## 4 Discussion
### Feasibility Module  
Using the feasibility module allows the evaluation of the OAR fDVH from first principles, assuming uniform target coverage with the prescription dose without needing an a priori plan database. This aspect represents a great advantage compared to other approaches (i.e., knowledge-based) as the required databases are usually characterized by a (large) number of high-quality plans for each protocol and treatment site, and any minimal changes in contouring or plan protocols could require the definition of a new database. In addition, the evaluation of fDVHs before plan development has been proven to improve plan quality, which outweighs by far the additional time [28]. This study uses the feasibility engine to highlight failure in specific dose constraints. In particular, the obtained results demonstrated the impossibility to achieve three requirements for the PCa case concerning the maximum dosage on the rectum, the percentage volume of the rectum receiving the 40 Gy, and the percentage volume of the bladder receiving the 65 Gy, as specified by AAPM TG 244 (AAPM Publications--MPPG-TPS). Through the resulting fDHVs, different objectives, indicated as \"mandatory constraints\", have been used as optimization goals before the calculation phase and allowed to obtain clinically acceptable solutions. No previous studies have been conducted on dosimetric verification using the AAPM cases with Pinnacle, so no comparison can be performed. Not fulfillingthe required constraints could be due to different dose computation algorithms among other TPSs, in which the scattering radiation could be managed differently from what is performed by the collapsed cone convolution algorithm used in Pinnacle. Moreover, the fDVHs indicated the further possibility of OAR sparing with respect to the required objectives, as already reported for other sites and schedules [22; 23]. For example, in the head and neck case, the dose limit used for the lips fell in the \"easy\" area of the correspondent fDVHs. However, even without changing the requirements, during the optimization phase, the OAR dose sparing was pushed beyond the specified constraints. This feature is unique and typical of Pinnacle autoplanning. It represents a significant advantage compared to other dosimetric planning engines, which have as a primary goal the achievement defined by the user as clinically effective. Of note, the feasibility module provides valuable guidance in treatment planning by defining achievable dose constraints based on patient-specific anatomy. Thus, one key limitation is the feasibility module's sensitivity to uncertainties in input data, particularly contouring errors. Variations in target and OAR delineations can propagate through the module, leading to unrealistic or overly optimistic constraints. This issue is especially critical in anatomically complex cases, such as HNCa, where inter-observer variability in contouring is more pronounced. To mitigate these challenges, strategies include the use of consensus guidelines and automated segmentation tools to reduce variability in contouring [39]. Another limitation is the assumption of uniformity in dose deposition within the feasibility module. In practice, dose delivery is influenced by factors like beam arrangement, modulation limits, and machine-specific characteristics, which may not align perfectly with the module's predictions. In addition, by comparing calculated DVHs with the fDVH, discrepancies may arise due to factors such as the different definitions of acceptable target coverage thresholds (e.g., 95% of the target volume receiving the prescribed dose for clinical plans and 100% of target coverage for the fDVH tool) and, as in our study, the related anisotropic nature of dose fall-off influenced by beam energy and control points weight.