When performing multimodal radiotherapy planning, an optimizer (36) concurrently optimizes a combined treatment plan that employs an intensity modulated radiotherapy (IMRT) device (30) and an intensity modulated proton therapy (IMPT) that respectively generate a photon beam and an ion beam for treating a volume of interest (18) in a patient (34). A simulator (40) iteratively generates multiple variations of a simulation model (44) according to optimization parameters that are varied by the optimizer (36) until the simulation model (44) satisfies user-entered treatment objective criteria (48) (e.g., maximum dose, does placement, etc.)