Monte Carlo simulation of a therapeutical proton beam line

Proton radiation therapy usually requires the use of beam modifying devices to ensure a homogeneous and conformal coverage of the target volume; A sharp lateral penumbra, a good flatness over the target volume and an appropriate range can be obtained by using suitable modifiers. These elements affect the spatial and energetic distributions of the therapeutical beam. On the basis of the Geant code, a Monte Carlo program has been developed in order to simulate the energy losses and the angular diffusions of the protons within these devices and the patient's structures. The capability of the program in evaluating dose distributions has been verified by comparison with measurements in water phantom. Some tissue substitute materials have been introduced in the beam line, upstream of the water tank, to investigate the perturbations they introduce in the proton range and in the particle fluence at the air-material interfaces. In general the model agrees well with the experiments, since in the worst case we find a difference of 1 mm in the penetration depth of the protons.