Dose corrections for low-density tissue inhomogeneities and air channels for 10-MV x rays

Methods of correcting for tissue inhomogeneities which consider only changes in the photon fluence are of limited usefulness for 10-MV x rays. Although there is normally transient electronic equilibrium on the central axis beyond the depth of maximum buildup in soft tissue, when the beam enters a low-density material the beam profile is degraded and there is a loss of lateral electron equilibrium which reduces the dose both within and beyond the inhomogeneity. This paper describes experimental measurements of the dose within and near large, low-density inhomogeneities simulating lungs and also behind air channels (where similar effects occur). Methods of calculating correction factors are discussed.