# Dose Calculation Algorithms for External Radiation Therapy: An Overview for Practitioners
## 6 Conclusions and Future Perspective  
In the present overview, we provided a general picture of the main dose calculation algorithms currently used in RT as well as the state of the art of the last-generation dose calculation algorithms, summarizing their underlying physical models and mathematical bases, and highlighting their strengths and weaknesses by referring to the most recent studies on algorithm comparisons.  
One may wonder what comes next and what are the future perspectives in relation to the modern RT techniques.  
A subject of increasing interest in RT planning is MR-imaging-based treatment planning [127], which aims at removing the planning CT scan and using in its place a magnetic resonance image (MRI) alone. This would allow for exploiting in a straightforward manner the intrinsic multi-spectral and intrinsic higher contrast of MRI compared to CT images [128,129], without the need for cumbersome registration processes [130,131]. Several techniques have been developed in order to introduce an MRI-only RT planning workflow [132]. The main sites clinically tested so far are the brain and prostate and a commercial solution has been developed by Philips for clinical synthetic CT generation for prostate patients (MRCAT pelvis). Despite the potential benefits of MRI-alone RT, clinical studies testing MRI-only planning techniques with larger patient cohorts are needed to bring MRI-only RT into clinical settings. In addition, the recent introduction of MRI-LINAC facilities requires dedicated techniques in terms of dose calculation engines to account for the secondary electron return effect in the MRI magnetic field [133].  
Dose calculation methods for hadrontherapy are still developing, similarly to the entire field. Faster calculation times are still necessary, especially for RBE-weighted doses. Online log-file-based dose reconstructions might enable further steps in adaptive therapy. A further speeding up of MC codes might enable routine treatment planning with increased accuracy, especially for heterogeneous target regions such as the lung.  
New modalities such as particle are therapy [134] require support from efficient dose engines, whereas novel delivery strategies such as spatially fractionated [135] or Flash [136] delivery, with an impact on radiobiological effects, would greatly benefit from explicit modeling, which could build on the existing RBE models already in clinical use. Although models for the increased efficacy of high-LET irradiation against hypoxia exist [137], they are not integrated into standard models and are not used in clinical practice.  
Eventually, next-generation dose calculation algorithms are expected to integrate biological criteria and models such as normal tissue complication probability (NTCP) [138] and tumor control probability (TCP) [139] models in order to optimize treatment plans on the basis of risk predictions, rather than on physical dose and dosimetric parameters [140; 141; 142]. In particular, a promising development for predictive models in the framework of dose engines is represented by the dosionic approach, characterized by the analysis of spatial patterns in dose distributions [143; 144; 145; 146; 147; 148; 149; 150], against the conventional point-wise parameters of dose-volume histograms (DVH) [151; 152] and their surface and mass variants (e.g., DSH [153; 154] and DMH [155]).  
This research received no external funding.  
Data Availability StatementData sharing not applicable.  
The authors declare no conflict of interest.