1. A method for simulating particle beam interactions for reducing computation time of particle dosage applied to a lesion, comprising: identifying a set of n functions F 1 , F 2 ,. .. , F n corresponding to a plurality of different physical aspects of a particle beam; performing simulations of each F i using a full physics model; selecting for each F a distribution function f i that models relevant behavior of the particle beam; reducing computation of the full physics model corresponding to each F i by replacing F i with the distribution function f i , wherein reducing computation includes: computing the particle dosage using at least one f; computing energy loss fluctuations in the particle beam; comparing a set of simulations wherein each f i replaces Its respective F i to determine if relevant behavior is accurately modeled; and selecting one of f i or F i for each n, for a Monte Carlo simulation based on a runtime and accuracy criteria; treating the lesion based on the computed particle dosage and the computed energy loss fluctuations.