Experimental evaluation of interface doses in the presence of air cavities compared with treatment planning algorithms

Abstract

A series of experiments were carried out to simulate air cavities in a polystyrene phantom. Dose was measured at air/polystyrene interface and as a function of depth. Results of experiments were compared to calculations done using three treatment planning systems. These systems employ Batho, modified Batho, and the equivalent tissue–air‐ratio methods for inhomogeneity corrections. The measured interface dose decreased by 55% for a 5 cm air gap, 5×5 cm 2 field size, and 6 MV photons. This has been attributed to a lack of electronic equilibrium and dispersion of secondary particles transported through the air gap. These results are at variance with predictions of calculations using three treatment planning systems, for which only a 10% decrease was calculated. This is because the calculation algorithms employed do not incorporate electron transport. Further experiments were conducted to study the contribution of scatter from sides of the walls of the cavities. Dose measurements as a function of depth were also performed to investigate the effect of primary fluence attenuation. The Batho algorithm did not show any sensitivity to the position of air gap sidewalls. This points to the need for proper inclusion of disequilibrium effects and shape of inhomogeneity.