## 2 Materials and Methods
### Procedure  
The ethics committees of Hospital Sant Joan de Deu in Barcelona approved this study. Before treatment, written informed consent was obtained from both the patient and her parents. The patient was informed about the procedure, the confidentiality of her data, and her right to withdraw from the study at any time. The study consisted of eight sessions in total. Five of these sessions were focused specifically on the VR intervention. The entire course of treatment spanned seven consecutive weekly sessions, which included the initial assessments and final evaluations. A follow-up assessment was conducted six months later. All sessions were administered by a female psychologist (see Table 1). The experimental design is detailed in the following information, as illustrated in Figure 1.  
The pre-treatment assessment session, lasting approximately one hour, involved several preparatory steps. Initially, a personalized avatar was created using frontal and lateral photographs of the patient. The avatar's silhouette was manually aligned with the patient's physical measurements. Simultaneously, the patient completed the pre-treatment questionnaires. Following these initial steps, the patient was immersed in the virtual environment, and the ET device integrated into the HMD was calibrated. The FBOI was then induced through visuo-motor and visuo-tactile stimulation techniques lasting five minutes (for further details, see [46]). Subsequently, measurements were taken of the patient's current body anxiety, fear of gaining weight, and sense of FBOI using a VAS. To assess body-related AB, the patient observed herself in the virtual mirror for 30 s, following the method described in [PERSON] et al. [14] and [PERSON] et al. [47], and her eye movements were monitored by the ET device. To ensure the patient maintained focus on her reflection and to minimize distractions, the researcher monitored the ET data in real time. This allowed the researcher to detect whether the patient's gaze strayed from the mirror image for extended periods, such as looking at the wall or door. In such cases, the researcher would gently remind the patient to refocus on her reflection in the virtual mirror. As a cover story, the patient was informed that this was part of a sensor calibration process to prevent any bias from knowing the true objective, which was disclosed only after the completion of the study to avoid influencing the patient's gaze patterns. Clinical sessions: ABMT + MET. Each clinical session, lasting approximately an hour, included ABMT followed by MET within the virtual environment. The exposure began with the patient's virtual body at her real BMI and progressed to avatars with incrementally increased BMI in subsequent sessions, culminating in a target healthy BMI. This target BMI was calculated based on the patient's age, sex, and pre-disorder BMI, following the growth charts by the World Health Organization [48]. Each session began with the calibration of the ET device and the induction of FBOI, followed by assessments of anxiety, fear of gaining weight, and FBOI using a VAS. ABMT: This component was adapted from the AB induction procedure proposed by [PERSON] et al. [49]. It involved the visual selection of geometric figures that matched specific body areas on the avatar. Participants focused on the body part highlighted by these figures for 4 s as they were progressively illuminated, aiming to balance attention