology, Maastricht University Medical Centre, The Netherlands; and Medical Physics
Unit, Department of Oncology, McGill University, Montréal, Canada

2. Innovation/Impact
New strategies using Dual energy CT (DECT) imaging are
investigated to optimize radiation treatment.
This review describes how DECT based methods can be used
for
• Electron density estimation
• Effective atomic number decomposition
• Contrast material quantification
Clinical radiotherapy applications are summarized with future
perspectives
• Improved dose calculation accuracy for brachytherapy and proton
therapy
• Metal artifact reduction techniques
• Normal tissue characterization

3. Dual energy CT imaging: technology and physics
Imaging equipment for dual-energy CT imaging
• Scanning techniques: rotate-rotate, dual source-dual detector, single source-dual
layer detector, single source-rapid kV switching and twin beam
• 80/140 kV, 100/140 kV or 70/150 kV
• Spectral CT: the extension of DECT from two to multiple energies (photon counting
detector)
• Low-dose: approximately equal to a single energy scan
Photon attenuation coefficient
decomposition
• Material decomposition algorithms
– Basis materials: Water and Iodine
– Projection based and image based algorithms
• First radiotherapy interest in DECT was for
brachytherapy
– Estimated the linear attenuation coefficient at
low photon energies
– Compared the atomic number and the
electron density based parameterization The energy dependence of μ for the basis materials water
and iodine, together with two special purpose DECT
polychromatic spectra that are produced by the X-ray tubes.

4. Dual energy CT imaging: technology and physics
Contrast material quantification and mono-energetic decomposition
• Lung perfusion or iodine uptake in specific lesions
• Virtual non-contrast image
• (Pseudo-)Mono-chromatic/energetic image
– Non-linear combination of the two kVp images
– Local spatial-frequency filtering approaches can improve the contrast-to-noise ratio
Example of a mono-energetic reconstruction of a lung cancer patient. Various keV energy levels are reconstructed ranging
from 40 keV to 180 keV. The bottom right panel shows the average Hounsfield Unit inside the region of interested together
with an estimate of contrast-to-noise ratio, showing that 75 keV was optimal for this patient.

5. Improved image quality
Tumor staging, delineation and characterization
• Mono-energetic CT reconstructions can improve subjective image quality
• The quantification possibilities of the iodine concentration in the lesion
• There is no literature yet that shows evaluations of delineation purposes in RT
Normal tissue characterization
• Material decomposition allows visualization of the ventilated parts of lung
• DECT ventilation imaging is not yet fully investigated.
• More quantitate evaluation of normal tissue for toxicity assessment and
outcome prediction
DECT based metal artifact reduction strategies
• The mono-energetic did not lead to significant improvements
• MV/kV DECT: MV image beam to fill in the sinogram space of the kV cone-
beam acquisition
Improved target tracking
• Fast sequential DE planar X-ray image (not CT) demonstrated the benefit of
bone subtraction to visualize lung tumors.
• Markerless lung tumor tracking base on DE fluoroscopy has been
investigated
• Multiple angles might be necessary to fully assess the tumor motion in CT
mode

6. Improved dose calculations
Brachytherapy
• A series of papers reported a significant improvement over tissue
characterization by DECT-based extraction of electron densities and effective
atomic number.
• Monte Carlo dose calculations for virtual phantoms based on DECT segmentation
agreed with ground truth simulation within 4% for 103Pd.
• The DECT based decomposition of abdominal soft tissues (lipid, protein and water)
did lead to reliable mass attenuation coefficients and mass energy absorption
coefficients.
Proton therapy and verification
• Accurate estimations of stopping power ratio (SPR), medium to water, are required.
• The accuracy and robustness was theoretically superior to SECT.
• Verification for proton beam delivery using PET that rely on measurements of the
decay of 11C and depend on prediction of tissue carbon content
• Improved tissue assignment from DECT may be beneficial to the verification
• Novel approaches
– prompt gamma imaging
– proto-acoustics.

7. Improved dose calculations
Photon therapy
• Electron density calibration approaches using DECT compared to SECT
imaging
– DECT allow better estimation (reducing dose uncertainties from 11% to 1% for
specific geometries).
• Virtual non-contrast enhanced images from DECT can avoid the forcing of
iodine enhanced structures
– Contrast enhanced image available for delineation purposes
– Non-contrast scan for accurate dose calculation
• DECT acquisitions can be performed in a dose neutral way compared to
standard SECT imaging.
Proton therapy treatment plan for a head tumor
optimized on the basis of a DECT image (left).
Absolute dose difference between the plan
optimized on DECT and a plan optimized on
SECT (right). The dose distributions of both
plans were recalculated on the same image to
evaluate the range difference. For both panels
the color bar is in percentage of the prescription
dose.

8. Take Home Message
DECT imaging has multiple opportunities to be implemented in
radiotherapy that could improve the accuracy of various parts of the
workflow in the future.
DECT imaging has found its way into the radiological department at the
stage of diagnosis
DECT will be utilized much more in the future because of the added
information that is acquired compared to single energy CT imaging in a
dose-neutral way.
There have been some preliminary investigations using DECT for
characterization of tumor and normal tissues for improved radiotherapy
• Better segmentation
• Functional imaging
DECT can reduce uncertainties in the dose estimation
• Improved dose calculations in both brachytherapy and particle therapy

9. N.B. Most vendors offer also the rotate-rotate DECT solution in which 2 sequential helical CT scans at different kVp settings are acquired.
Manufacturer and
model name
Technology of DECT
acquisition
kVp ranges for
DECT
Information (or
technical
specifications) by
manufacturer
Siemens Definition
Force, Definition
Flash
Dual Source - Dual Detector 70-150 kVp http://www.healthcare.siemens.com
/computedtomography/dual-source-
ct/somatom-
force/technicalspecifications
GE Revolution GSI Single Source – Rapid kV
switching
80 and 140 kVp http://www3.gehealthcare.com/en/pr
oducts/categories/computed_tomog
raphy/revolution_hd
Philips IQon CT Single Source – Multi-layered
detector
120 and 140 kVp http://www.healthcare.philips.com/m
ain/clinicalspecialities/radiology/solu
tions/IQon.html
Toshiba Acquilion
One Vision
Single Source – Sequential
gantry rotations at different
kVp
80 and 135 kVp http://www.toshiba-
medical.eu/eu/productsolutions/com
puted-tomography/aquilion-one-
visionedition-overview/
Siemens Definition
Edge
Definition AS,
Definition
Perspective
Single Source – Split beam
filter at target with single
detector (‘TwinBeam’)
Single Source – two
successive spiral scans at
different kVp
120 kVp (with
additional Au and Sn
filters to create two
spectra)
http://www.healthcare.siemens.com
/computedtomography/single-
source-ct/somatom-
definitionedge/technical-
specifications
Supplemental Table 1: Overview of the characteristics of
commercially available state-of-the-art DECT scanners
(status January 2016).