# Patient-Specific Dosimetry Evaluations in Theranostics Software for Internal Radiotherapy
## 4 Discussion  
In this study we compared Velocity Theranostics to OLINDA 2.0, with the aim to evaluate the advantages of using advanced software for absorbed dose assessment in IR. This comparison was performed in ideal conditions (i.e., acquisition/processing of a radioactive phantom) and in more realistic conditions, through a selection of patients chosen from a clinical trial. Clinical conditions are quite distant from the ideal conditions achievable in a phantom, namely in terms of biological kinetics, multiple sequential acquisitions of SPECT/CT images, image registration issues [21], image artefacts caused by patient motion, irregular volumes of interest, not homogeneous activity distribution. To be as close as possible to clinical situations, a synthetic dataset of images was created, generating kinetics for organs similar to clinical kinetics, starting from the image of an anthropomorphic phantom. This choice was driven by the need to have a time activity curve for organs similar to clinical cases (supposing a bi-exponential decay), as reference for the calculations of TIAs and absorbed doses. Furthermore, homogeneity of activity distribution inside inserts permits making the comparison between two different calculation approaches more robust. As reported in Table 2, the percentage differences in the phantom ranged from \(-0.9\%\) to \(10.2\%\) for the TIAs and from \(-4.4\%\) to \(5.7\%\) for absorbed doses. The data are plotted in Figure 4, showing a high correlation both for TIAs and doses (R\({}^{2}\) = 0.9968 and 0.9902, respectively). These results highlight the good agreement between the two calculation techniques, with no specific overestimation or underestimation. In relation to the analysis of clinical cases, data concerning the comparison between Velocity Theranostics and OLINDA 2.0 are plotted in Figure 5, showing a high correlation for the TIAs (R\({}^{2}\) = 0.9475) and a lower correlation for the doses (R\({}^{2}\) = 0.5856). Data related to the comparison between Velocity Theranostics and OLINDA 2.0 run with the same TIA values calculated by Velocity Theranostics (OLINDA(Theranostics TIA)) are reported in Figure 6. It shows a high correlation for the doses (R\({}^{2}\) = 0.9531) which is decisively stronger than the correlation shown in Figure 5 for the comparison with OLINDA 2.0, as described in Materials and Methods. Figure 8: Box-plots representing the absorbed dose percentage difference calculated in the cohort of 15 patients between Velocity Theranostics and both OLINDA 2.0 (A) or OLINDA(Theranostics TIA) in (B). Even the Bland-Altman plots for both comparisons (Figure 7) show the reduction in the percentage absorbed dose differences in the case of using the same TIAs as Velocity Theranostics. Similarly Figure 8 highlights the reduction of the absorbed dose percentage differences for the cohort of patients. [PERSON]'s coefficient of concordance confirmed better agreement with the calculations in the case of same TIAs of Velocity Theranostics being used. The main reasons for the dispersion of data for absorbed doses shown in Figure 5 can be attributed to different steps of dosimetry computation [6], which add uncertainties to the absorbed dose estimations. The registration of the images used for the dosimetry with OLINDA 2.0 was rigid, while Velocity Theranostics used deformable registration. As reported in [22], deformable algorithms provide a better registration than the rigid algorithm, generating an image more similar to the reference one. Aside from that, it was concluded that deformable registered images generate a structural similarity index measure (SSIM) more stable than the rigidly reconstructed images. The rigid registrations may be indeed difficult because of patient repositioning for sequential acquisitions or because of patient movements during acquisitions. Furthermore, rigid registration can be defined \"locally\" accurate, while deformable registration can be defined \"globally\" accurate, since it takes into account multiple internal organ movements.