Independent Monitor Unit Calculations in Intensity Modulated Radiotherapy

Sammanfattning: Independent dose calculations in radiotherapy are important as they help assure that the dose given to the patient is the same as the prescribed. The current version of software Radiation Verification Programme, RVP, does not take into account the effects that occur when a beam from a linear accelerator is moved off-axis. The aim with this project was to find a model for these effects, fit the parameters in the model with data from measurements and then to test the model in clinically relevant situations. Preferably the model should be easily incorporated into the software. Three variable parameters were used for the model; the slope, which is the proportional factor with which the dose increases with the distance, the width which affects the penumbra, and the leakage, which is the transmission through the field shielding. To fit the parameters for the model, measurements of fields placed off-axis were performed. This was done for two different linear accelerators; Varian Clinac 2100 and Elekta Precise, and one energy for both accelerators; 6 MV (Mega Voltage). Profiles were measured as to get the fields’ entire spread. The data gained from these measurements were used to fit the model’s parameters. The fitted model was first tested on profiles and then on IMRT fields in two dimensions. The model was simple, both in terms of measurements and implementation and showed good agreement for profiles measured through the central axis. The second step was to test the model for IMRT fields. The 2D arrays were compared using gamma comparison and with a 5 %/5 mm demand the pass rate was very high. Only a small number of fields were used and therefore no statistical certainty is guaranteed, but if further testing get similar results, the model can be used in clinical cases for dose calculations for beams off-axis and IMRT fields.