The effect of lateral electron disequilibrium on patient dose has been investigated. This has been achieved by dosimetry in lung and air cavity phantoms at megavoltage x-ray energies. The scatter function photon beam models for tissue inhomogeneity, such as the ETAR correction algorithm, currently implemented in commercial treatment planning systems do not predict the dose distribution accurately in many situations where lateral electron equilibrium does not exist. The lung phantom is made up of solid water slabs and lung analogue slabs. Using a thimble ionization chamber, a Markus ionization chamber and TLDs the problems of central axis dose reduction and penumbral flaring in lung for x-rays have been investigated. It is found that the ETAR correction predicts the dose at mid lung with varying degrees of accuracy depending on the field size. It was found that internal body cavities, depending on their size, experience underdose or overdose in the distal surfaces of the cavities when compared with the results predicted by an ETAR correction algorithm. Therefore, this energy is not recommended for use in situations where cavities arise / Master of Science (Hons)