In this study, dose distribution calculations for bidirectional interlaced microbeam radiation therapy (BIMRT) were performed with a detailed head phantom model and the Monte Carlo code MCNPX. Doses were calculated in intracranial targets of dimensions 20 x 6.8 x 20 mm(3) and 20 x 20 x 20 mm(3) and surrounding tissue for which interlacing arrays are composed of 5 and 15 microbeams, respectively. Simulations were performed with a realistic head phantom and a homogenized head phantom of the same outer shape to study the effects of the structure of the realistic phantom on dose distribution and to show how important it is to use realistic phantoms. Depth-dose profiles and dose falloffs at the edges of the targets were calculated for cases with and without an Au contrast agent deposited in the target region and surrounding tissue. The parallel pattern of the microbeam arrays was preserved through the head phantom which makes it possible to interlace microbeam arrays even at deep seated targets. As the dimensions of the target volume were increased, the valley dose values increased with the number of microbeams. This sets limits on the size and position of the target. The usage of gold as a contrast agent provided a substantial increase in target dose and decreased the skin entrance, maximum skull bone and maximum brain doses inevitable to produce the desired target dose. Short dose falloffs at the edges of the targets were preserved for all cases.