Example methods and systems for determining a treatment plan for a radiotherapy treatment system to deliver radiotherapy treatment to a patient are disclosed. In one embodiment, the method may comprise performing a particle transport simulation to initiate a first set of particle histories and a second set of particle histories from a radiation source of the radiotherapy treatment system to a treatment volume with multiple voxels representing an anatomy of the patient. The method may also comprise determining first cumulative costs associated with respective particle histories in the first set and second cumulative costs associated with respective particle histories in the second set. The method may further comprise selecting the first set or second set based on the first cumulative costs and second cumulative costs and determining the treatment plan based on the selected first set or second set.
