Method and system that allows patient dose response functions to be calculated, prior to treatment planning, for external photon beam radiotherapy. In one embodiment, for each location where a separate dose value is desired, referred to as a dose region, adjoint calculations are performed to calculate the adjoint solution fields associated with that dose region. Once potential beam directions are specified, a ray tracing process is performed to transport the adjoint solution fields out of the patient and to locations where treatment plan parameters may be specified. The output of this process is the dose response at each dose region resulting from a prescribed photon fluence, at a given location, direction, and energy.