Methods and systems are described for determining quantities of interest for particle therapy. An example method can comprise determining one or more first functions indicative of spectral fluence associated with a particle therapy. The one or more first functions can be based on a plurality of simulations of a particle beam. The method can comprise determining, based on the one or more first functions and a second function associated with calculating a quantity, data indicative of the quantity. The method can comprise causing output of the data indicative of the quantity.