Purpose: With intensity modulated radiation therapy (IMRT), the physician can prescribe, design and deliver optimized treatment plans that target the tumor and spare adjacent critical structures. The increased conformity of such plans often comes at the expenses of adding significant complexity to the delivery of the treatment. With volumetrically modulated arc therapy (VMAT), in addition to the modulation of the intensity of the radiation beam, other mechanical parameters such as gantry speed and dose rate are varied during treatment delivery. It is therefore imperative that we develop comprehensive and accurate methods to validate such complex delivery techniques prior to the commencement of the patient's treatment. Methods: In this study, a Monte Carlo simulation was performed for the high definition multileaf collimator (HD-MLC) of a Varian Novalis TX linac. Our simulation is based on the MCSIM code and provides a comprehensive model of the linac head. After validating the model in reference geometries, treatment plans for different anatomical sites were simulated and compared against the treatment planning system (TPS) dose calculations. All simulations were performed in a cylindrical water phantom as opposed to the patient anatomy, to remove any complexities associated with density effects. Finally, a comparison through gamma analysis of dose plane between the simulation, the TPS and the measurements from the Matrixx array (IBA) was conducted to verify the accuracy of our model against both the measurements and the TPS. Results: Gamma analysis of ten IMRT and ten VMAT cases for different anatomical sites was performed, using a 3%/3 mm passing criterion. The average passing rates were 97.5% and 94.3% for the IMRT and the VMAT plans respectively when comparing the MCSIM and TPS dose calculations. Conclusion: In the present work a Monte Carlo model of a Novalis TX linac which has been tested and benchmarked to produce phase-space files for the treatment head of the linac was used to produce a input phase-space to calculated dose deposition phenomena in different geometries for IMRT and VMAT treatment modalities. The control points defined for the MLC were replaced by blocks with the same characteristics and materials of the linac MLC to speed up the simulation time. With this technique a simulation of a typical IMRT case can be performed with a 10 computer cluster in about 1.02 hours in average. If the number of computer used is increased the computing time can be reduced even more which make our model suitable for clinical use as a second check method to compare the TPS dose calculated. Our results showed that for IMRT and VMAT deliveries with a HD-MLC, there is an average of 95.9% of the points have a gamma index less than 1 with our chosen criterion between our Monte Carlo simulations and the corresponding measurements and TPS calculations in a cylindrical water equivalent phantom. This Monte Carlo code can be used as pre-treatment, independent dose calculation verification for IMRT and VMAT deliveries.