The phase-space of a radiation source is a 7 parameter distribution;
entry position at a bounding surface, (x,y), trajectory, (u,v,w), energy
and charge Q.
Workers use highly detailed simulations of the therapeutic linacs to achieve consistency with measured data.
Some input parameters (e.g. electron potential) are not known and inevitably a degree of iteration is required.
CPU time required to simulate a therapeutic linear accelerator in sufficient detail to perform MC RTP is currently months. The storage requirements for the phase-space data generated is Gigabytes.
It would be useful to reduce this information requirement but a small phase-space data set may not have sufficient information to reproduce sharp features such as radiation penumbra.