## 3 Results  
As many as 32 pediatric patients met the inclusion criteria and were included in the study, for a total of 853 fractions, i.e., 27 per patient on average. The median age at treatment was 13.62 years (min--2.24; max--28.33). All patients but one, who received carbon ion therapy, were treated with protons. Three out of four patients (24/32, 75%) had head and neck malignancies; the remainder had extra-cranial tumours localized in the thorax (3/32, 9%), pelvis (4/32, 13%) or limbs (1/32, 3%). By looking at the mean across patients of the absolute values of the average corrections across fractions for each patient, it is evident that translations contribute the most in the systematic component of the positioning error, especially in the posterior-anterior (z, 3.26 mm) and caudal-cranial (y, 1.91 mm) directions. Corrections in the z directions were mostly (30/32) negative, i.e., couch down, while in the y direction they were mostly (26/32) positive, i.e., couch towards beam nozzle. Shifts in the x direction were, on average, lower (1.53 mm) and approximately equally distributed between negative and positive values. Regarding rotations, average values oscillate around 0.5 degrees. No clear relationships between the entities of shifts and room, age, site of lesion and anaesthesia are evident from the data (Table 1). Figure 1: Workflow followed to derive a normalized norm of the adjusted correction vector for the j-th treatment fraction of one patient. **Abbreviations. f**: number of fractions. **F\({}_{\mathbf{j}}\)**: j-th fraction. **N**: \(\mid\mathbf{V}^{\prime}\mid\) normalized with respect to \(\mid\)**V**\(\mid\). **P**: patient. **V\({}_{\mathbf{j}}\)**: current correction vector. **V\({}_{\mathbf{j}}\)**: adjusted correction vector of j-th fraction. 1...1: Euclidean norm. Two patients, namely, \"P8\" and \"P24\" from Table 1, both treated for extra-cranial malignancies, had corrections above 4 mm for all translational components and have been selected as representative for the pelvic district. Boxplots representing corrections of these patients for all fractions are reported in Figures 2a and 2b, respectively. As previously explained, the pre-alignment strategy is successful if the normalized value of the norm, i.e., N, is lower than 1. In the studied cohort, such an improvement (N \(<\) 1) was observed for most patients' fractions (781/853, 91.6%). For the remainder, 72/85  
By averaging the single fraction values across all patients, a reduction in the magnitude of the correction vector was evident. Instead, no definitive trends were observed regarding Nave across the different treatment fractions, with values oscillating between higher and lower benefits. On average, the magnitude of the CV was reduced by 46% (Figure 4a). Correcting translational components only was proven to be significantly more effective than correcting rotational components only (42.92% vs. 1.53%) (Figure 4b,c). Figure 2: Rotational and translational positioning error components for two representative patients, namely, P8 (**a**) and P24 (**b**) (cfr Table 1), across all treatment fractions. Both were 17 yo, treated for extra-cranial tumors, not in narcosis. NB The bottom and the top of each box represent the 25 th and 75 th percentiles of the samples, respectively. Their distance is the interquartile range. The red line inside the box is the sample median (skewness for the cases in which the red line is not centered inside the box). Black horizontal lines below and above the boxes are non-outlier minimum and maximum, respectively. Red crosses above maxima or below minima are outliers. Figure 3: Normalized norms (N) for all patients and fractions—full range (a), range 0–1 (b) and range \(>\) 1 (c). Color dots represent the values corresponding to the different patients’ fractions. Figure 4: Average normalized norms (N\({}_{\text{ave}}\)) across patients for all treatment fractions. Pre-correction is successful for all points below dotted line N\({}_{\text{ave}}\) = 1.