The subject matter described herein provides methods for developing an IMRT treatment plan for a radiotherapy system. In one aspect, the method can include providing initial treatment parameters. These parameters can include a number of isotopic beams, a transmission angle for each beam, a prescribed dose for a target, and dose volume histogram constraints. The method can further include determining an initial fluence map including one or more beamlets. The initial fluence map can specify a fluence value for each beamlet. The method can further include determining a delivery sequence for the fluence values in the initial fluence map. The delivery sequence can include one or more apertures formed by the leaves of a collimator. These apertures can have a specified size constrained to substantially less than a maximum size associated with the collimator, when the isotopic beam is near a critical structure. Related apparatus and systems are also described.