# The Role of a DirectDensity(r) CT Reconstruction in a Radiotherapy Workflow: A Phantom Study
## 4 Discussion
Reference dose distributions were calculated on reference Cheese phantom scans with the same metal insert densities but without the metal artifacts (SRP and TRP). These distributions were compared to those calculated on CT scans acquired with metal inserts at 120 kVp, using the 3D gamma index analysis and the MIM Maestro(r) software for graphical comparison. The gamma analysis results indicated that all of the distributions associated with a given metallic material were comparable with the distribution calculated on the corresponding reference phantom, since the gamma index was 100% for each comparison (Table 5). This result was confirmed by the analysis with MIM Maestro(r), which showed that all of the maximum percentage differences between the dose distributions were within the 3% (Table 6). Thus, the DD reconstruction was not affected by metal artifacts when we compared the treatment plan global quality in terms of gamma indices, however, there were visible differences (in the order of 0.5-1.6% of the local dose) in the surroundings of the metal inserts. The greatest differences were obtained by comparing the dose distribution in the reference phantom and the ones calculated in the presence of metals and reconstructed with the standard algorithm Qr40. This result may suggest a greater dependence of the standard algorithm in the presence of metal artifacts in the dose calculation, compared to the DD and DD + iMAR algorithms. Thanks to the spatial visualization of the distribution of dose differences, it was possible to identify regions where the differences were most concentrated. In all of the comparisons performed, differences were focused on the regions that were more deteriorated in the images, that is, the regions surrounding the inserted metal insert and the targets as well as along the striations of the metal artifacts (example in Figure 8). Furthermore, the adequacy of the DD reconstruction for dose calculations was tested by comparing, with MIM Maestro(r), the dose distributions calculated on the CT scans of the Cheese phantom with the stainless-steel insert acquired at 100 kVp and 140 kVp, both reconstructed with the DD algorithm. As in the metal ROI, the density override was performed, and possible relevant differences in dose would suggest an inadequacy of the DD reconstruction to be used for dose calculations in the presence of metal implants. The main differences between the two distributions did not exceed the 0.5% of the dose to the prostate and they were concentrated in the same regions as previously described, that is, near the metal insert, along the striations of metal artifacts and near the targets as can be observed in Figure 9. However, these discrepancies did not exceed the tolerance of 3%, thus the distributions were comparable and the algorithm invariance for the change in kVp can also be considered valid for dose calculations. This result matches the results of [PERSON] et al. [9], who compared the dose distributions calculated on the DD images at different kVp values without metal implants with a reference distribution obtained on standard reconstruction at 120 kVp. Almost all of the evaluated comparisons with the gamma analysis showed a passing rate greater than 99% with the acceptability criteria of 2 mm in space and 2% in dose. In the end, the robustness of the DD algorithm against a possible error in the selection of the calibration curve was investigated. Thus, the dose distribution calculated on the DD reconstruction at 120 kVp of the Cheese phantom including the stainless-steel insert was compared, using MIM Maestro(r), with the dose distribution calculated on the same image but intentionally selecting the wrong calibration curve associated with the DD reconstruction at 80 kVp. The main differences between the two distributions did not exceed the 0.3% of the dose to the prostate and they were located only near the structures simulating the seminal vescicles and the target (Figure 10).