# The Role of a DirectDensity(r) CT Reconstruction in a Radiotherapy Workflow: A Phantom Study
## 4 Discussion  
The relationships between the CT numbers and mass densities associated with each reconstruction algorithm and kVp value needed to perform dose calculation with the TPSs were obtained scanning the Gammex phantom with inserts of certified densities including metal inserts (Tables 1 and 2). The presence of metals induces artifacts on CT images (Figure 3), which could alter regions of interest in a clinical scan. The influence of metal artifacts on non-metallic regions of the Cheese phantom CT scans was investigated for each reconstruction algorithm used. No difference between acquisitions made in the presence or absence of metallic materials could be considered as significant in non-metallic regions, following the IAEA criteria [20] (Table 3). Therefore, the mean calibration curves were obtained by averaging the mean CT values in non-metallic ROIs over all the acquisitions associated with the same kVp value and reconstruction algorithm (Figure 6). This result was confirmed by [PERSON] et al. [12], who analyzed the influence of metal inserts on the images reconstructed with DD, concluding that the algorithm was not corrupted by the presence of such high densities. The mean calibration curves associated with the standard algorithm Qr40 represent the well-known trend of standard calibration curves, with a strong dependence on the kVp value set during the CT scan acquisition at both low and high densities (Figure 6). In contrast, curves related to the DD reconstruction algorithm showed this dependence only for high densities (Figure 6b), while at low densities (Figure 6a), the curves at different kVp values agreed with each other. This behavior is in agreement with other studies [11; 12; 21] and with the company's statement, which specifies that the algorithm may not be adequate to reconstruct the image with correct HUDD values in the regions of the image corresponding to materials with densities higher than that of the cortical bone (about 1.82 g/cm\({}^{3}\)) [10]. It is also worth noting that the curves associated with the combined DD-iMAR reconstruction were comparable for each kVp, with the curves associated to the DD reconstruction alone. Therefore, the combination with the iMAR algorithm does not affect the DD mechanism. Figure 10: The absolute maximum differences in dose, expressed as percentage of the dose to the prostate (100% = 70 Gy) between the dose distributions calculated on the DD reconstruction of the CT scan of the Cheese phantom with the stainless-steel insert acquired at 120 kVp using the correct calibration curve and the calibration curve related to the acquisition at 80 kVp, respectively. Blue shading corresponds to maximum differences of 0.5%. The analysis was conducted with MIM Maestro® and the differences were reported on the DD reconstruction of the Cheese phantom acquired with the stainless-steel insert at 120 kVp (axial section on the left, sagittal section at the upper right corner, and coronal section at the lower right corner). Evaluating the difference in HUDD between the 80 kVp and 140 kVp curves, no dependence of DD and DD-iMAR calibration curves on the kVp value could be detectable at low densities (in agreement with [PERSON] et al., [PERSON] et al. [11; 12], and [PERSON] et al. [21]), while the same analysis on standard calibration curves showed significant differences for the ROIs corresponding to the inner bone, B-200 bone, CB2-30%, CB2-50%, and cortical bone equivalent materials (Table 4). Moreover, it can be noted that this deviation was almost always greater, as expected, in the set of curves associated with the standard reconstruction, compared to the DD and DD-iMAR reconstruction sets, both at low and high densities (Figure 7). Given the criticality of the DD reconstruction in the presence of metallic materials, a clinical case of a prostate treatment in a patient with a hip prosthesis was simulated to verify the applicability of the algorithm in the radiation therapy workflow in all clinical situations.