Monte Carlo based investigation of the influence of accelerator head geometry on megavolt photon beam quality in radiotherapy

Sammanfattning: "This project concerns the precise characterization of one of the VARIAN accelerators used in the radiotherapy clinic at the Finsen Centre at Copenhagen University Hospital, Denmark. Detailed characteristics of the clinical beam incident on the patient are almost impossible to measure. Even though manufacturers provide the necessary information about the specific accelerator, the models are constantly improving and the individual purchasers often adjust the machines to match e.g. the characteristics of a previous or of an existing accelerator. One particular concern is that it is impossible to get accurate information about the primary electron beam, such as its energy and its radial intensity distribution, as it leaves the accelerator vacuum window and hits the bremsstrahlung target. With the Monte Carlo technique it is possible to simulate the radiation transport through the accelerator head and achieve a better understanding of the clinical beam. The accuracy of the simulated beams were validated by the agreement with measured dose distributions. In this project the photon beams from a Varian Clinac-23EX accelerator were investigated. This was done by simulating 6 and 18 MV photon beams for two different field sizes, 10 x 10 cm2 and 40 x 40 cm2, using the BEAMnrc and the DOSXYZnrc Monte Carlo code system. The linac geometry was used as input to the Monte Carlo code with specifications obtained from the vendor of the accelerator. Total dose measurements were performed in water using a Scanditronix-Wellhöfer RFA-300 beam scanner with an RK-8305 ionization chamber. To validate the Monte Carlo model for the photon-beam output from the Varian Clinac-23EX, measured and calculated relative depth-dose data along the central-axis and dose profile at two different depths, Dmax and 10 cm, were matched. This required some fine tuning of the incident electron beam parameters, such as its energy, energy distribution and radial intensity distribution. A good agreement between calculated and measured dose distributions was found, except near the surface for larger fields, particularly for the 18 MV photon beam. The final primary electron beam incident on the target, to get the best fit, was found to be monoenergetic with energies of 6.4 MeV and 17.5 MeV for the 6 MV and 18 MV photon beam, respectively. The optimal radial intensity distribution of the electron beams had a Gaussian spread with widths of 1.2 mm and 1.5 mm for the 6 MV and the 18 MV photon beam, respectively. This information will be important for future treatment planning, e.g. as a benchmark for clinical treatment planning systems. ii"