# The Role of a DirectDensity(r) CT Reconstruction in a Radiotherapy Workflow: A Phantom Study
## 2 Materials and Methods
### Dose Distributions  
To evaluate the effect of introducing metallic materials on the dose calculations, a prostate treatment in a patient with a hip prosthesis was simulated. When high-density materials are present, in most cases, our treatment planning procedure entails a density override in the CT scan regions corresponding to the materials. This approach allows for an accurate calculation to be performed because the material density is evaluated from the prosthesis technical specifications sheet. Furthermore, some TPSs (such as Pinnacle) are limited to a maximum value of HU, beyond which they saturate and are no longer able to associate the correct densities to the voxels involved, even if the implemented calibration curve includes higher values. The targets (i.e., prostate and seminal vescicles) and organs at risk (i.e., rectum and bladder) were contoured on the CT of the Multi-Modality Male Pelvic Phantom (Mod. 048A--CIRS, Norfolk, VA, USA), an anthropomorphic phantom dedicated to the simulation of this anatomical region (Figure 2). These structures were transferred onto the CTs of the Cheese phantom acquired at 120 kVp, with and without the metal inserts, and reconstructed using the cited algorithms (Figure 3). All of the mean calibration curves were implemented in the TPS Pinnacle (Philips Radiation Oncology Systems, Fitchburg, WI, USA) used for dose calculations. The CT acquired without metal inserts at 120 kVp and reconstructed with the Qr40 convolution kernel was selected as the reference. We defined the titanium reference phantom (TRP) as the reference CT with a density override simulating the presence of a titanium insert in the same way the steel reference phantom (SRP) was defined as the reference CT with a steel density override. Both the overrides were performed in the metallic ROI of the image (brown ROI in Figure 3a). Figure 1: The CT scans of the Cheese phantom reconstructed with the standard algorithm Qr40, acquired at 120 kVp: (**a**) without metal inserts; (**b**) with the stainless-steel insert with a nominal density of 8 g/cm\({}^{3}\); (**c**) with the titanium insert with a nominal density of 4.51 g/cm\({}^{3}\). In these images, a volumetric modulated arc therapy (VMAT) treatment plan was optimized and calculated with the 6 MV Elekta Synergy(r) (Elekta Oncology Systems, Crawley, UK) linear accelerator model in the TPS Pinnacle, setting the dose grid at 0.25 cm, the prostate dose at 70 Gy, and the dose to the seminal vesicles at 63 Gy, both in 28 fractions. Thus, two reference dose distributions were obtained (Figure 4a,b). Figure 3: The CT scans (transverse sections) of the Cheese phantom reconstructed with the standard algorithm Qr40 at 120 kVp: (**a**) without metal inserts; (**b**) with the stainless-steel insert; (**c**) with the titanium insert. Structures of the targets (prostate in red and vesicles in pink) and organs at risk (bladder in light blue and rectum in orange) are visible at the center of each phantom. The density override in the brown ROI of figure (**a**) allows two ideal phantoms to be obtained, simulating the presence of the stainless-steel insert and the titanium insert, respectively, but without artifacts. The override of densities was also conducted in the metallic ROIs of the other CT scans (**b**,**c**). Figure 2: The CT scan of the CIRS phantom simulating the pelvis region with the contouring of the prostate (red), seminal vesicles (pink), bladder (light blue), and rectum (orange). (**a**) Transverse plan; (**b**) sagittal plan; (**c**) coronal plan. The treatment plan optimized on the TRP was copied and recalculated on the images including the titanium insert acquired at 120 kVp and reconstructed with the Qr40, DD, and the combined DD + iMAR algorithms, selecting the correct calibration curve each time and overriding the density in the metal ROI, keeping the parameters of the treatment fields unchanged.