Assessing Photon Treatment Planning Using Relative Electron Density Map with Dual-Energy CT
Abstract
Purpose
Contrast-enhanced single-energy CT (SECT) scans provide superior image contrast and better differentiation of the target volume and vascular structures compared to non-contrast CT scans at the cost of deteriorated dose calculation due to the high density of the contrast agent. Dual-energy CT (DECT) can determine the relative electron density (RED) from a contrast-enhanced SECT as a replacement to capture a separate non-contrast CT scan for treatment planning. This elimination of the non-contrast scan can halve the number of necessary patient scans routinely acquired in clinical practice.
Methods
RED was derived from 90/Sn150kVp DECT obtained with a dual-source CT system and dose-neutral scan protocol compared with SECT. The clinical treatment plan was optimized on the pseudo mono-energetic images from the non-contrast DECT scan and forward calculated on the RED maps from both non-contrast and contrast-enhanced DECT scans. Five sets of pseudo mono-energetic images were derived from the contrast-enhanced DECT scans, among which physician review and selection of preferred scan was made for contouring purposes.
Results
Good agreement was observed in the dose calculated on the RED maps of the patient scans compared to that of the original planning images. The mean change in target volume coverages is <2% in GTV, <1% in CTV, and mean change is <40 cGy in maximum dose to brainstem. Moderate to high keV images (60 and 70 keV) were chosen for contouring, which provided better signal-to-noise and contrast-to-noise ratios.
Conclusion
Contrast-enhanced DECT scans can provide RED maps close to the ground truth derived from non-contrast DECT scans. Our studies showed that the dose forward-calculated on the RED maps from the DECT scan agreed well with the planning images. Therefore, it is possible to reduce the number of patient scans to acquire only one set of contrast-enhanced DECT scan for both contouring and treatment planning purposes.