## 2 Materials and Methods
### Planet(r) Dose  
Planet(r) Dose from DOSIsoft is a CE-marked software, which allows the user to perform dosimetry. Planet(r) was used under license at the 'Theagenio' Cancer Hospital of Thessaloniki, Greece on a dedicated workstation. Reconstructed SPECT/CT images with attenuation and scatter corrections were imported to the Planet(r) database. For every treatment cycle on every patient, a new study was created. SPECT/CT images of patient 4 for therapy cycle 1 were not available. In addition, patient 3 in the first cycle had only three timepoints at 4, 72, and 192 h post-Lutathera(r) administration, but was included in the study. Therefore, 23 dosimetry studies were created. The first step was to register all time points to the first time point (T0). The software offers rigid, elastic and manual registration modes. In this study, rigid registration was utilized. Registration was performed based on a selected region in the coronal plane. Our region of interest (ROI) was at the abdomen, and we aimed to minimize it while ensuring that the liver, spleen and kidneys were included in that ROI. In Figure 1, the left image illustrates the ROI in the CT at the first time point (T0). The middle image represents the ROI at the second time point, and the third image shows the final, registered images. To differentiate between the two CT images, the CT of T0 is depicted with green and the CT of T1 with purple. When the two images perfectly align, the overlapping areas do not exhibit these distinct colors. The process is repeated three times to register all time points (T1, T2 and T3) to T0. Consequently, the organs of interest (OOIs) were segmented, as seen in Figure 2, for patient 5. The segmentation in T0 was performed manually, slice by slice on the axial plane of the CT image by an expert radiologist. Hounsfield unit (HU) thresholds were applied for the liver and kidneys. For the latter, the HUs ranged from \(-\)20 to 150, while for the liver, from 0 to 150. Then, the contours were propagated to the next time points. Accurate propagation depends on successful registration, which may not always be achievable due to internal movements of the abdominal organs. As a result, manual corrections were made when it was necessary for T1, T2 and T3. The liver, the spleen and the right and left kidneys were segmented. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Modality** & **Resolution** & **Pixel Spacing** & **Slice Thickness** \\ \hline CT & 527 \(\times\) 527 & 0.977 \(\times\) 0.977 mm\({}^{2}\) & 2.5 mm \\ \hline SPECT & 128 \(\times\) 128 & 5.474 \(\times\) 5.474 mm\({}^{2}\) & 5.474 mm \\ \hline Dose Map & 128 \(\times\) 128 & 5.474 \(\times\) 5.474 mm\({}^{2}\) & 5.474 mm \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Data characteristics. A calibration factor (CF) was needed for the quantification of the SPECT/CT images. The calibration factor of the system was calculated in Bq/count depending on time per frame. SPECT images were taken with either 10 or 20 s per frame. The calibration factors were 86 and 43 Bq/count, respectively, for the two acquisition times. The injected activity for all patients was set to 7400 MBq. Time-activity curves (TACs) were generated for all segmented structures, using the Planet(r) software, as shown in Figure 3. Time-integrated activity coefficients (TIACs) in hours and time-integrated activity (TIA) in MBq-sec were calculated with the interpolation method, as implemented in Planet(r). S-values were calculated via the convolution of the absorbed dose voxel kernel (dose kernels) with density correction. Finally, the mean absorbed dose was calculated using the area under the curve (AUC), as specified by the fitting model. In this study, both bi- and tri-exponential fittings were applied.