This work is focused on coupling PLanUNC (PLUNC), a set of software tools for radiotherapy treatment planning (RTP), with MCNP5 Monte Carlo N-Particle transport code, utilizing the RANDO phantom as the patient model and the Elekta Precise linac as the irradiation source for comparison. Thus, the main goal of this paper is to compare the results obtained from the default calculations of the treatment plan software with those obtained via the implementation of MCNP5 calculations. Monte Carlo techniques have been proved to be a more accurate dose calculation aid than conventional treatment planning systems, having the only limitation of computer time. The implementation of MCNP5 calculations in a commercial RTP software aims to provide more accurate dose mapping of the patient in reasonable computer times. The results obtained in this paper represent a significant contribution in the development of RTP patient dose simulations.