Radiotherapy X-ray beam inhomogeneity corrections: the problem of lateral electronic disequilibrium in lung.

Accurate dose calculations in lung are important to assess lung and tumour dose in various radiotherapy cancer patients. Those patients of particular relevance are Ca lung and Ca Oesophagus patients because large volumes of lung are irradiated to high doses. In this paper, dosimetry results for megavoltage X-ray beams obtained in a lung phantom are compared with dose computations produced by (1) effective path length, (2) equivalent tissue-air ratio, (3) super-position/convolution and (4) Monte Carlo dose calculation methods. The mid-lung dose error at 10 MV for a 5 x 5 cm field is 10.0%, 6.7%, 1.9% and 0.6% respectively. Tests at the lower energy of 6 MV with a field size of 10 x 10 cm show a mid-lung error of only 2.0% for the equivalent tissue air ratio method. At this energy it appears that central axis dose voids are sufficiently small to enable the routine use of the equivalent tissue air ratio method. At the higher energies tested, 10 and 18 MV, this method is accurate. Superposition and Monte Carlo methods are presented which show good agreement with experimental results in a lung phantom even in regions of lateral electron disequilibrium.