Beam spreading is the foundation of proton therapy with large area and depth tumour.The characteristics of proton lateral beam distribution for dualring double scattering system are investigated using Monte Carlo multiparticle transport code Fluka and the results are compared with analytical computation

when the same radius of beam spreading is considered. It is found that the first scatterer is thinner than the results obtained from analytical computation and they are coincided better with thinfoil than thickfoil. For dualring double scattering system

as the thickness of foils increase

the radius of beam spreading increase

but the efficiency of beam use increase firstly and then decrease

the maximum value is about 27%.