Influence of lung tissue on the dose distribution of high energy photon beams.

Dose distributions in unit density (water equivalent), low density (lung equivalent) and heterogeneous (water with regions of lung equivalent) materials are described. While interest is focussed on the inhomogeneous situation the homogeneous cases will provide better understanding. In each case it is shown how primary and scattered photons as well as secondary electrons shape the depth dose curve and the lateral dose profiles. The influence of several parameters like lung thickness and density, distance from interfaces, field size, photon energy etc. is discussed. An investigation on dose calculation methods for inhomogeneous phantoms shows to which extent these are suitable for total body irradiation. Possible improvements require further systematical experiments. Because lung tissue differs from soft tissue only in its density, dose measurements in the lung can be handled in the same way as in tissue equivalent materials.