Since paediatric patients have higher life expectancy than adults and tend to receive higher secondary organ doses due to geometrical factors, secondary induced cancer due to out-of-field dose among paediatric patients is thus an increased concern. This work focusses on the influence of extrafocal dose in the out-of-field dose distribution using a paediatric anthropomorphic phantom. Monte Carlo simulations (Penelope) and Gafchromic EBT3 2D dose distribution measurement will be assessed.A 5-year pediatric phantom (hypothetical brain lesion) was irradiated by Varian TrueBeam 120HD MLC linac, using four 6MV Flattening Filter Free (FFF) static fields. A cranio-caudal beam was employed. The plan was simulated by Monte-Carlo (MC) simulation using PRIMO software. The 2D dose distributions at several distances from the isocenter were measured using Gafchromic EBT3 films. The results were evaluated using 2D Gamma index on PTW-Verisoft software. MC simulations were previously validated by gamma index method (98% of points <1; 2mm, 2%).Dose distributions were evaluated at 10, 15 and 17.5 cm away from the isocenter. The maximum local measured doses were 3.5, 1.6 and 1cGy per Gy at the isocenter respectively. A well-defined rectangular shaped dose distribution was noticed in the films located at 15 and 17.5 cm from the isocenter. The same shape was also detected in all the MC simulations of the same planes. The rectangular shape was not observed in the TPS (Acuros nor AAA). The center of this rectangular shape is aligned with the isocenter, and the width is consistent with the X-jaws opening direction. The dose values inside this rectangle are approximately 150% of the dose values in the surrounding regions.The rectangular shaped dose distribution was linked to the extrafocal dose due to the cranio-caudal beam, which propagates significantly in the out-of-field regions (lungs, heart, etc.), even after applying known methodologies to lower it such as using FFF modes; choosing lower-energy beams; and limiting the Y-field size.The authors acknowledge support from the project NORTE-01-0145-FEDER-000027, supported by Norte Portugal Regional Operational Programme (NORTE 2020), under the PORTUGAL 2020 Partnership Agreement, through the European Regional Development Fund (ERDF).