ESTRO 2023 - Abstract Book

S1596

Digital Posters

ESTRO 2023

one kVp resulting in suboptimal image quality for individual patients. Implementation of the DirectDensityTM (Siemens Healthineers GmBh Germany) reconstruction remedies this by generating one HN-D curve independent of kVp providing increased robustness to energy changes and allowing kVp optimisation. This work reports on linearity and energy change robustness investigations using DirectDensityTM. Materials and Methods HN–D calibration curves at all available kVp were generated using an Advanced Electron Density Phantom (Sun Nuclear Corporation, WI, USA) on a SOMATOM go.Open Pro (Siemens Healthcare GmbH, Germany) CT scanner. The curves were validated in the RayStation treatment planning system (RaySearch Laboratories, Sweden) using a Gammex RMI467 phantom. The kVp dependence of the Hounsfield Numbers (HN), for the Sm40 reconstruction, was investigated for 7 densities; 0.299 g/cc (Lung LN-300) to 1.925 g/cc (Cortical Bone). A representative mediastinal VMAT plan was created using the CIRS Model 002LFC IMRT Thorax Phantom (SunNuclear, USA). Dose was calculated using the Qr40 (at 120kVp) and the Sm40 HN-D calibration curves at the kVp stated above. CT patient scans (corresponding to pelvis, thorax, Head and Neck and Brain) acquired at 120kVp (Qr40), were reconstructed with the Sm40 kernel. Their clinical plan was recalculated for dose comparison.

Results DirectDensityTM produced linear HN-D curves with reduced energy dependence.

Fig1:

kVp

dependence

of

the

HU

number/CT

value

using

Qr40

and

Sm40.

For adipose, the mean HN (Sm40) =-70.8 (s.d. 7.2), equating to a density of 0.929g/cc (reference=0.960g/cc). For LN-300 the mean HN was -696.2 (s.d. 5.3) equating to a density of 0.304g/cc (reference=0.299g/cc). For cortical bone, the mean HN =983.9 (s.d. 28.8). This equates to a density of 1.984g/cc (reference=1.925g/cc). For bone inserts, an incidental finding where HN values decreased was observed at 110kVp