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, 5ϫ5 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.

Clinical implications of the anisotropic analytical algorithm for IMRT treatment planning and verification

“…Significant errors in the calculated dose distributions often result from such algorithms' poor modelling of situations in which electronic equilibrium does not exist, in particular in the presence of inhomogeneities [2,3,4] or when small fields are used [5,6]. More recently, the introduction of convolution-superposition (CS) algorithms that better account for electron transport has enabled improved estimation of dose distributions, particularly in the absence of electronic equilibrium.…”

Dosimetric verification of the anisotropic analytical algorithm for radiotherapy treatment planning

“…Historically, one of the most serious weaknesses in treatment planning systems has been their ability to accurately predict doses in the presence of inhomogeneities, particularly through poor consideration of electron transport [7,22]. Inaccuracies in dose calculation result in systematic errors in radiotherapy treatments and so are of particular importance [13].…”

Evaluation and Commissioning of Commercial Monte Carlo Dose Algorithm for Air Cavity

“…MC algorithm was able to predict the dose distributions with a higher accuracy [3]. Many researchers have investigated the effect of air cavities on the dose distribution and dose reduction near air cavity, depending on geometry, beam energy, and field size using various MC codes in water equivalent phantoms [4][5][6][7].…”