## 5 Conclusions  
Lung dosimetry tailored for \({}^{166}\)Ho radioembolization is a fundamental step in the treatment workflow for patients undergoing these procedures, especially in cases where involvement of the pulmonary compartment is significant enough to create a risk for the development of adverse effects such as radiation pneumitis.  
This study demonstrates that, due to the significant heterogeneity and low density of lung tissue, the radiation transport from radionuclide decay is substantial, making conventional dosimetric calculation methods inadequate for accurate dosimetric evaluation under these conditions.  
Furthermore, this study has shown that the complexity of the \({}^{166}\)Ho spectrum demands a proper dosimetric evaluation and morphological framework to correctly manage possible correlations between the heterogeneity of the absorbed dose distribution in lung tissue and potential adverse effects.  
The MC simulations comparing \({}^{166}\)Ho and \({}^{90}\)Y showed a significant difference in the spatial distribution of ADr between the two radionuclides, with greater ADr heterogeneity for \({}^{166}\)Ho due to its generally lower energy and complex decay spectrum.  
In this context, internal dosimetry using direct radiation transport simulation with Monte Carlo code has become an applicable approach in the clinical scenario, even with common hardware tools, with computation times fully compatible with everyday practice. Moreover, the advent of Monte Carlo codes on GPUs is already making the use of this type of dosimetric calculation tool agile and straightforward, with the potential to further implement personalized planning, reaching a complexity comparable to that of EBRT treatments, thereby positively impacting the future treatment and management of patients undergoing these therapies.  
Further understanding of the clinical role of direct MC simulations in lung dosimetry of liver radioembolization treatments could be improved by analyzing the impact of different clinical situations, evaluating patients with relatively high and various LS values.  
In assessing the probability of adverse effects, it is essential to adopt a robust and reliable method, such as direct MC simulation, to develop models for evaluating NTCP curves, potentially paving the way for more advanced and consistent planning strategies of the TRT treatments.  
**Supplementary Materials:** The following supporting information can be downloaded at: [[https://www.mdpi.com/article/10.3390/app15020958/s1](https://www.mdpi.com/article/10.3390/app15020958/s1)]([https://www.mdpi.com/article/10.3390/app15020958/s1](https://www.mdpi.com/article/10.3390/app15020958/s1)), 166 Ho S-value kernel for the soft tissue ICRU with a density of 1.04 g/cm\({}^{3}\) and lung tissue ICRU with a density of 0.296 g/cm\({}^{3}\) over 2.21 mm voxel size. Data are available in a csv-formatted file as position indices \((i,j,k)\) followed by the absorbed dose values expressed in mGy/MBq-s.  
**Author Contributions:** Conceptualization, E.D. and M.P.; methodology, E.D., N.L. and M.P.; software, E.D.; validation, E.D., A.P. and M.P.; formal analysis, E.D., A.P., B.C. and M.P.; investigation, E.D., A.P., N.L. and M.P.; resources, E.D., M.C., V.P. and M.P.; data curation, E.D., A.P., B.C. and M.P.; writing--original draft preparation, E.D.; writing--review and editing, A.P., B.C., N.L., M.C., V.P. and M.P.; supervision, B.C., V.P. and M.P.; project administration, M.P. All authors have read and agreed to the published version of the manuscript.  
**Funding:** This research received no external funding.  
**Institutional Review Board Statement:** Not applicable.  
**Informed Consent Statement:** Not applicable.  
**Data Availability Statement:** The original contributions presented in this study are included in the article and in the Supplementary Material; further inquiries can be directed to the corresponding author.  
**Conflicts of Interest:** The authors declare no conflicts of interest.