Monte Carlo study on a new concept of a scanning photon beam system for IMRT

Intensity-modulated radiation therapy (IMRT) is almost exclusively realized using a multi-leaf collimator (MLC). In this work we investigated alternative approaches to realize an IMRT - scanning photon beam system. The technical realization of this concept required investigating the influence of various design parameters on the final small photon beam. This was done using Monte Carlo (MC) simulation methods. The resulting photon beam that is to be scanned should have a diameter well less than 10 mm at a source-surface distance (SSD), and the penumbra should be as small as possible. A first draft for this system, based on the PRIMUS 6 MV accelerator at DKFZ (Deutsches Krebsforshungszentrum), was proposed and modeled using the BEAMnrc/EGSnrc MC code. We then proposed and studied a new geometry of the source-target-collimator system. Calculations were done for 108 particles, using an electron energy cut-off (ECUT) = 0.7 MeV, and a photon energy cut-off (PCUT) = 0.01 MeV. The influence of different collimator parameters, different target construction and various incident electron beam characteristics was studied. Calculations of the dose absorbed in the water were performed for 8 different collimators at a distance of 40 cm from the collimator exit, which is the medical requirement. Results of the dose distribution calculations are presented as photon beam profiles with the values of full width at half-maximum (FWHM) and penumbra (PM) for every beam profile. The influence of target construction was studied for different thicknesses of target and material minimizing electron contamination. The influence of the following characteristics of the incident electron beam was also investigated: size of electron beam, energy, displacement of the beam from the axis of target-collimator system, shape of the electron beam profile. The field dose distribution of the photon beam was calculated for the collimators giving the beam profiles. Basing on the work performed in this investigation, it will be possible to define adequate parameters for the target-collimator system as well as on the scanning electron beam for new IMRT system.