Comparison of experimental 3D dose curves in a heterogeneous phantom with results obtained by MCNP5 simulation and those extracted from a commercial treatment planning system

Full text: Standard planning systems used in radiotherapy treatments use determinist correlations to evaluate doses at the irradiation region. Nevertheless, it is well known that assigned dose with this type of planners can be problematic, especially when analyzing heterogeneous zones, as is the case inside human bodies. The present work is focused in quantifying the dose distribution in a heterogeneous medium irradiated by a 6MV and 15 MV photon beam from the Elekta Precise Radiotherapy Unit header, selecting a 20 cm x 20 cm and 10 cm x 10 cm field sizes. The heterogeneity consists of a low density slab of extruded polystyrene and it is placed inside a Scanditronix RFA-300 water cube-shaped phantom at 10 cm depth from the water surface which is located at a SSD of 100 cm. Its dimensions are 30 cm x 10 cm x 8 cm and it has a density of 0.0311 g/cm3. Dose shape inside the water phantom has been calculated, firstly, simulating the photon and electron transport with the Monte Carlo computer code MCNP5 and on the other hand, extracting the dose curves that the commercial planning software PCRT 3D establishes in the same irradiation conditions. Since the accuracy of the calculated results are highly dependent on the strictness of the Monte Carlo simulation model, all the physics parameters and geometry specifications of the radiotherapy unit and heterogeneous water phantom have been carefully selected, as well as the photon energy spectra definition used in the model. Results obtained in the simulation have been also compared with the relative depth dose in the central axis of the water phantom and profile curves measured experimentally at the Hospital Clinic Universitari de Valencia