The purpose of this study was to first validate a 3D-printed personalized pediatric phantom for use in out-of-field photon and proton dosimetry, and to secondarily assess the impact of phantom design on the accuracy of out-of-field dose measurements. This study presented a preliminary validation for the use of a personalized 3D printed phantom for out-of-field photon dosimetry and provided an initial test for use in out-of field proton dosimetry. This work also demonstrated that accuracy in dosimeter placement with respect to the field edge was of equal or greater importance than phantom design when measuring out-of-field dose for photon therapy.