This paper aims at evaluating the thermal effects produced by the gradient coils on patients carrying metallic hip prostheses when undergoing check-up in combined magnetic resonance imaging with a linear accelerator systems. The computations are performed by two noncommercial codes specifically developed by the authors for voxel based human models. The electromagnetic field problem is solved by a hybrid finite element-boundary element technique implemented in a massively parallelized GPU system. The temperature elevation due to the electromagnetic exposure is then evaluated through a finite element code. The computations are performed for a large number of situations, considering both radial and axial arrangements of the patient.