A MODEL FOR THE LATERAL PENUMBRA IN WATER OF A 200-MEV PROTON-BEAM DEVOTED TO CLINICAL-APPLICATIONS

An experimental approach for modelling the lateral penumbra of a proto n beam has been investigated. Measurements were made with a silicon di ode in a water tank. Several geometrical configurations (phantom posit ion, collimator-to-surface distance, collimator diameter, bolus thickn ess, air gap, etc.) and beam characteristics (range, modulation, etc.) have been studied. The results show that the lateral penumbra is almo st independent of the beam modulation and the diameter of the collimat or. The use of scaled variables for depth and penumbra allows us to re present the increase in penumbra with depth for any configuration with a second order polynomial function, provided that the penumbra at the entrance of the medium and at the depth of the range are known. (C) 1 997 American Association of Physicists in Medicine.