Radiation oncologists are particularly concerned about tumours growing on the surface of air cavities in the head and neck regions, which involve treatment with small x-ray fields. An inhomogeneous dose distribution exists within and beyond the cavity. This is caused by the loss of electron equilibrium and the attenuation of both the primary and scattered photons is altered. 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. Using a Markus ionization chamber and different solid water slabs to simulate different air cavities, it is found that internal body cavities, depending upon their sizes, experience underdose or overdose on the distal surfaces of the cavities when compared with the results predicted by an ETAR correction method for 6 MV and 18 MV x-ray beams. For an infinitely long air passage of dimensions 2 cm x 2 cm, the error in the ETAR correction for a 6 MV x-ray beam is 4.8%, 0.5% and 1.1% for the field size of 5 cm x 5 cm, 7 cm x 7 cm and 10 cm x 10 cm respectively. The ETAR correction is accurate to within 1.6% for a 6 MV x-ray beam provided that the field size is 5 cm across the cavity and greater than 7 cm along it.