Technological advancements have enabled the integration of magnetic resonance imaging with linear accelerators, introducing new dosimetry challenges arising from strong magnetic fields within the ionizing radiation field. Notwithstanding the traditional pengeom subroutine, distributed with the PENELOPE package, adequately tracks particles in the presence of external electromagnetic fields, it is limited to geometries defined by quadric surfaces. Conversely, established developments such as PenEasy, which handle voxelized geometries within the PENELOPE main code, are constrained in performing particle tracking with external electromagnetic fields. In response, the here introduced voxgeom emerges as an innovative geometry subroutine tailored to track particles within voxelized regions seamlessly integrated with the PENELOPE main code, even when accounting for external electromagnetic fields. Voxgeom considers each voxel of the geometry as a body in a 3D arrangement, whereas interfaces are treated completely analogue to the pengeom subroutine. Furthermore, allows the use of patient-specific information to establish a univocal relationship between each body and material files, and integrates the models provided by the PENELOPE package for simulating electron/positron transport with external EM fields. Successful performance is obtained comparing voxgeom with the well-validated pengeom geometry manager both in absence and in presence of external strong magnetic fields. Differences up to 2 % and 1 % are reported between both subroutines, for the homogeneous and inhomogeneous phantom with magnetic field, respectively. Moreover, parameters such as percentage depth dose at depth of maximum dose, 10, and 20 cm are indistinguishable. Finally, promising dosimetry outputs are obtained using voxgeom to characterize dosimetry effects due to the presence of magnetic fields as happens in the Elekta Unity MR-LINAC in a representative patient-specific clinical case.