###### Abstract  
The purpose of this retrospective study was to simulate a daily pre-alignment strategy to mitigate systematic positioning errors in image-guided pediatric hadron therapy. All pediatric patients (32 patients, 853 fractions) treated from December 2021 and September 2022 at our Institution were retrospectively considered. For all fractions, daily correction vectors (CVs) resulting from image registration for patient positioning were retrieved in the form of txt files from the hospital database. For each fraction, an adjusted correction vector (V\({}^{\prime}\)) was then computed as the difference between the actual one (V) and the algebraic average of the previous ones, as to simulate patient pre-alignment before imaging. The Euclidean norm of each V\({}^{\prime}\) was computed and normalized with respect to that of the corresponding V to derive N. Pre-correcting all the coordinate values led to a 46% average reduction (min 20%, max 60%) in CVs, considering the first 27 fractions (average value in this cohort of patients). Such a potential improvement (N \(<\) 1) was observed for the most patients' fractions (781/853, 91.6%). For the remaining 72/853 cases (8.4%), a remarkable worsening (N \(>\) 2) involved only 7/853 (0.82%) fractions. The presented strategy shows promising outcomes in order to ameliorate pediatric patient setup before imaging. However, further investigations to identify patients most likely to benefit from this approach are warranted. radiation oncology; particle therapy; pediatric oncology; imaging; setup +
Footnote †: journal:paradigm shift in the field of pediatric radiation oncology [3]. With a reduction in integral dose to normal tissues up to 60% compared to conventional photon RT due to its physical advantages, it holds the promise to reduce the incidence and severity of short-term and long-term toxicities [4]. If on the one hand the steep dose gradients allow for the greater sparing of healthy tissues, on the other, they make this treatment modality very sensitive to targeting uncertainties, arising when the position of the patient in the planning phase is not replicated identically in the treatment sessions [5; 6]. Any deviation from the planned position could result in the underdosage of target or overdosage of the organs at risk (OARs), undermining the efficacy of treatment and toxicity outcomes [7]. The initial patient positioning based on laser alignment marks on the mask is not accurate enough to ensure setup consistency [8]. As dose conformity and gradients increase, accurate patient alignment is therefore mandatory, and image-guided RT (IGRT) becomes a prerequisite [9; 10; 11]. The image guidance systems currently available in particle therapy facilities consist mainly of 2D orthogonal X-ray imaging systems, in-room 3D-computed tomography (CT) or on-board cone-beam CT (CBCT) [12], which enable one to readily identify setup uncertainties and organ motion, and hence improve the quality of treatment [13]. Daily acquired in-room images and digital reconstructed radiographs (DRRs) from simulation CT (sCT) are registered (i.e., overlaid), and the output of such a registration procedure is a numerical transformation that aligns the position of the patient at the time of simulation with the one at the time of treatment, with respect to the treatment isocentre [14]. Practically, this is achieved by correcting the position of the couch accordingly. Specifically, if the couch has six degrees of freedom (DOF), a correction vector (CV) featuring six components (rotational and translational) will be the output of the registration procedure. In HT, as well as in photon RT, each treatment fraction is normally preceded by setup verification that allows one to compensate for positioning errors.