Publikationsrepositorium - Helmholtz-Zentrum Dresden-Rossendorf

Purpose / Objective
Radiotherapy planning commonly requires an additional, ‘native’ CT scan for dose calculation if a contrast agent is used for tumor diagnostics and contouring. Iodinated contrast agents increase CT numbers (Hounsfield units) due to the large atomic number of iodine (Z=53), while electron density remains almost unchanged owing to its low concentration (Figure 1). With dual-energy CT (DECT), the impact of atomic number on CT image contrast can be removed, enabling the direct calculation of relative electron density (RED) for photon therapy and stopping-power ratio (SPR) for ion therapy, respectively. In this study, we are investigating the magnitude of the remaining impact of an iodinated contrast agent on DECT-derived RED/SPR and subsequent clinical treatment planning for both photon and ion therapy.

Material / Methods
As a first step, the effect of the CT contrast medium Imeron® 300 (Bracco Imaging Deutschland GmbH, Germany) on RED/SPR determination was investigated in a dilution series over a range of iodine concentrations between 0.3 and 300 mg/ml. CT images were acquired on a Somatom Definition Flash dual-source CT scanner (Siemens Healthineers, Forchheim, Germany) in single-energy (SECT, 120 kVp) and dual-energy (DECT, 80/140Sn kVp) scan mode. RED and SPR images were obtained (a) from SECT datasets by applying the respective calibrated Hounsfield look-up table and (b) from DECT datasets using the software application syngo.CT Rho/Z (Siemens) and an SPR calculation scheme previously validated by the authors in phantoms, biological material and patients.

Results
Calculating RED/SPR from a DECT dataset with typical contrast enhancement (max. 160 HU at 120 kVp corresponding to 6 mg iodine per milliliter) could limit the impact on both RED and SPR to 1% compared to 5-10% when using a contrast-enhanced SECT image (Figure 2). Consequently, dose calculation could be performed directly on DECT-derived RED/SPR images.

Conclusion
Dose calculation on a RED/SPR dataset derived from a contrast-enhanced dual-source DECT scan is conceivable. This can make additional native scans obsolete, thereby simplifying the treatment planning workflow and lowering the patient dose by 50% (one instead of two scans). A clinical trial is currently underway to investigate the role of contrast-enhanced DECT for patient radiotherapy planning.