## 2 Materials and Methods  
Between 2022 and 2023, we analyzed all 1252 VMAT plans delivered by two Varian TrueBeam machines at our institute, equipped with Varian Millenium 120-leaf MLCs. Treatment plans were optimized using the Eclipse TPS v15 (Varian Medical Systems, Palo Alto, CA, USA) with the Photon Optimizer (PO) and dose calculated with the Analytical Anisotropic Algorithm or Acuros XB algorithm. These plans were not part of the dataset used to develop our ML model [8]. Briefly, the model (xgboost) is an ensemble of boosted trees predicting the GPR of a VMAT arc with 3%/1 mm gamma criteria. In order to collect a sample size reasonable for analysis, ten patients from different regions were selected using the following filtering criteria:  
1. Average predicted GPR (pGPR): plans with a pGPR less than 95% averaged over the arcs, using a 3%/1 mm gamma criteria and a 10% dose threshold, were considered. 2. Minimum field size: fields larger than 5 \(\times\) 5 cm\({}^{2}\) were considered. Preliminary investigations showed that the ML model underestimated the GPR for arcs withsmaller field apertures. Moreover, a larger field size was crucial to mitigate the uncertainties in small field dosimetry and degradation of the EPID center [11; 13; 14]. 3. Case selection: brain and breast cases were excluded due to their specific requirements and the complexity of their treatment plans, which necessitate the intervention of an experienced planner. In particular, most brain cases are optimized with a dedicated algorithm (HyperArc; Varian Medical Systems, Palo Alto, CA, USA), while for breast cases a non-automatable procedure is required to account for the patient's breathing motion [15]. Table 1 summarizes the characteristics of the selected plans, all optimized with PO and dose calculated with AcurosXB. The treatment plans included various anatomical regions, from head to pelvis, and dose prescriptions. All constraints were defined according to the available literature and our internal experience. \begin{table}
\begin{tabular}{c c c c} \hline
**Treatment** & **Number of Cases** & **Prescription** & \multicolumn{2}{c}{**Clinical Constraints**} \\ \hline  & & Lenses & Mandible \\  & & \(\bullet\) & Dmax \(<\) 6 Gy \\  & & \(\bullet\) & D0.03 cc \(<\) 6 Gy \\  & & Cochlea & \(\bullet\) & Dmean \(<\) 35 Gy \\  & & \(\bullet\) & Dmean \(<\) 45 Gy \\  & & Brainstem & \(\bullet\) & V50 Gy \(<\) 70\% \\ Head and neck & 4 & 69.96 Gy/33 fr & \(\bullet\) & Dmax \(<\) 54 Gy \\  & & Temporal lobe & \(\bullet\) & Dmean \(<\) 45 Gy \\  & & \(\bullet\) & Dmax \(<\) 65 Gy \\  & & & \(\bullet\) & D0.03 cc \(<\) 65 Gy \\  & & & Parotids & \(\bullet\) & D0.03 cc \(<\) 45 Gy \\  & & & \(\bullet\) & Dmean \(<\) 26 Gy \\  & & & Oral cavity & \(\bullet\) & Dmax \(<\) 66 Gy \\  & & & \(\bullet\) & Dmean \(<\) 40 Gy \\  & & & & \(\bullet\) & D0.03 cc \(<\) 66 Gy \\ \hline  & & & Spinal cord & Liver \\  & & & \(\bullet\) & D0.03 cc \(<\) 40 Gy \\  & & & \(\bullet\) & Dmax \(<\) 45 Gy \\ Esophagus & 1 & 55 Gy/25 fr & \(\bullet\) & Dmax \(<\) 40 Gy \\  & & & Heart & Stomach \\  & & & \(\bullet\) & V30 Gy \(<\) 30\% \\  & & & \(\bullet\) & Dmean \(<\) 30 Gy \\ \hline  & & & Esophagus & Spinal cord \\ Mediastinum & 1 & 20 Gy/10 fr & \(\bullet\) & D0.1 cc \(<\) 48 Gy \\  & & & \(\bullet\) & Dmax \(<\) 48 Gy \\  & & & \(\bullet\) & D5 cc \(<\) 40 Gy \\ \hline \end{tabular}
\end{table} 
Table 1: Dose prescription and clinical constraints of the ten treatment plans identified. The constraints on the PTV were the same for all treatments: V95% \(\geq\) 95%, V100% \(\geq\) 50%, and V105% \(<\) 2%. The constraint for the CTV was V95% \(\geq\) 98%. The 10 plans were randomly split into two groups to optimize the replanning procedure. In particular, in the first phase, three plans were considered to identify the best strategies that balanced dosimetric quality, complexity, and deliverability. In the second phase, the remaining seven plans were replanned using the selected strategies to assess the overall improvement of the plans.