Purpose: This doctoral dissertation presents the methodology and results for the dosimetric evaluation of absorbed dose distribution calculation and optimization system Monaco, which uses a Monte Carlo simulation based algorithm. Monte Carlo based treatment planning system (TPS) are the most accurate approach for particle transport. They can calculate absorbed dose in terms of dose to media, Dm, as well as in terms of dose to water, Dw, which is retrospectively converted from Dm. The purpose of our study was to analyse differences between dose-tomedium and dose-to-water calculation options for Monaco TPS (Elekta, Sweden) and to propose a supplement to the existing verification methodology to make the verification of Monte Carlo based systems used for radiation oncology unambiguous and straightforward. Methods: Dosimetric evaluation for the two calculation modes was performed using several homogeneous phantoms, a semi-anthropomorphic and an anthropomorphic phantom. Measured absorbed dose distributions were compared to calculated absorbed dose distributions. Ionization chambers with different effective volumes and a two-dimensional detector with an ionization chamber array were used for measuring the absorbed dose. Additionally, measured absorbed dose distributions were compared to absorbed dose distributions calculated with two analytic calculation algorithms. As a supplement to the calculation algorithm evaluation in a point, a two dimensional dosimetric evaluation of absorbed dose distribution was performed. Different inhomogeneous measuring and computing geometries were designed to allow X-ray beams to pass through media of different densities on their path to the array of small volume ionization chambers. Measured absorbed dose distributions were compared to those calculated using gamma method which assesses the difference in dose for low dose gradient regions and spatial component for high dose gradient regions. Furthermore, we investigated the effect of dose-to-water and dose-to-medium calculation options built in Elekta Monaco TPS on intensity modulated radiotherapy (IMRT) dose distributions in clinical situations. Results: For a water equivalent medium and low density medium differences were statistically insignificant and it was confirmed that the most accurate absorbed dose distributions were calculated using an Monte Carlo based calculation algorithm. The statistically significant differences were observed for high density media between measured and calculated absorbed dose values and therefore several measuring/calculation geometries were designed for additional analysis. To overcome potential ambiguity on the correctness of the verification results in high-density media, we proposed a supplement to current comprehensive verification methodology for performance of algorithm built in the Monte Carlo based treatment planning systems. It is based on the physical principles which are ground for the concept of dose measurements of high energy X-ray beams used for radiotherapy. With this addendum Monte Carlo based calculation algorithms can be verified in described geometry for high-density media with higher level of confidence. Considerable difference among dose distributions calculated using respective calculation option obtained in semi-anthropomorphic phantom was also reported in clinical cases. Even though the discrepancies in tissue equivalent media (all target volumes and most of organs at risk) were not statistically significant, in bony structured organs at risk (mandible, cochlea) the discrepancies between two calculation modes was shown to be statistically significant. Conclusions: This dissertation presents the results of the dosimetric evaluation of the calculation algorithm which uses Monte Carlo simulation for absorbed dose calculation, built in a system for calculation and optimization of absorbed dose distribution Elekta Monaco. A supplement to current comprehensive verification methodology for performance of algorithm built in the Monte Carlo based treatment planning systems was proposed. Although proposed addendum to the verification methodology is not the answer to the dilemma which calculation option shall be used for radiotherapy treatment prescription, it provides additional tools for the verification of the TPS calculation algorithms based on the physics behind.