A multi-plane source model for out-of-field head scatter dose calculations in external beam photon therapy

Our purpose was to assess the out-of-field dose component related to head scatter radiation in high-energy photon therapy beams and then derive a multisource model for this dose component. For scattered photons, several planar sources have been defined, with number, location and tilt depending on the complexity of the field shape. In the absence of precise knowledge of out-of-field scattering characteristics, several assumptions are made to derive emission spectra and radiation intensity from measurements. Among these, the Compton formula is used to evaluate scattered photon energy and the Henyey-Greenstein phase function is used to evaluate the scattered photon angular distribution. For measured doses under out-of-field conditions, the average local difference between the calculated and measured photon dose is 10%, including doses as low as 0.01% of the maximum dose on the beam axis. This study demonstrates that the multi-plane source approach is suitable for accurate analytical modeling of the out-of-field dose component related to head scatter radiation. These results should be taken into account when evaluating doses to the remaining volume at risk in external beam radiotherapy planning.